The World Health Organization says measles has killed 35 children in Europe in the last year, calling it an "unacceptable tragedy" that deaths are being caused by a vaccine-preventable disease.

The figure is nearly a threefold increase since 2016, when measles killed 13 children in Europe. In 2015, it killed three.

In a statement Tuesday, the U.N. health agency said the most recent death was a 6-year-old boy in Italy, where there have been more than 3,300 cases and two deaths since last June. The highly contagious virus has also caused 31 deaths in Romania.

Despite decades of medical research confirming the vaccine's safety, fears about the vaccine have caused thousands of parents to avoid vaccinating their children across Europe. In May, Italy made 12 vaccines mandatory for children, to combat what the government characterized as "misinformation."

The U.S. has also seen an uptick in measles this year, with 108 cases reported to the CDC as of mid-June. That's more than the total of 70 cases reported in the country in all of 2016.

Measles is caused by a highly contagious virus that spreads through the air when an infected person coughs or sneezes. Without the vaccine, doctors estimate 90 percent of people who are exposed will become ill. Recent outbreaks have mostly been traced to travelers who contracted the virus overseas and then spread it to others who were not vaccinated.