Are you protected against measles? Do you need a booster shot? Everything you need to know about immunity

Hundreds of people have caught measles in the United States this year, with the majority of the cases tied to outbreaks of the disease. So far, Texas has been the hardest hit, with the highest number of reported illnesses.

According to data from the Centers for Disease Control and Prevention (CDC), 301 people in the U.S. have had confirmed measles infections this year, as of March 13. Additional, probable cases have been reported by various states, but they've yet to be independently confirmed by the CDC.

Kids under 5 years old account for 34% of the confirmed measles cases, with a further 42% affecting children and teens ages 5 to 19. In total, 17% of the cases have resulted in hospitalization.

There has been one confirmed death related to the outbreaks, and a second remains under investigation. The first fatal case was in an unvaccinated school-age child in Texas, marking the first death from measles in the U.S. since 2015. The second deadly case was reported in a New Mexico resident by local health authorities, but the CDC has yet to conclude its own investigation of the case, which would help confirm its connection to ongoing outbreaks.

Related: US measles outbreak tops 300 cases — what to know about the disease

The vast majority of the confirmed measles infections — 95% — have been in people who were unvaccinated or had unknown vaccination status. Additionally, most of the infections have been reported in Texas, with data from the Texas Department of State Health Services showing that 279 measles cases were identified between late January and March 18.

Experts say the Texas outbreak is likely related to low rates of vaccination in the state. For instance, data suggest that around 18% of school-age children in Gaines County have been exempted from at least one vaccine, and 191 measles cases have occurred in that county so far. Meanwhile, Gaines' neighboring counties of Dawson, Yoakum, Terry and Martin have reported 11, 11, 36 and 3 cases, respectively.

As measles continues to spread, here's what to know about the protection of the measles vaccine, how long it lasts, and how to know if you're immune to the disease.

How effective is the measles vaccine?

Measles can be prevented with the MMR vaccine, which protects against measles, mumps and rubella; or the MMRV vaccine, which additionally protects against the chickenpox-causing varicella virus. The CDC recommends that children receive their first dose of the MMR or MMRV vaccine between the ages of 12 and 15 months and their second dose between 4 and 6 years old.

(For measles protection, the MMR vaccine is generally recommended over the MMRV vaccine, because it's less likely to trigger fevers in children. For those who get the MMR shot, a separate vaccine is recommended to protect against chickenpox (varicella).)

Unvaccinated older children and adults are also advised to receive two doses of a measles vaccine, at least 28 days apart.

One dose of the MMR vaccine is 93% effective at preventing measles, while two doses are 97% effective, according to the CDC. The MMRV vaccine offers the same level of measles protection.

"It's probably one of the best vaccines that we have," Michael Head, a senior research fellow in global health at the University of Southampton in the U.K., told Live Science in an email.

"For example, the COVID-19 vaccines are very good at reducing mortality, but less good at preventing a mild COVID-19 infection," Head said. The measles vaccine is highly protective against measles infections of all severity levels.

Measles was declared eliminated in the U.S. in 2000 thanks to widespread vaccination against the disease. The CDC defines "elimination" as the "absence of continuous disease transmission for 12 months or more in a specific geographic area." Measles is not yet "eradicated" worldwide, meaning it still spreads regularly in other countries and can be imported to the U.S.

How long does the MMR vaccine's protection last?

The MMR vaccine provides long-lasting measles protection for most people, with immunity typically being lifelong after two doses, according to the CDC. However, one 2024 study published in the journal The Lancet Public Health reported that measles immunity does fade slightly over time.

"Like with COVID vaccines, antibody levels do reduce over time," Dr. Peter Chin-Hong, a professor of medicine at the University of California, San Francisco, told Live Science via email. Antibodies are protective immune proteins that appear in the days and weeks after a person receives a vaccine, but these proteins are expected to disappear over time.

"However there is likely continued immunity with memory immune cells that can be activated as needed (as we have seen in COVID vaccines)," Chin-Hong added. When exposed to the measles vaccine, the body generates "memory" cells in the immune system, which can quickly mount an immune response and make new antibodies if a person encounters the measles virus.

"In general, protection once immunized against measles, wanes very little," Chin-Hong said. The Lancet study suggested that, on average, measles immunity waned by about 0.039% per year after the age of 5.

Related: 'A political division, not a physical one, determined who got measles and who didn't': Lessons from Texarkana's 1970 outbreak

Does catching measles give you lifelong immunity?

Yes, if a person survives a bout of measles infection, they typically have lifelong immunity against the virus. However, even as the immune system mounts a powerful response against measles, its guard against other infections gets lowered.

Many studies find that measles infections induce a kind of "immune amnesia," in which memory cells tasked with remembering pathogens are lost and the body is unable to rev up its antibody responses, as it used to. This leaves a person at increased risk of other infections for years following their brush with measles.

