LONDON — Cases of measles, which appeared headed toward elimination in Europe, have soared this year, reaching their highest level in two decades across the Continent as vaccination rates fall short, the World Health Organization reported on Monday.

Until the 1990s, there were hundreds of thousands of measles cases in Europe every year, but aggressive vaccination programs pushed that steadily downward, to a record low of 5,273 in 2016. Last year, the figure jumped to 23,927, and in just the first half of 2018 there were more than 41,000 cases, 37 of them fatal.

The increase in Europe has come even as measles continues to decline worldwide.

Public health officials say that at least 95 percent of a population must have immunity to control the spread of measles, which experts call the world’s most contagious human disease. But in Europe, only about 90 percent of children receive the recommended two-dose vaccine in early childhood as recommended, the W.H.O. said, and last year, only Russia and a handful of small countries reached the 95 percent mark in young children. The rate in the United States was 94 percent.

Several European countries, including relatively wealthy ones like France, Belgium and Austria, were at 85 percent or less. And in many countries, vaccination rates for young children have actually declined somewhat in recent years.