In 2016, there were just 5,273 cases of measles recorded in the region—a record low. But last year that number soared to 21,315, with 35 deaths.

“Every new person affected by measles in Europe reminds us that unvaccinated children and adults, regardless of where they live, remain at risk of catching the disease and spreading it to others who may not be able to get vaccinated,” said Dr Zsuzsanna Jakab, the organization’s regional director for Europe.

“Over 20,000 cases of measles, and 35 lives lost in 2017 alone, are a tragedy we simply cannot accept.”

The countries with the biggest outbreaks were Romania and Italy, each of which saw more than 5,000 cases, and Ukraine, with more than 4,700 cases. According to the WHO, this may be down to “declines in overall routine immunization coverage, consistently low coverage among some marginalized groups, interruptions in vaccine supply or underperforming disease surveillance systems.”

The numbers in Europe are substantially higher than those in the United States, where the Centers for Disease Control and Prevention reported just 118 cases throughout 2017. However, some have expressed fears over a potential resurgence, thanks to President Donald Trump’s repeated insistence that there is a link between childhood vaccines and autism—a thoroughly discredited theory.

“Elimination of both measles and rubella is a priority goal that all European countries have firmly committed to, and a cornerstone for achieving the health-related Sustainable Development Goals,” said Jakab. “This short-term setback cannot deter us from our commitment to be the generation that frees our children from these diseases once and for all.”