The number of measles cases in the European region doubled last year to reach the highest level in 25 years, health officials say.
A joint report by the World Health Organization (WHO) and the UN children's fund, Unicef, said children under the age of five accounted for more than 40% of the cases reported in Europe and central Asia.
"Measles is back, and it's a wake-up call," Hans Henri Kluge, WHO regional director for Europe, said. "Without high vaccination rates, there is no health security."
The MMR vaccine - which immunises people against measles, mumps and rubella - is 97% effective in fighting off the dangerous virus.