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What Drives Vaccination Rates?

<h3>Is there any federal role in vaccine policy?</h3>
<p>The CDC's Advisory Committee on Immunization Practices (<a href="http://www.cdc.gov/vaccines/hcp/patient-ed/conversations/downloads/vacsafe-acip-color-office.pdf" target="_blank">ACIP</a>) serves as the nation's advisor on vaccine recommendations. Its 15 expert members meet three times a year to review studies, safety data, and other factors related to national vaccination matters. They also participate in working groups all year, as needed, to stay abreast of vaccine-related developments, including availability of vaccines and outbreaks of diseases.</p>
<p>In making their recommendations, ACIP members aim to ensure that specific populations receive vaccine protection at the safest and earliest possible age, before the members of those populations reach peak risk periods. All vaccines must be licensed by the FDA, a process that requires years of safety studies and other testing. Other factors that the ACIP considers include how many children would be at risk of a disease if large numbers of them aren't vaccinated, how a vaccine's effectiveness varies according to the age it's administered, and what the severity is of the target disease.</p>
<p>Some diseases, such as measles, are common and can involve serious complications; the measles vaccine is thus on the recommended childhood vaccination schedule in the United States. But a mosquito-borne disease called Japanese encephalitis, while common in rural areas of Asia, is not at all common in the United States; so although a vaccine for this disease exists, it is <a href="http://www.cdc.gov/vaccines/hcp/vis/vis-statements/je-ixiaro.html" target="_blank">not recommended</a> for U.S. citizens unless they're two months or older and will be traveling to certain parts of Asia.</p>
<h3>What are reasons for vaccine exemptions?</h3>
<p>At the state level, the default position across all 50 states is to require proof of up-to-date vaccination against a defined set of diseases for children who are entering kindergarten and for children age five and older who attend a state-run day care. But parents have three possible ways to opt out of having their children vaccinated: medical, religious, or philosophical/personal-belief exemptions.</p>
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<p>Children who need medical exemptions rely on a high level of immunization in their community for protection against vaccine-preventable disease, a concept known as herd immunity.</p>
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<p>Sometimes, a child needs an exemption for medical reasons—such as having a suppressed immune system or having an allergy to some ingredient in a given vaccine. Children who need medical exemptions rely on a high level of immunization in their community for protection against vaccine-preventable disease, a concept known as herd immunity. All states allow medical exemptions, typically with a doctor's letter.</p>
<p>But not all states offer exemptions for personal or religious beliefs. Where these other kinds of exemptions are an option, the requirements for justifying such requests vary considerably across the country.</p>
<p>All but two states, Mississippi and West Virginia, include religious exemptions in their vaccine policies. Possibly not coincidentally, as of mid-2014 Mississippi and West Virginia haven't seen a measles outbreak since the early 1990s.</p>
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<img alt="" class="richtext-image full-width" src="https://staging.pbs.org/wgbh/nova/media/images/vaccination-rates-01.width-990_cAbIKRk.jpg"/>
<figcaption>Exemptions for immunization can be easy or difficult to get, depending on the state.
        
        <div class="credit">© Mother Jones; source data from New England Journal of Medicine 2012</div>
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<p>As of 2014 there are 20 states that allow some kind of personal or philosophical opt-out from childhood vaccinations. Of those 20 states, eight—including Colorado, Rhode Island, and Arizona—have made it comparatively easy for a parent to claim a personal-belief exemption. In most cases, all a parent has to do is sign a form. But other states—including Texas, Minnesota, and Arkansas—have rules that are somewhat stricter. Those states require steps such as submitting a notarized form with a clinician's signature, sometimes with a letter explaining the reason for the exemption request.</p>
<p>The laxity or stringency of these requirements seems to affect exemption rates. States with tougher rules have <a href="http://www.ncbi.nlm.nih.gov/pubmed/22992099" target="_blank">lower exemption rates</a> and vice versa. And tightening existing rules appears to make a difference: When Washington State toughened its personal-belief exemption criteria to require a clinician's signature on the exemption form, exemption rates dropped 25% between 2009 and 2011.</p>
<h3>How do policies affect vaccine exemptions?</h3>
<p>Obviously, then, if public health officials want more parents to vaccinate their children, making nonmedical exemptions more difficult to get is a key tactic. But convincing state legislators of the wisdom of that approach isn't necessarily an easy matter. In fact, in recent years, some states—including include Kansas, Iowa, and Florida—have seen legislative attempts to make exemptions <em>easier</em> to obtain. A 2014 study of vaccine-related legislative proposals found that 36 bills regarding exemptions were introduced in 18 states between 2009 and 2012—and 31 of them were intended to <a href="http://www.ncbi.nlm.nih.gov/pubmed/24519303" target="_blank">ease the criteria</a> rather than toughen them. But to the likely relief of public health officials, none of the bills aimed at easing exemptions passed, while three of the five bills tightening restrictions were enacted into law.</p>
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<img alt="Vaccine bills" class="richtext-image full-width" src="https://staging.pbs.org/wgbh/nova/media/images/vaccination-rates-02.width-990_Rbrn8sK.jpg"/>
<figcaption>A number of bills to change vaccine policy have been introduced in various states throughout the country.
        