Other complications of measles infection include a rare but deadly central nervous system condition called subacute sclerosing panencephalitis (SSPE), which can develop seven to 10 years after the infection. Some people can also experience brain swelling (encephalitis) during a measles infection, which can cause convulsions, permanent hearing loss and intellectual disability.

The measles vaccine provides lifelong immunity against the virus without these dangerous complications associated with measles infection.

Is there a way to check if you're immune to measles?

If you've received two doses of the MMR vaccine, you're considered immune for life in most cases. Additionally, if you've had a confirmed case of measles in the past, you're likely immune for life.

If someone is still unsure whether they've either been vaccinated or had the disease, there's a test that checks for measles-specific antibodies in your blood. The type of measles antibody it screens for belongs to a broad class called immunoglobulin G (IgG), which protects the body from infection by binding to pathogens.

If enough measles-specific IgG antibodies are present, compared to a standard threshold, a person has immunity.

"You can check an antibody level with the help of a health care provider," Chin-Hong said. The test will give you a number back that's called a "titer"; this is a measure of your antibody levels. However, the test will not pick up memory cells, Chin-Hong said, so it could miss that additional layer of evidence that you're immune to measles.

Should anyone get a booster for measles, if they've been fully vaccinated in the past?

If a person is unsure about how protected they are against measles, they can get a booster shot of the vaccine to bolster their immunity.

"It is perfectly safe to have another dose," Head said. "The recommendation to get an extra dose will typically come from public health teams, and it's always best to see the advice of a healthcare worker if anyone is unsure."

If someone has had two doses of the measles vaccine in their lifetime, there is likely no need for a booster shot. The exception is a group of people who received the measles vaccine in the 1960s, who would need an additional MMR vaccine to ensure their protection.

That's because the measles vaccine used in the U.S. between 1963 and 1967 was less effective than modern measles vaccines. Unlike the current shots, which contain live-but-weakened viruses, the old shot contained an inactivated virus, so it was completely "killed." In response to the old vaccine, the immune system did not develop long-lasting memory cells, unlike the vaccines used today. Many people who received the old vaccine lost their immunity within a few years, leaving them vulnerable to measles later in life.

In addition to people who received only this old vaccine, anyone who has gotten only one dose of the MMR or MMRV vaccine is recommended to get the recommended second dose.

The CDC started recommending two doses of the MMR shot in 1989, Chin-Hong noted, so those vaccinated before that time may need a second dose.

Individuals traveling to areas experiencing measles outbreaks, including within the U.S., can also consider getting another dose even if they're fully vaccinated, Chin-Hong said.

Additionally, health care workers, immunocompromised people and those who live with them, and college students should check their vaccination records to confirm they've received two MMR doses, and perhaps get an immunity test if they are unsure, Chin-Hong suggested. Individuals can speak with their health care provider about getting a booster shot, if they're interested in receiving one.

Can vaccinated people still catch measles sometimes?

Fully vaccinated people may still catch measles, but only in rare circumstances.

About 3% of fully vaccinated people may still develop measles symptoms after an exposure to the virus, according to the CDC. However, people who have been fully vaccinated tend to experience milder symptoms than unvaccinated or partially vaccinated individuals, and they are also less likely to spread the virus to others.

These "breakthrough" cases become more likely in the context of large outbreaks or high-exposure settings — when an individual has prolonged, close contact with an infected person. About 5% of total measles cases in the U.S. are breakthrough cases, according to the CDC.

Additionally, people with weakened immune systems — due to medical conditions or medications — might not maintain full protection, even if they're fully vaccinated. So they can be more prone to breakthrough cases.

In the current Texas outbreak, there were a "handful" of people who contracted measles after having only one shot," Chin-Hong noted. In a study that looked at measles cases in California between 2000 and 2015, most of those infected were unvaccinated, but 11% of the total cases were among people who had gotten two doses of measles vaccine. The fully vaccinated group did have milder disease, however.

How high do vaccination rates in a community need to be to reach "herd immunity"?

Herd immunity refers to when a high percentage of a population is immune to a disease, making it difficult for the infection to spread. Reaching this collective level of immunity protects even those who can't be vaccinated, such as newborn babies, pregnant people or immunocompromised people.

Measles is one of the most contagious diseases in the world, and because of that, at least 95% of a given population needs to be immune to prevent major outbreaks.

"Measles is so infectious, with very high levels of transmission. Typically, we refer to 95% coverage as the magic number of that 'herd immunity threshold,'" Head told Live Science. "At this level of coverage, there may still be individual case[s], but the majority of the population is protected, and the outbreaks will remain small and very localised."

MMR coverage among U.S. kindergartners has declined in recent years, according to the CDC. In the 2019-2020 school year, 95.2% of kindergartners had received two doses of the MMR vaccine, but in the 2023-2024 school year, that coverage dropped to 92.7%. That marked the "fourth consecutive year" that national coverage had been below the targeted 95%.

Several states — including Arizona, Utah, Colorado, Oklahoma, Georgia and Florida — had less than 90% coverage during the 2023-2024 school year.