The World Health Organization listed "vaccine hesitancy" -- defined as the reluctance or refusal to vaccinate, despite the availability of vaccines -- among the top 10 health threats of 2019.
Vaccination is one of the most cost-effective ways to avoid disease, preventing 2-3 million deaths annually, according to the organization. An additional 1.5 million more deaths could be prevented if vaccines were more widely available globally.
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Measles is one disease that was nearly eliminated by vaccinations, but there has been a 30 percent increase in cases globally, according to WHO. The organization says that people who choose not to vaccinate threaten "to reverse the progress made in tackling vaccine-preventable diseases."
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WHO notes that not all of these cases are due to vaccine hesitancy, but that it plays a role in the spike. The reasons why people choose not to vaccinate are complex, the organization says, and include complacency, inconvenience accessing vaccines and a lack of confidence in their safety or effectiveness.
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Concerns about the safety and effectiveness of vaccines have abounded for several centuries, especially before pathology and proper sanitation were well-understood. The anti-vaccination movement has seen a resurgence in the U.S. since 2007, according to the Measles and Rubella Initiative, largely due to celebrities' influence.
Visit the World Health Organization's website for more information about vaccine safety.
FILE - In this Monday, Aug. 6, 2018, file photo, a health worker prepares a syringe with a vaccine against measles in Rio de Janeiro, Brazil. (AP Photo/Leo Correa, File)