by Vivian Chou
figures by Daniel Utter
If you have been following the 2016 US presidential elections, you are, in all likelihood, aware of the controversy surrounding mandatory childhood vaccination. Vaccines have risen to the limelight in recent years, but their history is much longer than that. Ever since the first vaccination was scientifically documented in 1798 , they have reshaped the landscape of human health and medicine. The impacts of vaccines have ranged from the 1979 eradication of polio in the US  and the 1980 eradication of smallpox worldwide , to prevention of cancer of the liver  and the cervix . In fact, vaccines have been so influential that some scientists consider them among the greatest successes in public health .
But not everyone shares this optimistic view of vaccines. Today, anti-vaccinationists (sometimes known as “anti-vaxxers”), who include parents, politicians, religious leaders, and more, strongly oppose vaccination, particularly childhood vaccination. At its core, the anti-vaccination movement argues that vaccines are unnecessary, ineffective, or dangerous. But is this truly the case? A closer look at the evidence surrounding vaccinationists suggests that while the anti-vaccinationist ideals are compelling, a more accurate picture of vaccination may be found elsewhere.
What vaccines do: the science at the heart of a controversy.
When a pathogen , such as harmful bacterium or virus, infects the body for the first time, the immune system  will quickly recognize it as an “invader” based on molecular features specific to the pathogen (known as antigens) (Figure 1A). In response to these antigens, the body produces molecules called antibodies  that enable our immune system to track down and kill the pathogen.
The first time the body encounters a pathogen, it takes days to assemble enough antibodies to defeat the infection. In that time, the pathogen has the opportunity to attack the body, causing us to experience symptoms of illness. After this first encounters, the body will memorize what the antigens look like, so that if the same pathogen strikes again, we will be able to launch a stronger, faster defense against future invasions.
Vaccines teach the body to recognize a pathogen. Specifically, vaccines contain the same antigens as pathogens (Figure 1B), but in a weakened or dead form, so that the body can learn what the pathogen looks like and produce antibodies (Figure 1C) in a safe and comparatively controlled manner. Thus, when the body encounters the real pathogen, it is already trained and ready to eliminate the pathogen before it can do any damage (Figure 1D).
Importantly, a vaccine protects not only the individual to whom it is administered, but also the entire population. When the number of immunized individuals within a population reaches a critical threshold, herd immunity  is conferred (Figure 2). Herd immunity protects the entire population, even those who are not vaccinated are protected from disease. The percentage of the population that must be immunized to achieve herd immunity varies for individual diseases, with thresholds for common diseases ranging from 75-94% . Herd immunity is crucial to protecting those who are not eligible for vaccines, such as infants, pregnant women, and immunocompromised adults. This means that while vaccines may seem like a personal choice, vaccination protects the entire population—and accordingly, failure to vaccinate could have negative population-level consequences.
Are vaccines truly that effective?
Though no vaccine is 100% effective, many come quite close. For instance, two doses of the chickenpox vaccine are 98% effective against any form of chickenpox . In fact, many vaccines against deadly diseases – such as smallpox , yellow fever , measles - are over 95% effective, as long as they are administered on schedule. History has also attested to the effectiveness of vaccines. Immediately following the licensing of the measles vaccine in 1962, the number of measles cases in the US dropped dramatically  (Figure 3).
Some anti-vaccinationists have claimed that the decrease in measles shortly after the vaccine was introduced is purely coincidental, and that other measures, such as improved hygiene and sanitation, are mainly responsible for the decline in measles and other than diseases. If this were true, we would expect all diseases to have begun their most drastic declines around the same time. However, the sharpest drop in polio cases occurred shortly after the licensing of the polio vaccine in 1955 , in the late 50’s — years before the measles decline occurred (Figure 2). Similarly, cases of rubella declined sharply after 1969 , when the vaccine was first licensed (Figure 3).
The observation that a decline in the number of disease cases occurs right after the introduction of the vaccine for that disease argues very strongly that it is, indeed, the vaccine which is responsible for the reduction in disease. While improvements in sanitation and healthcare were doubtless beneficial, the availability of vaccines was still the major determinant of the spread of a disease.
A tale of autism and vaccines
Among the most popular arguments within the anti-vaccination movement is that vaccines can do more harm than good – for instance, by causing autism , a developmental brain disorder known to impair social interaction and communication.
