The number of measles cases in Europe has hit a record high of more than 41,000, the World Health Organization says.

That means that, little more than halfway through the year, 2018 is already the worst year on record for measles in Europe in a decade, WHO said. So far, at least 37 people have died of the highly contagious virus.

A lack of vaccines is partly to blame in some affected countries, but in others, it’s the refusal to get kids vaccinated on time that is driving epidemics, WHO said.

“Following the decade’s lowest number of cases in 2016, we are seeing a dramatic increase in infections and extended outbreaks,” Dr. Zsuzsanna Jakab, WHO Regional Director for Europe, said in a statement.

More cases in any country means more cases can be spread by travelers, including to the U.S. Measles has been eliminated in the U.S., but every year, the virus is imported by travelers and can spread quickly in areas where people are not properly vaccinated.

Most of the cases in Europe are in Ukraine, which had had 23,000 cases, WHO said.

The number of measles cases varies greatly from year to year. “So far, the highest annual total for measles cases between 2010 and 2017 was 23,927 for 2017, and the lowest was 5,273 for 2016,” WHO said.

“Seven countries in the region have seen over 1,000 infections in children and adults this year (France, Georgia, Greece, Italy, the Russian Federation, Serbia and Ukraine).”

With a safe and effective measles vaccine available, even one case is too many. Tragic to see such high numbers of #measles cases in both the #US and #Europe. #VaccinesWork https://t.co/t290VdMzKW — Sabin Vaccine Inst. (@sabinvaccine) August 20, 2018

In the U.S., the Centers for Disease Control and Prevention has reported 107 measles cases as of the middle of July this year. Case counts also vary widely in the U.S. from year to year and range from 55 cases in 2012 to 667 in 2017. Last year, 118 cases were reported by CDC.

“This partial setback demonstrates that every person who is not immune remains vulnerable no matter where they live, and every country must keep pushing to increase coverage and close immunity gaps,” WHO’s Dr. Nedret Emiroglu said in a statement.

WHO said that 95 percent of the population must have received at least two doses of measles vaccine to prevent outbreaks. Some parts of Europe have that, while others are below 70 percent, WHO said.

“We can stop this deadly disease. But we will not succeed unless everyone plays their part: to immunize their children, themselves, their patients, their populations — and also to remind others that vaccination saves lives,” Jakab said.

Social media and vaccine opposition

The U.S. has its own pockets of under-immunized people. For instance, a report published in June found that more than a quarter of kindergarten students in Camas County, Idaho, lack at least some of their vaccines, mostly because the state makes it easy for parents to simply choose not to vaccinate their children.

“A social movement of public health vaccine opposition has been growing in the United States in recent years; subsequently, measles outbreaks have also increased,” pediatrician Dr. Peter Hotez and colleagues wrote in their report, published in the Public Library of Science journal PLoS Medicine.

When one person has #measles, 90 percent of the people they come into close contact with will become infected, if they are not already immune. #VaccinesWork! https://t.co/Arbo3hY7dv pic.twitter.com/N1xHY4emAX — World Health Organization (WHO) (@WHO) August 20, 2018

In Europe and the U.S. alike, social media has helped spread incorrect information about vaccine effectiveness and safety. Italian authorities say vaccine critics have helped lower rates there from 90 percent in 2010 to 85 percent in 2015.

Despite the record number of cases in Europe, WHO says vaccination against measles is a success story.

“Global measles deaths have decreased by 84 percent worldwide in recent years — from 550,100 deaths in 2000 to 89,780 in 2016,” WHO says.

Measles epidemics are also causing trouble in Brazil, where the Brazil Health Ministry reports 1,053 measles cases and five deaths this year.

The Pan American Health Organization (PAHO), part of the WHO, says indigenous Brazilians, including members of Amazon-dwelling Yanomami communities, are suffering outbreaks.

At least 126 measles cases, including 53 deaths, have been reported among Yanomami just across the border in Venezuela.

Measles is one of the most contagious viruses known. It spreads in the air and almost everyone who has not been vaccinated will develop an infection if they are exposed.

Symptoms of measles generally begin a week to 14 days after someone is infected. They include a high fever, cough and the signature red rash of the infection. Some people can develop encephalitis, a potentially fatal brain inflammation.