People who choose not to vaccinate has emerged into a global health threat in 2019, said the World Health Organization.

“Vaccine hesitancy,” which WHO describes as “the reluctance or refusal to vaccinate despite the availability of vaccines” is among the organization’s top global threats for the year, according to a report released Tuesday.

WHO said vaccines prevent 2-3 million deaths a year, and improved global coverage on vaccinations could stop an additional 1.5 million deaths annually.

The agency cites measles, which has seen a 30 percent increase in cases worldwide. “The reasons for this rise are complex, and not all of these cases are due to vaccine hesitancy,” said WHO in a statement. “However, some countries that were close to eliminating the disease have seen a resurgence.”

In the U.S., the number of young children who aren’t getting vaccinated is rising. Between 2001 and 2015, the number of unvaccinated babies and toddlers has quadrupled, according to the Centers for Disease Control and Prevention. 

Some parents opt not to vaccinate discredited belief vaccines are linked to autism. The CDC says there is no link, and there are no ingredients in vaccines that could cause autism.

Last October, Amanda Cohn, a senior adviser for vaccines for the CDC’s National Center for Immunization and Respiratory Diseases, said uninsured children are “highly represented” among kids who don’t get vaccines.

“While we know parental choice clearly plays a role, we also see in this report that access does seem to be an issue,” Cohn said in a statement.

The U.S. has seen a surge in the measles, which the CDC had considered eradicated in 2000. Earlier this year, an outbreak of the measles reached more than 100 people and across 21 states. 

Among other global health threats to watch this year, according to WHO: air pollution and climate change, noncommunicable diseases such as diabetes and cancer, and the global flu pandemic.