“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.”