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Measles Cases Are Popping Up in U.S.: Are You Immune?

Here are five things you need to know about the highly contagious virus

spinner image Little girls bare back overlaid with a line graph made of a measles rash cases of the measles are on the rise

Flu, COVID and RSV aren’t the only viruses concerning health experts this winter. Several cases of measles — an illness that was effectively eliminated from the U.S. more than 20 years ago — have popped up in more than a dozen states, including Florida, Virginia, Georgia and Pennsylvania. Between Dec. 1, 2023 and Feb. 22, 2024, a total of 35 cases have been reported, according to the Centers for Disease Control and Prevention (CDC).

That number may not seem particularly alarming, but “even one case of measles is something that we should all sit up and pay attention to,” says Patricia A. Stinchfield, a nurse practitioner and president of the National Foundation for Infectious Diseases. “And the reason for that is, it is the most contagious and easily transmittable virus that we have.”

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Here’s what you need to know about the measles as new cases surface.

1. Measles is highly contagious

Like many other viruses, measles spreads through droplets released into the air when an infected person coughs or sneezes.

But unlike other common viruses, measles is so contagious that up to 90 percent of people who are close to an infected person will become infected if they are not immune to the virus, according to the CDC.

“It doesn’t have to be cough right in your face,” Stinchfield says. Tiny virus particles can survive in the air for two hours, where they “circulate around and bounce over to this person and that person, and before you know it, you’ve exposed a lot of people,” she says.

What’s more, a person infected with measles can spread the virus four days before the most obvious symptom — a telltale rash — appears.

2. Many older adults have immunity  

If you’ve had the measles — and you probably have if you were born before 1957, the CDC says — it’s unlikely you’ll get it again. You’re also considered immune if you’ve had the MMR vaccine, given for measles, mumps and rubella. According to the CDC, two doses of MMR is about 97 percent effective at preventing measles — and that protection lasts a lifetime.

“So I would say the older population is, either from disease or vaccination, most likely in good shape,” Stinchfield says.

Still, no vaccine is 100 percent effective, and about 3 out of 100 people who are fully vaccinated for measles will get it if exposed to the virus, the CDC says. Fully vaccinated people who get measles typically experience milder symptoms and are less likely to spread the disease to other people.

Can’t remember if you’ve had the illness or the vaccine? Talk to your doctor. The CDC says there is no harm in getting another dose of the MMR vaccine, even if you have prior immunity. 


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3. Without population immunity, measles can spread 

Because the virus spreads so easily, you need a high level of immunity in the community — or about 95 percent of the population immunized — to prevent ongoing transmission, says John Schieffelin, M.D., an infectious disease physician and associate professor of pediatrics at Tulane University School of Medicine in New Orleans.  

Research shows that the U.S. is under that threshold. A CDC report published in November found that during the 2022-2023 school year, MMR vaccination coverage among kindergartners was about 93 percent, down from 95 percent in pre-pandemic years. This translates to roughly 250,000 kindergartners who are at risk for measles, the report’s researchers say. In some states, measles vaccine coverage among children is under 85 percent.

Vaccine misinformation is partially to blame for this dip, Stinchfield says. So is the pandemic, since many non-urgent doctor’s appointments were skipped or delayed, and so routine vaccinations went missed. (The first dose of the MMR vaccine is typically given around the first birthday; the second dose is around kindergarten entry.)  

If vaccination rates continue to decline, measles could reestablish itself in the U.S., health experts warn. “Any kind of drop in our vaccine rates, we need to pay attention to and get them back up where they belong,” says Stinchfield, who adds that grandparents can help play a role by talking to their children and grandchildren about vaccines.

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4. Measles can be dangerous, even deadly

Schieffelin says a common misconception is that measles isn’t a big deal, since virtually everyone used to get infected. But that is not the case.

Common symptoms — which include a fever, a cough, a runny nose — can cause a person to feel “miserable,” says Stinchfield, who helped take care of hospitalized children during a large measles outbreak in Minnesota in the 1990s.

Three to five days into the illness, a rash breaks out and spreads from the top of the head down the body. At this point, a fever can spike to more than 104 °F, the CDC says.  

The thing with measles, though, is that it’s not uncommon for the illness to turn severe. About 1 in 5 unvaccinated people in the U.S. who get measles end up in the hospital, according to the CDC. Complications range from dehydration to pneumonia to encephalitis, or swelling of the brain.  

Measles can also mess with your immunity to other illnesses, Stinchfield says — a phenomenon referred to as immune amnesia.

Young children are at increased risk for these complications; so are older adults, Schieffelin says. “As we get older, we don’t tolerate [viruses] quite as well,” he says.

It’s estimated 136,200 people worldwide, mostly children, died from measles in 2022, according to a report from the CDC and the World Health Organization.

5. Unvaccinated travelers can spread measles

Most people in the U.S. are protected against measles with the vaccine, but the virus hasn’t been eliminated worldwide. In fact, the global vaccination rate for both MMR doses hovers around 74 percent, according to the CDC. In some countries, it’s as low as 66 percent.

The CDC says measles outbreaks are occurring in every region of the world and that “the increased number of measles importations seen in recent weeks is reflective of a rise in global measles cases and a growing global threat from the disease.”

An unvaccinated traveler who picks up a case in another country can bring it to the U.S. and spread it to other unvaccinated people, Stinchfield says. In a community with low vaccination rates, the disease will spread like wildfire, she adds.

Measles “is still out there,” Schieffelin says. “And by not being vaccinated, you really do run the risk of getting sick as well as your family members getting sick.”

Editor's note: This story, originally published Jan. 24, 2024, has been updated to reflect new information.

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