LONDON — Measles has returned to four European countries this year amid a “dramatic resurgence” in the disease on the continent, the World Health Organization said on Thursday, a reversal fueled in part by a rising wave of people who are refusing to be vaccinated.

The number of cases in the first half of the year in the European region doubled in comparison with 2018 — with 90,000 people infected, compared with 44,175 in the first six months of last year, the W.H.O. said in a statement.

“We are backsliding; we are on the wrong track,” Dr. Kate O’Brien, the director of the W.H.O.’s immunization department, said at a news conference.

The W.H.O. monitors the elimination of the disease across the region, and countries with continuous transmission of the virus for 12 consecutive months are no longer considered measles-free.