        <div class="credit">© Mother Jones; source data from Saad Omer, Emory University</div>
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<p>California is among the states that have sought to follow Washington State's lead and toughen requirements for nonmedical vaccine exemptions. The Golden State is home to hot spots of vaccine opt-outs, with rates higher than 60% in some schools. And <a href="http://www.cdph.ca.gov/programs/immunize/Documents/ChildhoodImmunizationCoverageCA2006-08.pdf" target="_blank">personal-belief exemptions for California kindergartners tripled</a> between 1979 and 2008. In response, in January 2014, a new California law came into effect, requiring parents to obtain a clinician's signature when claiming a personal-belief exemption. In signing the bill, however, California Governor Jerry Brown told the Department of Public Health to add a religious exemption option that would not require a clinician's signature. This addition concerned some public health experts in California, not least because exemptions in that state had already climbed at least 12% from 2012 to 2013, before the law came into effect. Time will be the final arbiter of whether exemptions go up—a sign that the requirements made opting out easier—or go down—a sign that the <a href="http://www.forbes.com/sites/emilywillingham/2013/11/04/could-california-law-to-boost-vaccine-uptake-end-up-reducing-it/" target="_blank">bill had the intended effect</a>.</p>
<h3>Is there a role for publicity?</h3>
<p>When legislative measures fail or require backup, health authorities in some states are turning to a kind of naming-and-shaming policy. Not only do they determine which locales have high exemption rates, but they also publicize that information. Colorado, which has a relatively high rate of kindergartners with vaccine exemptions, is considering this tactic.</p>
<p>The effect of publicity can go beyond simply informing state residents about where herd immunity might be low. Exemption enforcement often lies in the hands of local school personnel. Studies suggest that even in states like Massachusetts and Missouri, which disallow personal-belief exemptions, almost 20% of schools allowed them anyway. Publicizing hot spots of vaccine exemptions could spur improved oversight at the local level.</p>
<h3>How is policy set on the front lines?</h3>
<p>Some pediatricians have decided not just to encourage parents to have their children be vaccinated—and sign the exemption form for those who decline to do so—but also they have refused to take patients whose parents opt out of having them vaccinated. This practice is <a href="http://www.ncbi.nlm.nih.gov/pubmed/15867060" target="_blank">not in keeping with the policy</a> of the American Academy of Pediatrics, however. But given that an outbreak in southern California was traced to a physician's waiting room, doctors feel they have good reason for strict constraints of this nature. Some physician practices don't mince words in communicating their belief in the importance of vaccination. One California practice, for example, notes that, "by not vaccinating you are putting your child at unnecessary risk for life-threatening illness and disability, and even death."</p>
<p>The threat to others is relevant to the policy discussion as well, some have argued, suggesting that the repercussions of not vaccinating should go beyond calling out schools with low vaccination rates. One bioethicist, Arthur Caplan, <a href="https://blogs.law.harvard.edu/billofhealth/2013/05/23/liability-for-failure-to-vaccinate/" target="_blank">has proposed</a> that families who opt out of vaccination should be held liable if an outbreak is traced to their door.</p>
<p>Vaccine policy also finds its way into the courts. New York City allows religious exemptions, for example, but bars unvaccinated children from attending public school if another student at the school has a vaccine-preventable illness, such as chickenpox. In response to a suit from some parents whose unvaccinated children were barred from school on these grounds, a judge <a href="http://law.justia.com/cases/federal/district-courts/new-york/nyedce/1:2012cv00098/325949/39" target="_blank">ruled in favor of the city</a>. He cited a <a href="http://supreme.justia.com/cases/federal/us/197/11/case.html" target="_blank">1905 U.S. Supreme Court ruling</a> that a man who was fined $5 for refusing the smallpox vaccine during an outbreak did, in fact, have to pay up.</p>
<h3>Does money matter?</h3>
<p>A legal threat might influence parents who opt out of vaccination for philosophical reasons, and courts and local authorities may draw exemption lines in the sand. But vaccine refusal isn't always a matter of philosophy and may not even be an active choice. Sometimes, it's about money and time. Research suggests that so-called "exemptions of convenience" also contribute to low vaccination rates, particularly when signing a form is simply cheaper and easier for struggling families than finding the funds to pay for childhood vaccinations and taking time off of work to take their child to the doctor.</p>
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<p>Studies suggest that children with vaccine exemptions are as much as 22 times more likely than vaccinated children to contract measels in an outbreak.</p>
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<p>While no one can buy parents more time, the 2010 Affordable Care Act (ACA) might take care of the cost barrier. The law requires that health plans offered under the ACA cover all childhood immunizations administered by an in-network provider, with no deductibles or copayments. The federal government also offers the <a href="http://www.cdc.gov/vaccines/programs/vfc/index.html" target="_blank">Vaccines for Children program</a>, which provides no-cost vaccines for those who meet eligibility requirements.</p>
<h3>Why do exemptions matter?</h3>
<p>Whether people are cajoled, encouraged, or mandated to have their children vaccinated, the need to keep exemptions at a minimum is real. Studies suggest that children with vaccine exemptions are as much as <a href="http://www.ncbi.nlm.nih.gov/pubmed/11135778" target="_blank">22 times more likely</a> than vaccinated children to contract measles in an outbreak. And although recent pertussis outbreaks can be traced in part to waning immunity in children who did receive the vaccine, one study showed that children who were not vaccinated had an <a href="http://www.ncbi.nlm.nih.gov/pubmed/19482753" target="_blank">six-fold greater risk</a> of contracting the disease than did fully vaccinated children.</p>
<p>According to a 2014 report, the <a href="http://www.ncbi.nlm.nih.gov/pubmed/24590750" target="_blank">recommended U.S. schedule of vaccinations</a> against 13 diseases saves 42,000 lives each year, prevents 20 million cases of illness, and saves $13.5 billion in direct costs and $68.8 billion in indirect costs. Regardless of the focus of cost calculations—lives or money or both—vaccines are clearly the better policy and public-health bet.</p>

Each state sets its own policies for vaccination requirements and exemptions, and they vary widely across the U.S.  

A 2012 measles outbreak in the United Kingdom sent about 700 people to the hospital.

Tracking Disease Outbreaks

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<p>Snow's detecting presaged the sort of footwork that epidemiologists <a href="http://www.cdc.gov/EIS/">still conduct today</a>. They begin with an "<a href="http://www.cdc.gov/ophss/csels/dsepd/SS1978/Glossary.html#caseIndex">index case</a>," the patient who first comes to the attention of health workers, and then "map" that person's contacts and activities. That is, they backtrack along the web of interactions associated with the index case in the hope of limiting a wider outbreak. A <a href="http://www.pbs.org/wgbh/nova/body/disease-detective.html">NOVA Online interactive</a> gives you a chance to walk through this process and trace the origin of an infection, just as a working epidemiologist does.</p><h3>How do "disease detectives" work?</h3><p>Plain old detective footwork is critical in tracking and reining in a disease outbreak. The work can start with a report of a single case. Fortunately, the diseases that are of most concern to public health officials, like measles, must be reported: Clinicians are required to notify the appropriate public health authorities whenever they <a href="http://www.nlm.nih.gov/medlineplus/ency/article/001929.htm">encounter a suspected case.</a></p><p>In the United States, the index case of a vaccine-preventable disease like measles tends to be an unvaccinated person who traveled overseas, picked up the virus, and carried it home. A measles-infected person can be contagious <a href="http://wwwnc.cdc.gov/travel/yellowbook/2014/chapter-3-infectious-diseases-related-to-travel/measles-rubeola">as early as four days</a> before showing symptoms. Thus public health workers have to identify as many as possible of an infected person's connections during the days before a confirmed diagnosis is made—including, for example, fellow plane passengers. In 2013, disease detectives were kept busy with several U.S. measles outbreaks. Of the 159 cases in these outbreaks, 99% were traced to an overseas traveler who returned to the States infected but unaware of that fact. Disease detecting also showed that 82% of the cases involved people who were unvaccinated for measles.</p><p>In their investigations, epidemiologists often find that a low vaccination rate in a given population is a factor in the outbreak of a vaccine-preventable disease. Sometimes, the role of this factor is obvious because the outbreak happens in a culturally defined community with low vaccination rates. This scenario unfolded in a 2014 measles outbreak in Ohio among the Amish, many of whom are never vaccinated. But there are also places where cultural bonds aren't as obvious, such as Marin County, California, a region that is home to pockets of parents who have declined to have their children vaccinated; unlike the Amish community in Ohio, this is a scattered aggregation of people who can't be as easily traced by distinctive cultural connections.</p><p></p>
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<img alt="" class="richtext-image full-width" src="https://staging.pbs.org/wgbh/nova/media/images/disease-outbreaks-01.width-990_57U9zRg.jpg"/>
<figcaption>The distribution of vaccinated and unvaccinated people can influence the course of an outbreak.
        