This argument was popularized by British surgeon Andrew Wakefield , “the father of the anti-vaccine movement” in his 1998 article in The Lancet  that claimed a link between autism and the measles, mumps and rubella (MMR) vaccine. The article describes a study of twelve children who had begun exhibiting symptoms of autism, as well as gastrointestinal issues, shortly after receiving the MMR vaccine . The paper, and Wakefield’s bold assertions to the media of an MMR-autism link, sparked immediate controversy and quickly became a rallying point for parents opposed to the MMR vaccine, or vaccinations in general.
Within months of the Wakefield study, other scientists conducted independent investigations, which failed to corroborate Wakefield’s findings . Additional studies from the UK in 1999 , Finland  and Denmark in 2002 , and Japan in 2005  similarly failed to confirm the Lancet article. In 2014, Australian scientists analyzed the combined results of ten prior studies — collectively involving over 1.2 million children — and similarly found no relationship between vaccines and autism . Most recently, a US study published in April 2015 has once again shown that the MMR vaccine does not increase the risk of developing autism, even for those who are genetically predisposed to the disorder .
In 2004, British investigative journalist Brian Deer presented evidence that Wakefield may have falsified data in his publication . Questions against Wakefield’s findings continued to mount, until January 2010, when the General Medical Council ruled that Wakefield guilty of serious misconduct , and the Lancet retracted the original 1998 article . A few months later, Wakefield was removed from the UK medical register and barred from practicing medicine in the UK .
The enduring popularity of the autism-vaccine story may be fueled by stories within the anti-vaccination movement itself. Parents have shared stories of children who began developing signs of autism shortly after receiving vaccines, and conclude that vaccines are in fact the cause of autism. However, the onset of autism typically occurs at 2-3 years of age , which is around the same age that children receive most of their vaccines.
Additionally, anti-vaccinationists supporting Wakefield’s views often point to the rise of autism diagnoses in the US  – more than ten-fold what it was in the 1970s and 1980s – as evidence of an autism-vaccine link. In actuality, the causes of the increase in autism diagnoses are far more complex.
The rise in autism diagnoses is due, in part, to changes in the definition of autism. In general, the definition of autism has become increasingly detailed and inclusive, and thus, more children today are diagnosed with the disorder . In fact, the same symptoms that would be considered “autistic” today may have led to a different diagnosis, such as mental retardation, several decades ago.
Interestingly, the city of Yokohama, Japan has not administered the MMR vaccine to any children born since 1993. Yet, autism rates have increased even among these unvaccinated children . Thus, even as scientists continue to untangle the exact causes of autism, one thing remains unchanged: no one has yet found a scientifically sound and robust link between vaccines and autism.
Are there dangers of vaccines beyond autism?
Besides autism, anti-vaccinationists are often wary of other potential ill effects of vaccines. In some cases, these concerns are reasonable. Many vaccines are not recommended for individuals who have serious allergies to the ingredients, and for those with a weakened immune system due to diseases such as HIV/AIDS or cancer . Additionally, the seasonal flu vaccine can cause life-threatening symptoms in people with Guillain-Barré syndrome . In rare cases, shoulder injury related to vaccine administration (SIRVA) , causing prolonged pain and stiffness, can result when vaccines are injected too high up on the arm. However, while there are certainly situations where vaccines can cause severe side effects, they are quite rare.
The most common side effects of vaccines, such as discomfort at injection site, fatigue, and fever, are minor and temporary . Fevers can very occasionally worsen into seizures after vaccination . While many parents worry that fever or febrile seizure are dangerous, neither is life-threatening nor permanently damaging. Febrile seizures are not rare: up to 5% of children experience them within their lifetime when they experience fevers from a variety of causes, such as the flu and cold — not just from vaccines . In fact, vaccines may even prevent febrile seizures, as they protect against many diseases associated with febrile seizures, such as measles and chickenpox, among others.
For many, the question of whether or not to vaccinate is a daunting one. The vaccination debate is plagued with a myriad of contradictions and conflicts, and it can be difficult to navigate these turbulent waters. However, with careful investigation and analysis of the available evidence, an accurate picture may begin to take shape.
Vaccines, like any other medical technology, have advantages and pitfalls alike. History and science have repeatedly shown that the advantages of vaccination far outweigh the pitfalls, and that many of the negatives can be avoided with proper judgment. Yet mistrust of vaccines has persisted, despite ample evidence that they are safe, potent, and effective, and further evidence that soundly counters claims such as causing autism. And despite all the vocal opposition to vaccines, none of the attempts to prove the ineffectiveness or dangers of vaccines have stood up over time and rigorous testing.
