More than 1,100 measles cases have been reported in the United States this year, and the highly contagious virus has put people at risk while they wait in ERs and doctor’s offices — but also at a Shakira concert in New Jersey, at a mall in Minnesota, at a Buc-ee’s store in Texas and in airports and planes in several states.

“Travelers can catch measles in many travel settings including travel hubs like airports and train stations, on public transportation like airplanes and trains, at tourist attractions, and at large, crowded events,” the US Centers for Disease Control and Prevention said in a recently updated travel notice. “Infected travelers can bring the disease back to their home communities where it can spread rapidly among people who are not immune.”

It’s a situation that may feel unfamiliar for US travelers. In 2000, measles was declared eliminated from the US due to highly effective and coordinated vaccination efforts. That year, only 85 cases were recorded, all of which were imported. But at least 1,157 measles cases have been reported this year, according to a CNN tally using data from state health departments. Only one other year since 2000 has had more cases: 2019, when there were 1,274.

The vast majority of this year’s cases have been part of an ongoing multistate outbreak centered in West Texas. The outbreak appears to be stabilizing, but there is growing concern around the risk of additional spread due to summer travel, said Lori Tremmel Freeman, chief executive officer for the National Association of County and City Health Officials.

At the epicenter of the measles outbreak in Gaines County, “that community has either become immune by contracting measles and developing immunity from the disease, or in some cases, they’ve gotten vaccinated,” Freeman said, but she added that “there’s a cautionary tale,” because of summer travel.

“Heavy travel is anticipated,” she warned. “The travel spread could continue throughout the rest of the country as these groups become mobile and still carry the disease.”

The CDC’s health notice emphasizes that all international travelers should be fully vaccinated with the recommended two doses of the measles-mumps-rubella (MMR) vaccine and that infants ages 6 months to 11 months who are traveling should receive an extra dose.

Dr. Peter Hotez, co-director of the Texas Children’s Hospital Center for Vaccine Development, said this recommendation for infants was also given by the Texas Department of State Health Services in the 10 counties that have seen increased measles transmission during this year’s outbreak.

“That means, then, that you’re going to wind up giving three doses instead of two doses. Usually, you give one dose [to children between] 12 to 15 months, a second dose at 4 to 6 years [old]. Now, you’re going to give three doses,” he said.

But he emphasized that that this extra dose is still safe: “The MMR vaccine is one of the safest vaccines we know.”

Neither the Texas health department nor the CDC recommends the MMR vaccine for adults born before 1957, Hotez notes, riding on the assumption that measles exposure was common in the first half of the 20th century and that exposure would have led to enduring immunity. However, there are exceptions for health care workers serving a measles outbreak area, for whom two MMR doses are recommended regardless of birth year.

Hotez highlights that vaccine recommendations may change as outbreaks continue, stressing the importance of staying aware of updates from local health officials.

The MMR vaccine is not recommended for pregnant women, infants younger than 6 months and people who have severely weakened immune systems.

Although the risk of exposure is generally low, people who can’t be vaccinated should speak with their doctor about their individual risk tolerance, said Dr. Scott Roberts, associate medical director for infection prevention at the Yale School of Medicine. “A lot of this depends on their access to therapeutics at their travel destination, like are they going to an area where immunoglobulins may be available?”

Immunoglobulins, also known as antibodies, are naturally produced by the body as a part of immune responses. They play a crucial role in identifying and neutralizing foreign substances like bacteria and viruses and are specific to each pathogen. People who have a weakened immune system can get immunoglobulins, typically at a hospital or clinic, if they’re exposed to measles.

Historically, Roberts said, the most common source of US measles outbreaks has been non-immune US residents who traveled abroad, were exposed to the virus and then brought it back. Although some international travelers have brought measles into the US, this makes up a minority of cases.

Although no country requires proof of MMR immunization for travel, Roberts advises travelers to “double-check their immunity status before traveling internationally.”

This immunity may involve a prior measles infection, birth before 1957, lab test results of measles immunity or written documentation of MMR immunization.

Travelers should check their status at least 6 weeks before they leave, he said. Full immunity includes the two doses of MMR vaccine at least 28 days apart as well as the two weeks it takes to build up final immunity after the last dose.

Two doses of the MMR vaccine are 97% effective against measles infection, so vaccinated travelers who suspect that they have been exposed to the virus shouldn’t need to worry, Roberts said. Although that leaves a 3% chance of infection, symptoms are significantly lessened with the vaccine.

If you are unvaccinated and suspect that you may have been exposed, Roberts says you should see a doctor as soon as possible. “Depending on how soon you are after exposure, you can still do something,” he said. You may be able to get immunoglobulin or even the MMR vaccine to help lessen symptoms.

Other than verifying immunity, Roberts advises typical travel guidance such as packing commonly used and prescribed medications, as well as keeping a list of hospitals or clinics at your destination and other first-aid supplies. When traveling internationally, Roberts recommends checking the country-specific travel health notices on the CDC website to stay aware of additional vaccinations and medications needed for certain destinations.

Roberts advises travelers wash their hands frequently, avoid people who are sick when possible, stay in well-ventilated areas and wear masks if they are at high risk of severe illness, staying up to date with vaccinations, especially those who were unable to get vaccinated because of severe immunodeficiency. These practices don’t just help prevent measles infection, they help curb infection from other circulating pathogens such as Covid-19 and the flu.

Measles is one of the most contagious germs around. In one case 10 years ago, four travelers were infected by a person at the same airport gate. “This highlights how contagious [measles] is,” Roberts said. “Even airports and terminals are risky.”

The ongoing measles outbreak is not to be taken lightly, Hotez said.

“In the 1980s, measles was the single leading killer of children globally. It [led to] 3 million deaths annually. Now, through the GAVI Vaccine Alliance and other programs, we’ve brought it down to around 100,000 deaths, but it’s still a lot. And we’re seeing what happens in West Texas. Ten percent, 20% of kids are being hospitalized, measles, pneumonia, neurologic injury. We’ve had over 90 hospitalizations so far in Texas and two measles deaths.”