        <div class="credit">Getty Images</div>
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<h3>What is the role of geographic overlap?</h3><p>Because religious or cultural practices don't always exactly delineate clusters of unvaccinated people, disease detectives also examine the geographic overlap between vaccine exemption rates and outbreaks. Typically, outbreaks tend to concentrate where there are high rates of nonmedical vaccine exemptions—that is, people who have declined to be vaccinated for a reason <a href="http://www.ncbi.nlm.nih.gov/pubmed/20929352">not supported by medical necessity</a>. For example, the three largest measles outbreaks in the United States between 2001 and 2008 arose in populations with high rates of vaccine exemptions for personal beliefs. Researchers have found similar patterns of overlap for pertussis, or whooping cough.</p>
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<h3>How do epidemiologists keep ahead of microbes?</h3><p>Epidemiologists can also track whether or not microbes are changing enough to evade vaccine-triggered immunity. These molecular investigations involve interrogating microbes instead of people, using sophisticated tools to sequence a germ's DNA or proteins to see if it's changing its molecular appearance.</p><p>In addition, different regions of the world can host different types of a given microbe. These region-specific molecular clues can be as telling about where someone became infected as the person's travel history—contributing to the information needed to limit an outbreak.</p><h3>What happens after an outbreak ends?</h3><p>Footwork and geographic overlap analysis are among the methods that epidemiologists can use to trace, track, and try to limit an outbreak. But their work doesn't end there.</p><p>Even after an outbreak has ended, disease detectives still have plenty to do. Using the information gathered during the outbreak, they can compare disease rates among vaccinated and unvaccinated people. That calculation tells them exactly how risky it is to be unvaccinated when an infectious disease threatens a community. And this work isn't simply a math exercise. With these numbers, public health officials can then set target thresholds for immunization rates in a community. They can also use these numbers in public-health communications to inform people of the risks of forgoing vaccination.</p><p>Such data crunching has revealed a clear finding: Being unvaccinated greatly boosts a person's risk of infection. According to one study, for example, children whose parents choose not to have them vaccinated against chickenpox face a nine-fold greater risk of contracting the disease than do vaccinated children. A similar study of pertussis revealed a risk of getting the disease about <a href="http://www.ncbi.nlm.nih.gov/pubmed/19482753">six-fold greater</a> for unvaccinated children. And going unvaccinated for measles increases <a href="http://www.ncbi.nlm.nih.gov/pubmed/11135778">as much as 22-fold</a> the risk of contracting that disease.</p>
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<p>How do epidemiologists figure out the role of low vaccination rates in preventable diseases like measles or pertussis?</p>

An infant with pertussis is given an oxygen mask.

Why is Whooping Cough Back?

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<h3>What is whooping cough?</h3><p><i>Bordetella pertussis</i> bacteria cause whooping cough by sticking to the cells lining the lungs and producing a toxin that blocks lung function. The main symptom of pertussis is a gasping cough characterized by a whooping sound that gives the disease its common name. The cough can be intense enough to cause vomiting or break ribs. Although adults and older children usually survive the disease—while enduring considerable discomfort—it can be fatal in infants, the age group with the highest pertussis death rate.</p><p>Antibiotics can cure the disease if the infection is identified early enough, but there is no treatment that can neutralize the pertussis toxin once it's present in the lungs. An infected person first shows symptoms of a common cold, including sniffles and a low-grade fever; but once the characteristic cough develops, it can last for weeks. One nickname for pertussis is the "<a href="http://www.cdc.gov/pertussis/fast-facts.html">100-day cough</a>."</p><p>Pertussis kills about 300,000 people each year worldwide, most of them infants. According to the U.S. Centers for Disease Control and Prevention (CDC), infants under one year old not only are at greatest risk of death from the disease, but they also have the highest reported rate of pertussis. Even in older children, the consequences of the disease can be serious; one in 20 children who contract pertussis has nervous system damage. The disease, which passes from person to person through the air, is highly contagious; one infected person can spread the disease to more than a dozen other people. An infected person is contagious even when symptoms are indistinguishable from those of a cold, before the "whooping" cough begins.</p><p>In the United States, pertussis affected more than 200,000 people a year in the late 1930s and early 1940s. The advent of a vaccine against the disease in the 1940s led to a drop in those numbers, to a low point of just over 1,000 cases in 1976. Then, in the 1990s, the numbers <a href="http://www.sciencenews.org/article/whooping-cough-bounces-back">started rising again</a>.</p><p></p>
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<img alt="U.S. Pertussis cases" class="richtext-image full-width" src="https://staging.pbs.org/wgbh/nova/media/images/whooping-cough-02.width-990_8xo8vMK.jpg"/>
<figcaption>Pertussis cases in the United States, 1950-2013
        