Many questions in science and medicine are confusing and frustrating, but fortunately, the question of vaccination need not be one of them. Because for vaccines, the verdict is already in: guilty of being safe, effective, powerful, and highly recommended.
Vivian Chou is a Ph.D. student in the Biological and Biomedical Sciences Program at Harvard Medical School.
Selected references for further reading
”Pathogen.” Science Daily. Accessed December 2015.
Zimmermann, KA. “Immune System: Diseases, Disorders, and Functions.” Livescience. October 2014. Accessed December 2015.
Willingham, E & Helft L. “What is herd immunity?” www.pbs.org. September 2014. Accessed December 2015..
”Vaccines are effective.” www.vaccines.gov. Accessed December 2015.
”What is autism?” www.autismspeaks.org. Accessed December 2015.
Deer, B. “Andrew Wakefield and MMR: the investigation.” The Sunday Times. February 2004. Accessed December 2015.
Willingham, E. “Increase in autism diagnoses not an increase in autism. Forbes. www.forbes.com. July 2015. Accessed December 2015.
”Childhood vaccines and febrile seizures.” CDC. www.cdc.gov. Accessed December 2015.
Full list of references
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”Keeping the US polio free.” CDC. www.cdc.gov. Accessed December 2015.
”Smallpox.” WHO. www.who.int. Accessed December 2015. .
”Hepatits B.” WHO. www.who.int. Accessed December 2015.
”HPV vaccine information for young women.” CDC. www.cdc.gov. Accessed December 2015.
Schlenoff, DC. “What are the 10 greatest inventions of our time?” Scientific American. www.scientificamerican.com. November 2013. Accessed December 2015.
”Pathogen.” Science Daily. www.sciencedaily.com. Accessed December 2015.
Zimmermann, KA. “Immune System: Diseases, Disorders, and Functions.” Livescience. www.livescience.com. October 2014. Accessed December 2015.
Mandal, A. “What is an antibody?” News Medical. www.news-medical.net. Accessed December 2015.
”Community immunity (“herd” immunity).” NIH. www.niaid.nih.gov. Accessed December 2015.
Willingham, E & Helft L. “What is herd immunity?” PBS NOVA. www.pbs.org. September 2014. Accessed December 2015..
”Varicella vaccine effectiveness and duration of protection.” CDC. www.cdc.gov. Accessed December 2015.
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”Yellow fever.” WHO. www.who.int. Accessed December 2015.
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 ”Vaccines are effective.” www.vaccines.gov. Accessed December 2015.
 ”Poliomyelitis.” CDC. www.cdc.gov. Accessed December 2015.
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 in The Lancet journal. www.thelancet.com.
 Wakefield, A, et al. “RETRACTED: Ileal-lymphoid-nodular hyperplasia, non-specific colitis, and pervasive developmental disorder in children.” Lancet. 1998.
 Peltola, H, et al. “No evidence for measles, mumps, and rubella vaccine-associated inflammatory bowel disease or autism in a 14-year prospective study.” Lancet. 1998.
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 Deer, B. “Andrew Wakefield and MMR: the investigation.” The Sunday Times. February 2004. Accessed December 2015.
 Triggle, N. “MMR scare doctor ‘acted unethically,’ panel finds.” BBC. news.bbc.co.uk. January 2010. Accessed December 2015.
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 Dohemy, K. “Autism cases on the rise; reason for increase a mystery.” WebMD. www.webmd.com. March 2008. Accessed December 2015.
 Willingham, E. “Increase in autism diagnoses not an increase in autism. Forbes. www.forbes.com. July 2015. Accessed December 2015.
 ”Who should NOT get vaccinated with these vaccines?” CDC. www.cdc.gov. July 2011. Accessed December 2015.
 ”Guillain-Barré syndrome.” MayoClinic. www.mayoclinic.org. Accessed December 2015.
 Zhang, S. “Why are cases of shoulder injuries from vaccines increasing?” Wired. September 2015. Accessed December 2015.
 ”Side Effects from Vaccines.” Immunize for good. www.immunizeforgood.com. Accessed December 2015. .
 ”Febrile seizures following childhood vaccinations, including influenza vaccination.” CDC. www.cdc.gov. Accessed December 2015. .
 ”Childhood vaccines and febrile seizures.” CDC. www.cdc.gov. Accessed December 2015.