Which makes the encounter just the latest indication that the next president might be willing to discard science and medical research on vaccination in favor of debunked myths. There’s a real risk that Trump — who has expressed doubt about the safety of immunizations in the past — could politicize vaccines, undermining trust in one of the great public health interventions in human history.

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The success of immunization programs in the United States should not be taken for granted. It took decades of hard work by doctors, the Centers for Disease Control and Prevention, and state and local health departments to get vaccination rates to where they are today: over 90 percent for vaccines against polio, hepatitis B, chickenpox, measles, mumps and rubella.

Even a modest decrease in those rates could be enough to cause future outbreaks. And because political polarization can push people to change their belief in basic facts, making vaccines a political cause for one side or another may lead to exactly that.

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The modern era of immunization programs began in the early 1960s, when President John F. Kennedy, Robert Kennedy Jr.’s uncle, signed the Vaccination Assistance Act. This law allowed the CDC to support local health departments in immunization-related activities.

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In the late 1960s, after several measles outbreaks, there was a push for state laws requiring children to be vaccinated before starting school. By the early 1980s, all states had mandatory immunization laws in place (although all but three states allow exemptions for religious or “philosophical” reasons). Over the decades, with bipartisan support, federal funding for vaccines grew.

These efforts led to a gradual increase in vaccination rates among U.S. children. But outbreaks of measles between 1989 and 1991 demonstrated that those rates weren’t consistent everywhere or in every demographic group. In 1991, President George H.W. Bush’s administration announced a goal of achieving immunization levels of 90 percent or more for children under 2 years old. That effort received a major boost from the Clinton administration’s Childhood Immunization Initiative and Vaccines for Children Program. By 2001, the 90 percent target had been achieved for most of the recommended vaccines.

Since the late 1990s, though, there has been a countervailing trend of vaccine refusal. Low state-level refusal rates are masked by substantial local clustering of vaccine refusers — who provide the tinder for outbreaks to start and spread like wildfire.

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Until relatively recently, vaccines enjoyed broad and bipartisan support in most of the country. A 2014 Pew Research Center survey found that party identification and ideology had only a weak effect on whether people believed that vaccines were safe (though there was a “medium” effect on whether they should be required).

But no president in recent history has called the science supporting vaccination into question. Despite Kennedy’s family and political lineage, if Trump takes up the issue, the public discussion around vaccines could quickly become politicized.

Political polarization influences the basic facts voters believe in, research and polling have found. Just look at climate change: One survey last fall found that only 16 percent of conservative Republicans accept that virtually all the scientists who study the issue agree that human activity is mostly responsible for warming climate. Researchers have found a close correlation between GOP opposition to policy proposals to alleviate climate change and a refusal among Republican voters to believe in the scientific consensus on the issue.

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If vaccination becomes closely associated with political identity, the high vaccination rates required to maintain what experts call “herd immunity” might be at risk. Herd immunity provides unvaccinated people with indirect protection from infections by decreasing the likelihood that they will encounter someone who is infected. Herd immunity thresholds (i.e., what percentage of the population needs to be vaccinated to prevent outbreaks) vary by disease and population, but they range from 75 percent to 95 percent for most common diseases preventable with childhood vaccines. So even if only a small segment of Trump’s supporters are persuaded by his rhetoric not to vaccinate their children, the result could be dangerous.

Experience in other countries suggests that vaccine confidence can decrease precipitously once challenged, and that it takes time and substantial effort to get the public back on board. In the most notorious example, vaccination rates in Britain declined after a 1998 article in the journal Lancet pushed a purported link between the measles vaccine and autism. The measles vaccination rate, approximately 92 percent in the year before the paper, fell to a low of 80 percent by 2003. It took until 2013 for the rate to return to its earlier level. Meanwhile, the frequency and size of measles outbreaks increased. The article was eventually retracted and found to be fraudulent, and its lead author lost his medical license.

In another case, after a scare over side effects of the whooping cough vaccine in 1974, the vaccination rate for that disease in England and Wales dropped from over 80 percent to a low of 31 percent in 1978, and incidents of whooping cough increased. Despite efforts by public health authorities, the rate did not rebound to its pre-1974 level until 1988.

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Here in the United States, my research has shown that vaccine refusal is associated with outbreaks of preventable diseases such as measles and whooping cough . In fact, the Disneyland measles outbreak in 2015 was substantially linked to vaccine refusal.

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The United States has one of the strongest vaccine safety monitoring systems in the world. Safety is evaluated in clinical trials before a vaccine is approved by the Food and Drug Administration. Even after approval, scientists continue to monitor the safety of vaccines using large-linked databases — the biggest of which contained information for 178 million individuals by 2014.

Data gathered by these monitoring systems is published in peer-reviewed journals. The evidence base is periodically compiled and evaluated by independent scientific bodies. The most credible of these is the National Academy of Medicine, which has conducted several reviews of vaccine safety, all of which have supported the continued use of vaccines. Notably, the reviews are conducted by highly qualified physicians and scientists — not politically appointed activists.

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Because of the overwhelming evidence backing their safety, doctors and scientists strongly support vaccines. The American Academy of Pediatrics, the American Medical Association and the Infectious Diseases Society of America not only support immunization in general, they also endorse adhering to the vaccine schedule recommended by the CDC.

Trump, however, has challenged that schedule, saying in a 2015 presidential primary debate that he wanted “smaller doses over a longer period of time.” Worse, he has repeated the false claim that vaccination was causing autism.

Still, perhaps there is one area where scientists and the next administration can find common ground: investment in vaccine safety research. Most vaccine proponents are also big supporters of such research. While the CDC, the FDA and — to some extent — the National Institutes of Health have been conducting rigorous vaccine safety research, the introduction of new vaccines requires further expansion of monitoring by physicians and scientists. More money would help that effort.

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Vaccines are one of the most successful advances in public health of all time. Among American children born between 1994 and 2013, 322 million cases of illness and 732,000 deaths will be prevented by vaccines over their lifetime. Threats to vaccine confidence can result in a large number of avoidable illnesses and even loss of life.