        <div class="credit">© Tangled Bank Studios; source data from Centers for Disease Control</div>
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<h3>Is there a resurgence of pertussis?</h3><p>By the first decade of the 21st century, pertussis outbreaks had started to increase in number and size. California has been slammed with two major outbreaks: one in 2010 that hit 9,120 people and left 10 infants dead, and one in 2014 that as of August involved 7,503 cases and <a href="http://www.cdph.ca.gov/programs/immunize/Pages/PertussisSummaryReports.aspx">three infant deaths</a>.</p><p>Whooping cough cases have always waxed and waned in natural cycles, peaking every three to five years. Since the 1990s, though, the number of cases has been on a <a href="http://www.cdc.gov/pertussis/surv-reporting.html">steady climb</a>. Doctors are recognizing the disease earlier and more often, now that it's back on their radars, and testing for it has improved. But these aren't the only reasons for the increase. Another important factor was a change made in 1991 in the <a href="http://www.popsci.com/science/article/2013-05/fyi-are-unvaccinated-kids-really-causing-resurgence-whooping-cough">kind of pertussis vaccine</a> used in the United States.</p>
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<h3>How has the pertussis vaccine changed?</h3><p>The first pertussis vaccine contained inactivated but complete bacterial cells. Typically, vaccines contain bits of the target microbe, which trigger our immune systems to mount a defense against them and—most importantly—remember them in case of future invasion. The whole cells in the old vaccine offered the immune system a huge selection of pertussis-specific targets to recognize and remember.</p><p>But then some people became concerned because the vaccine was causing seemingly alarming reactions, including fever-induced seizures and fainting. Even though these reactions didn't cause any lasting damage, the relatively high rate of these reactions (about one in every 1,000 doses of the vaccine resulted in a seizure event) prompted the development and approval of a new version of the pertussis vaccine. Instead of using the whole bacterial cell, it offers the immune system only select bits of the cell that trigger an immune defense. Because it doesn't rely on complete cells, the new version is called an acellular vaccine.</p><h3>Is the newer pertussis vaccine less long-lasting?</h3><p>It turns out that the host of molecules in the whole-cell vaccine probably triggered a stronger and longer-lasting immune response than the newer, acellular vaccine does. Those of an age to have received the newer version exhibit shorter-lasting immunity than those who got the whole-cell vaccine. The numbers are clear: With whole-cell vaccination, cases nationwide stayed in the low four digits for decades. With the advent of the acellular version, the number of U.S. cases climbed again, reaching almost 50,000 in 2012. Meanwhile, in countries such as Thailand that continued to use the whole-cell vaccine, case numbers have remained low and have even continued to decline.</p><p>As a result of the waning immunity of the acellular vaccine, U.S. preteens now receive an additional inoculation, at age 11 or 12 years. The first five shots, given from infancy through age four to six years, are in a formulation known as DTaP (for diphtheria, tetanus, and acellular pertussis). The sixth shot, the preteen booster, is referred to as the Tdap—an existing tetanus and diphtheria booster, now with pertussis added in.</p><h3>Is pertussis a moving target?</h3><p>The new version of the pertussis vaccine might have had yet another unanticipated effect: opening the door to changes in the bacteria. Some recent samples of the <i>B. pertussis</i> bacterium from infected people contain cells that no longer make the molecules used in the vaccine to trigger the immune system. Without these recognizable molecules, the altered bacteria can <a href="http://www.ncbi.nlm.nih.gov/pubmed/24655754">slip right past</a> the immune system's defenses. The upshot could be that an already relatively weak vaccine is becoming even weaker.</p><h3>What is the role of vaccine refusal?</h3><p>According to the CDC, about 5% of children in the United States have received less than three shots in the DTaP vaccine series. Parental refusal to vaccinate their children against pertussis has been tagged as <a href="http://www.scientificamerican.com/article/vaccination-opt-outs-found-to-contribute-to-whooping-cough-outbreaks-in-kids/">one factor</a> in the 2010 California outbreak. Regions where more parents requested vaccine exemptions for nonmedical reasons were <a href="http://www.ncbi.nlm.nih.gov/pubmed/24082000">2.5 times more likely</a> to also see a high number of pertussis cases.</p><p>Protecting a community against pertussis requires that at least 92% of the population be immune. Given the complexities of waning immunity, as well as a weaker pertussis vaccine than was previously used in the United States, maintaining that threshold to achieve "<a href="/wgbh/nova/body/herd-immunity.html">herd immunity"</a> is more critical than ever—especially to protect infants, who are the segment of the population most vulnerable to the disease.</p>
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<p>Pertussis—which can be fatal in infants—is on the rise due to changes in the vaccine, and to more people opting out.</p>

The Autism-Vaccine Myth

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<h3>What was the impact of the alleged vaccine-autism connection?</h3><p>What the public didn't know in 1998 was that the now-retracted study, which involved just 12 children, would turn out to have some serious flaws—and even to contain <a href="http://www.bmj.com/content/342/bmj.c7452">apparently falsified data</a>. The 12 years between its publication and its retraction, however, left a lot of time for the unfounded and never-confirmed vaccine-autism link to take hold in the minds of worried parents—and thus for vaccination rates to suffer.</p><p>In 1997, the year before the paper was published, measles vaccination rates in the United Kingdom were over 91%. They started to fall in 1998 and in 2003-2004 reached a nadir of just 80%, although rates were even lower than that in specific areas. Only in recent years have MMR vaccination rates started climbing again in the U.K., reaching about 90% in 2013.</p><p></p>
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<figcaption>As a result of a paper published in 1998, MMR vaccination rates dropped Europe. The dotted line indicates the year in which Wakefield and colleagues published a paper alleging a connection between the MMR vaccine and autism.
        
