The recent outbreak of measles in the United States prompted health officials and doctors to remind folks about the importance of vaccinations. But a new online survey reveals that almost half of American adults doubt the safety of these biomedical preventives, reports the American Osteopathic Association (AOA).

The survey of more than 2,000 adults found 45% of Americans surveyed harbored reservations about immunizations because of negative news from at least one source. The top three sources were online articles, past questionable behavior by the pharmaceutical industry, and information from medical experts.

Survey participants were also asked to choose a statement that best represented their feelings about vaccine safety and efficacy. A total of 82% of respondents were in favor of vaccines, while 8% picked responses that communicated serious distrust and 9% were undecided.

But even these small proportions of vaccine skeptics could significantly damage public health if the uncertainty causes more people to avoid vaccinations, experts say. For example, diseases such as measles require that as much as 95% of the population be vaccinated in order to achieve herd immunity (or community immunity), says Paul Ehrmann, DO, an osteopathic family physician.

Because of medical problems, such as allergies, illness or a weakened immune system, some people cannot be vaccinated. This is why community immunity is crucial to maintaining overall public health. If the rest of the population is vaccinated, these inoculations help to protect the most vulnerable.

According to Rachel Shmuts, DO, a perinatal psychiatrist, human psychology and social media amplification are responsible for the spread of negative attitudes about vaccines. “From an evolutionary perspective, humans are primed to pay attention to threats or negative information,” she said. “So it makes sense that people hold onto fears that vaccines are harmful, especially when they believe their children are in danger.”

But the medical and scientific communities must stand up to such fears and use evidence-based medicine and policy changes to change behaviors, Ehrmann said.

Nevertheless, convincing people of the safety, efficiency and necessity of vaccines is a challenge, especially once they believe otherwise. Shmuts pointed out that despite the relatively small number of people who believe vaccines are dangerous, online support groups solidify these beliefs, “making them less susceptible to influence from their neighbors and real-world communities.”

Click here to learn about how the U.S. measles outbreak hit a record high.