The current measles outbreak has infected 21,000 people in Europe. U.S. officials are concerned that overseas travelers could bring it here.

Multiple measles outbreaks in Europe have resulted in 21,000 people contracting the virus and 35 deaths over the past year.

This fourfold increase of European measles cases has put a spotlight on the possibility of a measles outbreak starting in the United States via travelers or an unvaccinated American returning from a trip abroad.

Measles is one of the most easily transmissible viruses in the world. For people who aren’t vaccinated, 90 percent of them will contract the virus when exposed to it.

Zsuzsanna Jakab, PhD, the World Health Organization’s regional director for Europe, called the European measles cases a “tragedy” in a statement.

“Every new person affected by measles in Europe reminds us that unvaccinated children and adults, regardless of where they live, remain at risk of catching the disease and spreading it to others who may not be able to get vaccinated. Over 20,000 cases of measles, and 35 lives lost in 2017 alone, are a tragedy we simply cannot accept,” Jakab said.

The spike in measles cases wasn’t limited to one country or one area of Europe. Romania (5,562 cases), Italy (5,006 cases) and Ukraine (4,767 cases) reported the most infections.

In total, 15 countries reported outbreaks where at least 100 people were affected, including Germany, Greece, the United Kingdom, and France.

Health ministers from 11 countries are expected to meet later this year to discuss how to eliminate measles and rubella by 2020.

“Elimination of both measles and rubella is a priority goal that all European countries have firmly committed to, and a cornerstone for achieving the health-related Sustainable Development Goals,” Jakab said. “This short-term setback cannot deter us from our commitment to be the generation that frees our children from these diseases once and for all.”

The measles vaccine works well, providing 97 percent effectiveness after two doses. But that only stops outbreaks if enough people get the vaccine.

Pockets of people who are unvaccinated either by choice or due to a lack of robust vaccination programs can increase the risk of a major outbreak.

Herd immunity — or having enough people vaccinated in a population to prevent an outbreak — is especially important for measles. It’s so infectious that the virus can linger for hours in the air.

Having enough people vaccinated can help protect both infants under 1 year who are too young to be vaccinated and others who may be immunocompromised, such as people getting treatment for cancer or people living with HIV.

In the United States, 91 percent of young children receive vaccinations for measles nationwide. This makes it more difficult for large-scale outbreaks to happen.

However, some pockets of the country have larger numbers of unvaccinated people. This increases the outbreak risk dramatically if a person with measles visits these areas.

Recent outbreaks in the United States have often been attributed to either unvaccinated travelers or Americans who traveled abroad and contracted the virus before arriving back in the country.

In 2015, a large measles outbreak that started at Disneyland in California is believed to have originated by an international visitor who may have contracted the disease and spread it mostly to people who weren’t fully vaccinated.

Last month, the New York City Health Department issued a warning that people in certain areas of the city may have been exposed to measles after an Australian tourist with the disease visited the city.

Dr. William Schaffner, an infectious disease expert at Vanderbilt University Medical Center in Tennessee, said that some people still don’t understand the severity of the disease.

“This is just terribly sad,” he said of the European outbreaks.

Schaffner said before the measles vaccine was discovered, approximately 400 to 500 children died from measles every year in the United States.

A 2015 study found that even children who survive measles may have a compromised immune system for years after the infection.

Schaffner said that while national vaccination rates are high, public health officials are constantly on guard for the disease, especially when there’s an outbreak in other parts of the world.

He said they may be particularly on guard in the summer, when more children travel due to school vacation.

“It can spread at any time,” he told Healthline. “More often than not it’s the children who are the transmitters, children from abroad come to the U.S.… our children who are unvaccinated sometimes go abroad [and] pick it up.”

Schaffner said he did see signs that more European governments are instituting increased regulations that promote vaccinations, which could hopefully diminish the risk of measles outbreaks here and abroad in the future.

“Slowly but surely, the Europeans are instituting measures, legislation to get all children vaccinated against measles,” he said. “But none of them have the ‘no shots, no schools’ legislation we have had.”

If families with infants want to travel to Europe, Schaffner said they may have some options to go safely.

He said if a family with a young baby is traveling to an area with a measles outbreak, the infant can get the measles vaccine as early as 6 months old to provide protection. They’d then be recommended to get the regular two measles vaccine doses to provide full protection.