        <div class="credit">© Tangled Bank Studios; data from the National Health Service of the United Kingdom</div>
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<h3>The vaccine-autism question: A timeline</h3><p>Here are some highlights along the scientific journey the vaccine-autism hypothesis made from its 1998 publication in The Lancet to today. The American Academy of Pediatrics has compiled an <a href="https://www2.aap.org/immunization/families/faq/vaccinestudies.pdf">expanded list of relevant studies</a>.</p><p><b>1998</b>-The Lancet publishes a paper by Wakefield et al. titled "Ileal-lymphoid-nodular hyperplasia, non-specific colitis, and pervasive developmental disorder in children." A <a href="http://briandeer.com/wakefield/royal-video.htm">press conference and interview</a> in which Wakefield says it is his "feeling that the…risk of this particular syndrome developing is related to the combined vaccine, the MMR, rather than the single vaccines" set off a media storm.</p><p><b>1999</b>—A U.S. Food and Drug Administration review of the vaccine preservative thimerosal, which contains mercury, finds no evidence of its causing neurological harm, but the agency recommends the precautionary removal of the agent from vaccines administered to infants.</p><p><b>2001</b>—Thimerosal is removed from childhood vaccines on the U.S. market.</p><p>—The Journal of the American Medical Association publishes "Time trends in autism and in MMR immunization coverage in California" by Dales et al. These authors looked for and found <a href="http://www.ncbi.nlm.nih.gov/pubmed/11231748">no association</a> between MMR vaccination rates in young children and increased autism occurrence.</p><p>—Pediatrics publishes "No evidence for a new variant of measles-mumps-rubella-induced autism." by Fombonne et al. In this study of 96 children with pervasive developmental disorders, the authors found <a href="http://www.ncbi.nlm.nih.gov/pubmed/11581466">no evidence</a> of a specific syndrome being related to whether or not they had received the MMR vaccine.</p><p><b>2002</b>—The British Medical Journal publishes "Measles, mumps, and rubella vaccination and bowel problems or developmental regression in children with autism: population study" by Taylor et al. This study of 473 children with autism who were born between 1979 and 1998 offered "further evidence against a link between MMR and autism." The authors found <a href="http://www.ncbi.nlm.nih.gov/pubmed/11850369">no differences</a> in regression or bowel symptoms in those born before and after the introduction of the MMR vaccine.</p><p>—Pediatrics publishes "Neurologic disorders after measles-mumps-rubella vaccination" by Makela et al. This study of 535,544 children in Finland found <a href="http://www.ncbi.nlm.nih.gov/pubmed/12415036">no link</a> between MMR vaccination and autism.</p><p>—The New England Journal of Medicine publishes "A population-based study of measles, mumps, and rubella vaccination and autism" by Madsen et al. These authors describe <a href="http://www.ncbi.nlm.nih.gov/pubmed/12421889">"strong arguments" against the hypothesis</a> that the MMR vaccine causes autism, based on an analysis of data from 537,303 children in Denmark, 82% of whom had received the MMR vaccine.</p><p><b>2003</b>—The Journal of the American Medical Association publishes "Association between thimerosal-containing vaccine and autism" by Hviid et al. These authors, using data from 467,450 children born in Denmark, <a href="http://www.ncbi.nlm.nih.gov/pubmed/14519711">found no link</a> between autism risk and vaccines containing thimerosal.</p><p><b>2004</b>—Of the 13 authors on the 1998 Wakefield paper, ten formally retract its interpretation, stating a wish to make clear that the paper <a href="http://www.ncbi.nlm.nih.gov/pubmed/15016483">established "no causal link"</a> between the MMR vaccine and autism.</p><p>—The Institute of Medicine of the United States' National Academies issues a report from its Immunization Safety Review Committee, which concludes that <a href="http://www.iom.edu/Reports/2004/Immunization-Safety-Review-Vaccines-and-Autism.aspx">vaccines don't cause autism</a>.</p><p><b>2007</b>—The New England Journal of Medicine publishes "Early thimerosal exposure and neuropsychological outcomes at 7 to 10 years" by Thompson et al. These authors looked for links between early thimerosal exposure and neuropsychological outcomes in a group of 1,047 children aged seven to ten years and found <a href="http://www.ncbi.nlm.nih.gov/pubmed/17898097">no association</a>.</p><p><b>2010</b>—The Lancet, after an extensive investigation, including <a href="http://briandeer.com/mmr/lancet-summary.htm">investigative work</a> by journalist Brian Deer, issues a formal retraction of the 1998 Wakefield paper.</p><p>—The U.K,'s General Medical Council strikes Andrew Wakefield from the medical register (the U.K. equivalent of stripping a medical license), concluding, according to reports, that he had been "dishonest, irresponsible, and showed callous disregard for the distress and pain of children" in conducting the <a href="http://www.telegraph.co.uk/health/7095145/GMC-brands-Dr-Andrew-Wakefield-dishonest-irresponsible-and-callous.html">MMR-autism study</a>.</p><p>—Pediatrics publishes "On-time vaccine receipt in the first year does not adversely affect neuropsychological outcomes" by Smith et al. This study of more than 1,000 children born between 1993 and 1997 found that on-time vaccination was associated with <a href="http://www.ncbi.nlm.nih.gov/pubmed/20498176">better performance</a> on a number of neuropsychological tests.</p><p>—Pediatrics publishes "Prenatal and infant exposure to thimerosal from vaccines and immunoglobulins and risk of autism" by Price et al. This study of over 1,000 children found <a href="http://www.ncbi.nlm.nih.gov/pubmed/20837594">no increase</a> in autism outcomes among children who had received thimerosal-containing vaccines, compared to those who had not.</p><p><b>2013</b>—A major measles outbreak in Wales is linked to regional response to the 1998 Wakefield paper, which led to reduced measles vaccination rates in the region affected by the outbreak.</p><p>—The Journal of Pediatrics publishes "Increasing exposure to antibody-stimulating proteins and polysaccharides in vaccines is not associated with risk of autism" by DiStefano et al. This study of more than 1,000 children found no link between the number of antigens in vaccines and autism, and thus <a href="http://www.immunizeforgood.com/fact-or-fiction/too-many-too-soon">no support</a> for the "too many, too soon" <a href="http://www.ncbi.nlm.nih.gov/pubmed/23545349">school of thought</a>.</p><p><b>2014</b>—Vaccine publishes "Vaccines are not associated with autism: An evidence-based meta-analysis of case-control and cohort studies" by Taylor et al. This meta-analysis of ten studies covering more than 1.2 million children finds <a href="http://www.ncbi.nlm.nih.gov/pubmed/24814559">no links between autism and MMR vaccination</a>, mercury, thimerosal, or vaccination generally.</p>
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<h3>Why has the myth persisted?</h3><p>Two fears powered the unfolding of these events. One was a fear of the unfamiliar ingredients in vaccines, including the mercury-containing preservative thimerosal, which as of mid-2014 was still used in some multidose flu shots but was otherwise phased out of routine childhood vaccines in the United States starting in 2001. The other was a fear of autism, an anxiety fostered by media stories pitting emotional appeals by high-profile anti-vaccine advocates against statistically based reports by medical researchers.</p><p>No link with autism—not to mercury, not to thimerosal, not to any vaccines, including the MMR—has been found.</p><p>These fears persisted even as evidence mounted that they were completely unfounded. Scientific verification relies on a process of testing and confirmation, not on a single observation. Researchers sincerely grappled with the question of a vaccine-autism link in numerous studies following publication of the 1998 Lancet paper. Some of these studies analyzed data from millions of people, in the quest to see if vaccines and autism might be linked. The overwhelming scientific consensus is that they are not. The Lancet paper specifically addressed a possible association between the MMR vaccine and autism, but later studies also looked at other vaccine-related factors, such as the mercury-containing preservative thimerosal, which is not used in the MMR vaccine. Thimerosal initially raised concerns because mercury is neurotoxic. However, <a href="http://www.immunizationinfo.org/issues/thimerosal-mercury/vaccines-and-autism-2009">no link with autism</a>—not to mercury, not to thimerosal, not to any vaccines, <a href="http://www.immunize.org/catg.d/p4026.pdf">including the MMR</a>—has been found.</p><p>In some fraction of the American population, however, the belief in a link remains. One reason is a coincidence of timing: children are routinely vaccinated just as parents begin to observe signs of autism. Most vaccines are administered during the first years of life, which is also a period of rapid developmental changes. Many developmental conditions, including autism, don't become apparent until a child misses a milestone or loses an early skill, a change that in some cases can't help but be coincident with a recent vaccination.</p><p>Adding to such concern is the fact that, sometimes, vaccinations can lead a child to develop a high fever and accompanying <a href="http://www.ninds.nih.gov/disorders/febrile_seizures/febrile_seizures.htm">febrile seizures</a>. Such seizures are temperature-related and don't cause lasting damage. A tendency to experience febrile seizures <a href="http://www.healthychildren.org/English/health-issues/conditions/fever/pages/Febrile-Seizures.aspx">runs in families</a>, and about one in 20 young children will have one at some point. According to the U.S. Centers for Disease Control and Prevention, most febrile seizures happen <a href="http://www.cdc.gov/vaccinesafety/concerns/febrileseizures.html">when a child is sick</a> rather than after a vaccination, though the MMR vaccine is associated with a <a href="http://www.ncbi.nlm.nih.gov/pubmed/22336803">slightly increased risk for febrile seizures</a>. Children with and without autism have these febrile responses, but since their timing may coincide with emerging signs of autism, that can link the two incidents in a parent's mind, even though there is no causal relationship.</p><h3>What do we know about what contributes to autism?</h3><p>Largely ignored during this prolonged vaccine-autism controversy was clear evidence of a strong genetic influence on autism. For example, if one identical twin has been diagnosed with an autism spectrum disorder (ASD), the other twin is from <a href="http://www.ncbi.nlm.nih.gov/pubmed/21727249">60%</a> to <a href="http://www.ncbi.nlm.nih.gov/pubmed/24794370">90%</a> more likely to also be diagnosed with an ASD. Also known is that males are more likely than females to be diagnosed with autism and that the older a father is, the higher the likelihood of a diagnosis of autism in his offspring.</p><p></p>
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<img alt="Modern DNA sequencing" class="richtext-image full-width" src="https://staging.pbs.org/wgbh/nova/media/images/autism-vaccine-myth-02.width-990_3iV49YX.jpg"/>
<figcaption>Modern DNA sequencing is uncovering the myriad of genetic mutations that can lead to autism.
        
