The U.S. is in the midst of one of its largest measles outbreaks since the disease was declared eliminated from the country in 2000. Its elimination two decades ago meant that measles no longer circulates on its own in the U.S. But outbreaks can occur there when a traveler—such as an infected tourist returning from a country where measles has been spreading—introduces the virus.
As of May 2 the Centers for Disease Control and Prevention reported that so far this year there have been 131 cases in 21 states, stretching from California and Washington State to New York State and New Jersey. Measles is possibly the most contagious disease that can infect humans: it’s airborne and remains in the air several hours after an infected person leaves an area, and it stays on surfaces for up to two hours. Just one person with measles can infect nine out of 10 nonimmune individuals who encounter that person’s aerosols. And it’s estimated that in an unvaccinated population, one person with measles would infect about 12 to 18 people on average. Epidemiologists estimate that at least 95 percent of a population needs to be vaccinated to prevent measles from spreading. But the total measles immunization coverage of U.S. kindergarteners has dropped below that, to 93 percent.
Many experts worry that the U.S. is at risk of losing its measles elimination status, which occurs if an outbreak lasts longer than a year.
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“That threat is real,” says Patsy Stinchfield, a registered nurse and president of the National Foundation for Infectious Diseases. Even in states that do have at least 95 percent vaccination coverage, smaller communities within the state may be undervaccinated, Stinchfield says.
Here’s how to know if you’re protected.
How do you know if you have immunity to measles?
People born before 1957 are presumed to be immune to measles because they had it in childhood. Those born in 1957 or later, however, are likely protected only if they have been vaccinated. Initially, U.S. children received just one dose of the measles, mumps and rubella (MMR) vaccine, but that changed after a big measles outbreak that occurred from 1989 to 1991 and resulted in a reported 55,622 cases in the country. The outbreak killed 123 people and led to a congressional hearing.
Public health officials determined that a single MMR dose was not enough to prevent outbreaks. So in late 1989 both the American Academy of Pediatrics and the CDC began recommending that all children—including older children who received their MMR shot as toddlers—receive two doses of the vaccine.
“Most people born after 1975 would have received the second dose as part of their routine vaccinations,” says CDC spokesperson Jasmine Reed. “Most adults born between 1957 and about 1975 likely have had only one dose.”
How many doses of the measles vaccine are needed for immunity?
Vaccination against measles is highly effective in preventing infection. One dose of the MMR vaccine is 93 percent effective at preventing measles; it protects the vast majority of people. A second dose increases the effectiveness to 97 percent, capturing more of those who didn’t gain full immunity from the first dose.
How can you find your vaccination records?
You can start by asking your parents or former pediatrician, if that’s an option. If not, every state, as well as Washington, D.C., and most U.S. territories, maintains an immunization registry called an immunization information system (IIS). You can contact your state’s IIS to see if it has your immunization records. The CDC and the nonprofit group Immunize.org have additional tips on locating your vaccination records.
What if you’ve never been vaccinated against measles?
If you’re an adult born in 1957 or later who has never received an MMR dose, you should get two doses, four weeks apart, Stinchfield says. Or if you’re unsure if you were vaccinated, especially if you work in health care or childcare or in a long-term care facility, it’s worth making sure you’re protected by getting the vaccine.
If you’ve only had one dose of the measles vaccine, do you need another?
It depends on your risk factors.
When the CDC recommended that all children receive a second dose, it did not advise catch-up doses for most vaccinated adults. But the agency does recommend that the following adults get a second dose because they may be at higher risk during an outbreak: students in colleges, universities, trade schools and other post-high school institutions; health care workers born during or after 1957; and anyone traveling internationally.
Others who may want to consider a second dose during a measles outbreak are those who work around or have close contact with immunocompromised individuals who are unable to be vaccinated and those living in a group setting, such as barracks, says Erica Johnson, an infectious disease physician at Johns Hopkins Medicine. Stinchfield adds that those who work in a long-term care or childcare facility may also want to consider a second dose.
If you’re still concerned because there’s an outbreak in your area, Johnson recommends talking to your health care provider about your personal risk.
How can you find out if you have immunity to measles?
When you get a vaccine, your body produces proteins called antibodies that recognize the pathogen. “You’re basically teaching the immune system to remember a particular pathogen so that when the immune system sees that pathogen again, it remembers it and responds much more quickly,” Moss says.
You can get some sense of your immunity level with a blood test to check your antibody titers, or the concentration of antibodies in your blood. But that may not always be helpful. Often the results will be “indeterminate,” Stinchfield says. This could mean that you have insufficient immunity or that you do have protection but that the test did not show it. The immune system also has another vaccine response called cell-mediated immunity. It stores a memory of the pathogen in three other cell types: memory B cells, the factories where antibodies are made; memory T cells, which facilitate the immune response against the pathogen; and long-lived plasma cells in the bone marrow, which continually produce antibodies throughout a lifetime. Even if you have low antibody levels, these other immune cells can still protect you.
Drawing blood to check your antibody titers therefore may not give you the information you want to know. “It’s probably just as easy or easier to get an additional dose,” Moss says. Johnson and Stinchfield agree. “A third shot is not going to hurt anyone,” Johnson says.
Even if you’re vaccinated, can breakthrough infections happen?
In most cases, the vaccine confers lifelong protection. Secondary vaccine failure — in which someone develops the disease despite being immunized because of waning immunity—can occur, but it’s uncommon. Even in such cases of secondary vaccine failure, the illness people experience is very mild, Moss says. “They don’t get very sick because they have some protection.”
Who cannot get vaccinated against measles?
The MMR vaccine is a live vaccine. It contains an extremely weakened version of each virus that’s not potent enough to make most people sick but can still activate the immune system. Live vaccines are not recommended, however, for pregnant people or anyone who has an active immunodeficiency diagnosis (such as HIV) or who is on immunosuppressive therapies, such as chemotherapy, steroids or medications for certain rheumatologic conditions. The CDC has a longer list of other groups who should either delay the vaccine or skip it. If you’re uncertain whether a medication you’re taking makes it risky to get the vaccine, talk to the doctor who prescribed it.
There are no measles outbreaks in my area, but I plan to travel overseas. Do I need a measles vaccine dose?
The CDC recommends that any person planning to travel abroad should make sure before their trip that they have gotten two doses during their lifetime. “Measles transmission is different in different places, and if you’re uncertain about whether or not you’ve been adequately vaccinated for measles, that’s a good time to get another dose,” Johnson says.
Is there anything else I should do to protect myself?
If you’re a parent, make sure your kids are vaccinated, Moss says.
Ensuring both you and your children are immunized against measles is also important for protecting your community through herd immunity, in which a highly immune population makes it harder for a disease to spread and reach people who are not protected. “That’s how we protect immunocompromised people who can’t get vaccinated” and those who are too young to be vaccinated, Moss says.
It also protects those who are vaccinated but still at risk. If full vaccination is 97 percent effective, that means 3 percent of people “will have not made protective antibodies” even after two doses, Stinchfield says. “So we always have vulnerable people in our community.”