DEAR MAYO CLINIC: How long does it take to become immune from measles once one has gotten the measles vaccine? And given the current outbreak, would it be beneficial for me to get a booster vaccine even though I was vaccinated against measles as a child?

ANSWER: The measles vaccine takes effect within weeks of receiving it, and once a person has developed immunity to measles, it lasts a lifetime. Measles is an extremely contagious illness that can lead to serious and sometime fatal complications. The measles vaccine is safe and highly effective at preventing this illness.

All children 18 and younger should receive two doses of the measles vaccine. It’s recommended that the first dose be given when a child is 12–15 months old. Children younger than 1 year don’t develop a good response to the vaccine because their mother’s immunity not only protects them from getting the disease, but also from responding to the vaccine. The second dose is recommended before a child starts school, around ages 4–6.

Immunity to measles develops in 10–14 days after the first dose. Studies have found that 93% of recipients receive full immunity with the first dose. A second dose ensures that more than 97% are immune. The measles vaccine does not wear off over time. So even during an outbreak, you don’t need another dose of the measles vaccine if you already have evidence of immunity to the illness.

Evidence of immunity includes written documentation of adequate vaccination. For low-risk adults who are 19 or older, adequate vaccination is one or more doses of the measles vaccine on or after the first birthday. Adults who are at higher risk for measles, including college students, health care personnel and international travelers, need two doses of the vaccine at least 28 days apart to be vaccinated adequately.

In addition, almost everyone born before 1957 has immunity to measles and does not require additional vaccination. That’s because the measles vaccine was first produced in the early 1960s. Before that, measles was common, so it’s likely people who were children before 1957 had measles and as a result have immunity from disease.

One of the measles vaccines given in the 1960s does not count. Current and past valid measles-containing vaccines are live, attenuated vaccines. The “killed” or inactivated version used from 1963 through 1967 does not count. If you received unspecified measles vaccine doses during those years, it’s not possible to know which version was used and you should discuss this with your health care provider.

As an adult 19 or older, if you’re unsure about your vaccination status or your immunity to measles, talk to your health care provider. Records showing the dates of your measles vaccination serve as proof of immunity. For those who lack records, getting a dose of the vaccine is safe, even if you were previously vaccinated. Alternatively, a blood test can confirm if you already have immunity as the result of vaccination or illness.

Having immunity to measles is crucial because the illness can lead to serious medical complications, particularly in young children. Complications can include bacterial ear infections, bronchitis, laryngitis, croup and pneumonia.

Measles is highly contagious. It’s caused by a virus that replicates in the nose and throat. When someone infected with measles coughs, sneezes or talks, infected droplets spray into the air, where other people can inhale them. The infected droplets stay in the air or they may land on a surface, where they remain infectious for several hours. You can contract the virus simply by breathing the air in the room or touching your mouth, nose or eyes after touching an infected surface. About 90 percent of people who don’t have immunity to measles and are exposed to the virus will become infected.

Not all people can receive the measles vaccine. These include infants younger than 12 months, pregnant women, and those whose immune systems don’t work properly. Those people depend on everyone else who can get the vaccine to do so. That way, the vaccine not only protects the people who receive it, it also safeguards the health of vulnerable individuals in the community. — Dr. Robert Jacobson, Pediatric and Adolescent Medicine, Mayo Clinic, Rochester, Minnesota

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• More measles cases: CDC says largest outbreak since measles eliminated in 2000 published 4/25/19
• Infectious Diseases A-Z: The 3 Cs of measles published 4/22/19
• Infectious Diseases A-Z: Why the measles virus is so contagious published 4/9/19
• Infectious Diseases A-Z: Battling a myth about measles vaccine published 4/1/19
• Infectious Diseases A-Z: Measles outbreaks continue in the US published 2/25/19
• Consumer Health: Measles — contagious and preventable published 2/1/19

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