        <div class="credit">© Tangled Bank Studios</div>
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<p>Nonetheless, autism has proven to be very difficult to pin down genetically in the way that single-gene disorders, such as cystic fibrosis and Huntington's disease, have been traced. The reasons for this difficulty have recently become clearer thanks to advances in DNA sequencing technology that have made it possible to compare the genomes of individuals with autism to those of individuals without autism, as well as to those of their parents. It is now evident that there are many genetic paths to autism, and that some mutations leading to autism are not inherited but arise spontaneously in reproductive cells or during development. These mutations help to explain <a href="http://www.hhmi.org/biointeractive/decoding-autism-puzzle">how autism can appear in families</a> that previously had no history of the condition.</p>
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<p>Some mutations associated with autism are single changes in the DNA code that make up genes. Other kinds of mutations include extra copies or deletions of larger stretches of chromosomes that include multiple genes.</p><p>When investigators have homed in on specific genes, they have identified numerous candidates, many of them associated with the <a href="http://www.ncbi.nlm.nih.gov/pubmed/21129364">formation or function of the brain</a>. These include genes that play roles in neural development and structuring, nerve signaling, and speech and language processing. Still, because of the rapid proliferation of genetic data regarding autism, scientists are still discovering the ways in which many other mutations lead to autism—a task that will likely take years.</p><p>The emerging scientific picture of autism is a condition that <a href="http://www.ncbi.nlm.nih.gov/pubmed/24670167">begins during fetal development</a>, as a result of both <a href="http://www.scientificamerican.com/article/autism-genetic-complexity/">genetic</a> and <a href="http://sfari.org/news-and-opinion/news/2014/environment-genetics-may-contribute-equally-to-autism-risk">environmental influences</a>.</p><p>It is now very clear that vaccines are not among the environmental factors that cause autism. But scientists are eager to understand how genetic and environmental influences may interact and to explain how these factors play a role in the broad spectrum of symptoms manifest in different people with autism.</p>

<p>How do we know that vaccines don&#x27;t cause autism, and what do we know about what does contribute to the condition?</p>

Herd, or community, immunity is the result of a high immunization rate.

Courtesy: The National Institute of Allergy and Infectious Disease (NIAID)

What is Herd Immunity?

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<h3>What is "herd immunity?"</h3><p>Just as a herd of cattle or sheep uses sheer numbers to protect its members from predators, herd immunity protects a community from infectious diseases by virtue of the sheer numbers of people immune to such diseases. The more members of a human "herd" who are immune to a given disease, the better protected the whole populace will be from an outbreak of that disease.</p><p>There are two ways an individual can become immune to an infectious disease: by becoming infected with the pathogen that causes it or by being vaccinated against it. Because vaccines induce immunity without causing illness, they are a comparatively safe and effective way to fill a community with disease-resistant people. These vaccinated individuals have protected themselves from disease. But, in turn, they are also protecting members of the community who cannot be vaccinated, <a href="http://www.vaccines.gov/basics/protection/">preventing the chain of disease</a> from reaching them and limiting potential outbreaks. Every vaccinated person adds to the effectiveness of this community-level protection.</p><p></p><h3>What do thresholds have to do with herd immunity?</h3><p>The microbes that cause disease all have different infectious features. Some, like measles and influenza, pass from person to person more easily than others. Some tend to have more severe consequences in specific demographic groups. For example, the symptoms of pertussis, or whooping cough, are distressing at any age but can be fatal in infants, the age group with the highest death rate from pertussis. Each of these features—such as transmissibility and severity—affects a given disease's threshold, or the minimum percentage of immune individuals a community needs to prevent an outbreak.</p><p>To set a threshold, epidemiologists—experts in infectious disease transmission—use a value called "basic reproduction number," often referred to as "R0." This number represents how many people in an unprotected population one infected person could pass the disease along to. For example, R0 for measles is between 12 and 18, while for polio, it is between five and seve. The higher this number is, the higher the immunity threshold must be to protect the community. Because measles is extremely contagious and can spread through the air, for example, the immunity threshold needed to protect a community is high, at 95%. Diseases like polio, which are a little less contagious, have a lower threshold—80% to 85% in the case of polio.</p>
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<p>The general concept of an immunity threshold seems simple, but the factors involved in calculating a specific threshold are complex. These factors include how effective the vaccine for a given disease is, how long-lasting immunity is from both vaccination and infection, and which populations form critical links in transmission of the disease. The collective differences in these factors result in different thresholds for different diseases (see below), with a significant factor being R0.</p><p></p>
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<img alt="R0 and threshold level" class="richtext-image full-width" src="https://staging.pbs.org/wgbh/nova/media/images/herd-immunity-02.width-990_49f6zg6.jpg"/>
<figcaption>Relationship between R0 and threshold level needed for herd immunity
        
