In Ukraine, the seeds of vaccine skepticism fell on particularly fertile ground. Just 35% of residents are fully vaccinated against COVID-19, and only 1% more are partially vaccinated — among the lowest such rates in Europe, according to data from Oxford University. Childhood immunizations for diseases like measles and polio are among the continent’s lowest as well.

That gives public health officials reason for worry as more than 3.6 million Ukrainian refugees have poured into other countries and millions more are displaced within Ukraine, often hunkered down in crowded, frigid places without clean water or electricity.

“COVID doesn’t need crowded refugee camps to thrive,” said Dr. Chris Beyrer, the director of the Center for Public Health and Human Rights at the Johns Hopkins Bloomberg School of Public Health. “Crowded gyms, crowded buses and crowded trains will be enough.”