        <div class="credit">© Tangled Bank Studios; data from Epidemiologic Reviews 1993.</div>
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<h3>Why is herd immunity important?</h3><p>Human communities were once relatively small and isolated. Diseases certainly broke out, but their transmission ended wherever geography limited a populace's mobility. But today, our chains of connection traverse the globe—reaching across oceans and over mountain ranges, pervading immense cities and remote villages—linking us all into one vast, interactive human herd. Almost no one anymore lives in isolation from such connections.</p><p>These chains of human interaction have resulted in more potent chains of disease transmission. The only thing that can break a chain of transmission is a disease-resistant link. The chicken pox vaccine offers an example of the effectiveness of disease-resistant links. After the chicken pox vaccine debuted in the United States in 1995, deaths rates from chicken pox dropped by as much as 97%. Significantly, even though the vaccine is not administered to infants, <a href="http://www.ncbi.nlm.nih.gov/pubmed/21788222">no infants died from chicken pox</a> in the United States between 2004 and 2007. These tiniest, most vulnerable links in the chain of human connections avoided exposure thanks to herd immunity.</p>
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<p>When enough people are immune to a disease, the &quot;herd&quot; protects those who cannot safely receive a vaccine.</p>

Preventing HPV-Caused Cancers

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<h3>What is HPV?</h3><p>Although people sometimes refer to HPV as if it were a single virus, it's really a group of viruses, not all of which cause cancer. Of the 120 identified HPVs, about 50 are sexually transmitted—that is, they are passed from person to person by skin-to-skin contact involving the genitalia. The remaining HPVs are not sexually transmitted and cause common warts on other parts of the body, such as the fingers and toes.</p><p></p>
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<img alt="HPV warts" class="richtext-image full-width" src="https://staging.pbs.org/wgbh/nova/media/images/hpv-vaccines-cancer-01.width-990_abuqJPd.jpg"/>
<figcaption>Some types of HPV can cause warts, including on the hands or feet.
        
        <div class="credit">© Getty Images</div>
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<p>Some of the <a href="http://www.nlm.nih.gov/medlineplus/hpv.html">40 sexually transmitted HPVs</a> can lead to cancers of the cervix, vulva, vagina, anus, penis, or throat; the others aren't associated with cancer but can cause genital warts.</p><p>Each type of HPV is distinguished by its outward structure, a molecular "face" that interacts with the human cells that the virus infects. These different faces are known as types, and HPV types are designated by numbers.</p><h3>What diseases does HPV cause?</h3><p>Two types of the virus, HPVs 16 and 18, cause about 70% of all cervical cancers, at least in populations that have been studied. Another two, HPVs 6 and 11, are responsible for nine out of ten cases of genital warts. The only known preventive is the HPV vaccine, the first of which was <a href="http://www.fda.gov/BiologicsBloodVaccines/Vaccines/ApprovedProducts/ucm111283.htm">approved for use in the United States in 2006</a>.</p><p>Genital warts are <a href="http://www.chop.edu/service/vaccine-education-center/a-look-at-each-vaccine/hpv-vaccine.html">not life threatening</a> but can prove embarrassing and are uncomfortable to have removed. At any point in time, one in 100 sexually active adults in the United States has visible genital warts, according to the U.S. Centers for Disease Control and Prevention (CDC).</p><p>An intrepid researcher named E.V. Ullmann confirmed the infectious nature of HPVs in the 1920s. He experimented on himself by taking extracts from a patient's throat warts, which HPV also can cause, and injecting the fluid into self-made wounds on his arm, triggering wart growth there nine months later.</p><p>Confirmation of an HPV-cancer link came many decades later, in the 1980s, when technology finally made it possible to identify HPV DNA in tissue samples from women with cervical cancer. If viruses like HPV persist in the body for a long time, they can cause cancer by <a href="http://www.ncbi.nlm.nih.gov/pubmed/15479788">disrupting processes that limit cell division</a>, so that cells divide out of control. Warts result from a more controlled and limited overgrowth of cells on the outer layer of the skin.</p><p>A loss of cellular control that leads to cancer can be deadly. Cervical cancer kills about 300,000 women worldwide and 4,000 women in the United States annually. Each year, about <a href="http://www.cdc.gov/std/hpv/STDFact-HPV-vaccine-young-women.htm">12,000 women in the United States</a> learn that they have this cancer.</p><p>HPV 16 is also responsible for up to nine out of ten cases of HPV-caused throat cancers and is the <a href="http://www.cancer.gov/cancertopics/factsheet/Risk/HPV">most frequently detected HPV</a> so far in anal cancer. These cancers occur in both men and women. HPV-associated oral cancers have been on the increase for the last three decades. Unless preventive measures are taken, the incidence of HPV-related oral cancers is <a href="http://www.nature.com/news/hpv-sex-cancer-and-a-virus-1.14194">expected to continue to rise</a>.</p><p></p>
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<img alt="" class="richtext-image full-width" src="https://staging.pbs.org/wgbh/nova/media/images/hpv-vaccines-cancer-02.width-990_UZgCkcS.jpg"/>
<figcaption>Number of cancers caused by HPV worldwide each year
        
        <div class="credit">Courtesy: National Institutes of Health</div>
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<h3>How are HPVs spread?</h3><p>HPVs are the most common cause of sexually transmitted infections in the world. At any given time, 20 million people in the United States—about 6% of the population—have some type of HPV infection. Being young is a risk factor; about half the people who contract an HPV are between the ages of 15 and 24. Although many HPVs are transmitted sexually, you don't have to have sex to pick them up; direct skin-to-skin contact with the genitals of an infected person is <a href="http://www.chop.edu/service/vaccine-education-center/a-look-at-each-vaccine/hpv-vaccine.html">sufficient to result in transmission</a>. The use of latex condoms may reduce the risk of transmission but <a href="http://www.nlm.nih.gov/medlineplus/hpv.html">doesn't completely eliminate it</a>.</p><p>HPVs are so common that, in the United States, most sexually active people acquire an HPV infection at some point in their lives. Many infected people never know they have an HPV because <a href="http://www.cdc.gov/vaccines/hcp/vis/vis-statements/hpv-gardasil.html">they fight it off without ever showing symptoms</a>. But sometimes, HPV evades the body's defenses and persists for years, leading to cancer.</p>
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<h3>What are the facts about HPV vaccination?</h3><p>There are two approved HPV vaccines; both contain protein molecules specific to different HPV types. After they're injected, these proteins trigger the immune system to construct a memory of the molecular "face" of those types. If a real HPV shows up later, the body is able to recognize the protein—on the face of the actual virus this time—and <a href="/wgbh/nova/body/immunity-and-vaccines.html">crush the invasion</a>.</p><p>Both approved vaccines, <a href="http://www.cdc.gov/vaccines/hcp/vis/vis-statements/hpv-gardasil.html">Gardasil</a> and <a href="http://www.cdc.gov/vaccines/hcp/vis/vis-statements/hpv-cervarix.html">Cervarix</a>, target the two most common cancer-causing strains identified so far: HPVs 16 and 18. But Gardasil also targets two more strains—6 and 11—adding protection against the HPVs that cause most genital warts.</p><p>The schedule for administering the vaccines is the same: a three-shot series over the course of six months, usually beginning at age 11 or 12. The rationale behind the target age is to get people vaccinated before they start engaging in sexual behaviors, including skin-to-skin genital contact, that can lead to an HPV infection.</p><p>But people who don't receive the three-dose series as scheduled can often get it later. The CDC's recommendations for individuals who haven't completed the series by the time they turn 13 vary slightly depending on whether they receive Gardasil or Cervarix, but the agency still recommends either vaccine for girls and women aged 13 to 26 and Gardasil for some groups of boys and men. In teens and young adults who have become sexually active, the vaccine can't stem an infection they've already acquired, but in those who have not yet encountered the types of HPV the vaccine covers, immunization still offers some protection. The HPV vaccine can be given at the same time as other vaccines.</p><h3>What are side effects of the HPV vaccine?</h3><p>The HPV vaccine has proved controversial because of rumored side effects, including blood clots, strokes, and seizures. The vaccine even came in for comment during the 2011 political season, when Republican presidential hopeful Michele Bachmann erroneously asserted that it was linked to intellectual disability. Subsequent reports <a href="http://www.cbsnews.com/news/katie-couric-hpv-vaccine-show-criticism-valid/">in the popular press</a> reinforced the impression that the vaccine can cause alarming side effects.</p><p>These claims commanded the attention of researchers, but studies have shown <a href="http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3805482/">no cause-and-effect relationship</a> between the vaccine and such side effects. A huge 2014 study of more than 1.6 million women, for example, found no link between Gardasil and a dangerous clotting disorder.</p>
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<p>Evidence has shown that HPV vaccinated girls are no more likely to be sexually active than unvaccinated girls.</p>
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<p>There is one known side effect of the HPV vaccine, however—fainting. The CDC says that adolescents are more prone than other age groups to fainting after a vaccination. The fainting itself doesn't indicate anything serious but carries a risk of falls and injury, so resting for 15 minutes after the vaccine is administered is suggested for tweens and teens.</p><p>The just-vaccinated preteen or teen who faints might be reacting to one of the most widely known negative features of the HPV vaccine: It hurts, possibly because it has a <a href="http://www.chop.edu/service/vaccine-education-center/prevent-hpv/hpv-questions-answers.html">higher concentration of salt</a> than other vaccines. Fortunately, the pain doesn't seem to deter adolescents from completing the shot series.</p><h3>Does HPV vaccination affect sex-related behaviors?</h3><p>Because HPV infection is a sexually transmitted disease, some have expressed concern that the vaccine might affect sexual behaviors—especially that girls who've received the vaccine would be more likely to engage in sex at an early age. Evidence has shown these worries to be unfounded. Vaccinated girls are no more likely to be sexually active than unvaccinated girls, and vaccination is <a href="http://www.ncbi.nlm.nih.gov/pubmed/24488747">not associated</a> with riskier sexual behavior.</p><p>What is worrisome, however, is that many girls incorrectly believe that the vaccine protects them against sexually transmitted diseases other than HPV, which it does not.</p>
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<h3>How effective are the HPV vaccines?</h3><p>Both vaccines effectively prevent most cervical, anal, and oral cancers, and Gardasil prevents most genital warts as well. Both vaccines have even been shown to protect unvaccinated girls and young women against HPV infections, thanks to a concept known as "<a href="/wgbh/nova/body/herd-immunity.html">herd immunity</a>." In a population of young women in Cincinnati, for example, within four years after the vaccine was introduced, rates of infection with an HPV type covered by the vaccine had dropped by 70% among vaccinated women and had dropped by 50% among unvaccinated women. Nationally, the vaccine has shown similar results, with a 56% drop in vaccine-type HPV infection rates among teen girls in the four years after the vaccine debuted, even though only a about third of such girls had received the full series of shots at that time.</p><p>Yet unfortunately, despite this documented success, the U.S. vaccination rate has remained well below the CDC's goal; the agency's target is to see 80% of girls aged 13 to 17 vaccinated, but as of 2013 just under 38% had received the full series.</p><p>But not all populations may benefit equally from the existing two vaccines. The different types of HPV vary in their geographic incidence, and some strains cause cancer more often in certain populations. HPV 16 is relatively common worldwide, for example, but HPV 18 is <a href="http://www.ncbi.nlm.nih.gov/pubmed/15894666">twice as prevalent</a> in cervical lesions diagnosed among women in North America as among women South and Central America.</p><h3>What's next for the HPV vaccine?</h3><p>There is the possibility of new vaccines that will cover more strains of HPV. Maximum coverage from the current vaccines is four strains of the virus, two of which cause cancer. New vaccines that are in development could cover up to nine strains, including seven that cause cancer.</p><p>And for those with anxiety about the pain associated with the HPV vaccines, there is more potentially good news: The shot series may be reduced, thanks to evidence that <a href="http://www.cancer.gov/newscenter/newsfromnci/2013/HPVdose">two shots can be just as effective as three</a>.</p>

<p>What are human papilloma viruses, why do they cause cervical cancer, and how well does the HPV vaccine work?</p>

Emily Willingham