Two doses of the MMR vaccine protect most people against measles for life. Published estimates put one dose at about 95% effective against measles, and two doses at about 96% to 97%.
If you are wondering how well your own vaccination holds up, here is what the numbers and the health agencies say.
What is measles?
The CDC’s MMR vaccine information statement describes measles as causing fever, cough, runny nose, and red, watery eyes, commonly followed by a rash that covers the whole body. It can lead to seizures, ear infections, diarrhea, and pneumonia. Rarely, it can cause brain damage or death.
The FDA’s measles vaccination update adds that measles spreads through the air. It says that if one person has it, 90% of the close contacts who are not vaccinated or otherwise immune will also become infected.
How effective is one dose compared with two?
As of October 2026, the CDC and FDA pages describe MMR protection in general terms and do not give a percentage. The figures below come from published reviews and a health service page.
| Source | What it reports for measles | Doses |
|---|---|---|
| Cochrane review, as reported by Respiratory Therapy (April 24, 2020) | One dose 95% effective; two doses around 96% | 1 and 2 |
| UCLA Health “Ask the Doctors” column (April 2, 2025) | Two-dose series 97% effective | 2 |
| HSE, Ireland’s health service (page last reviewed 07/03/2024) | After 2 doses, around 99% of people are protected against measles and rubella | 2 |
The Cochrane review looked at 51 effectiveness studies covering 10 million children. Its figures show the number of cases falling from 7% in unvaccinated children to under 0.5% in children who got one dose.
The numbers differ because each comes from different studies and measures. Each one describes study results across groups of children, not a test of your own immunity. The Cochrane review rated its certainty for MMR effectiveness as moderate, because the studies were not randomized. It still said “the effects are very large.”
How long until the MMR vaccine starts working?
The HSE’s MMR vaccine page says: “Protection starts within 2 weeks of having the vaccine.” Ireland’s schedule is its own, so the routine ages in the next section are the CDC’s US schedule.
How long does MMR protection last?
The CDC says most people who are vaccinated with MMR “will be protected for life.” The FDA says most people who get the recommended two doses “will have lifelong protection.” The UCLA column says anyone who has had two doses “has strong lifetime protection.” These are the agencies’ and columnist’s statements. No single study in this area measures one person’s immunity across decades.
Can I still get measles if I was vaccinated 30 years ago?
Most people who had two doses are protected for life. The FDA says they will “never get sick with measles, even if they’re exposed to the virus.” A vaccine that is 97% effective still leaves a small share of vaccinated people unprotected, so a shot decades ago is not a guarantee in every case. This is our reading of the numbers.
If you had only one dose, or cannot find your records, the UCLA column describes an MMR titer test. It is a blood test that screens for antibodies to measles, mumps and rubella. Results are typically available within one to three days, and a positive result indicates immunity. Ask your clinician whether a titer test makes sense for you.
Do adults need an MMR booster?
The CDC’s statement says older children, adolescents and adults “also need 1 or 2 doses of MMR vaccine if they are not already immune to measles, mumps, and rubella.” It adds that a third dose “might be recommended for certain people in mumps outbreak situations.” The UCLA column says unvaccinated teens and adults are advised to get two doses, separated by at least one month.
What age is worst for measles, and when is MMR given?
The FDA says children younger than 5 years and adults older than 20 years are more likely to suffer measles complications. UCLA Health adds that infants too young for the vaccine are at particular risk.
The CDC’s routine schedule gives the first dose at 12 through 15 months and the second at 4 through 6 years. The FDA says its approved MMR vaccines, M-M-R II and Priorix, are for people ages 12 months and older. For infants 6 to 11 months old who will travel outside the United States, the CDC says a dose before travel is given, and those children still get two more doses at the recommended ages.
So 9 months falls only in that travel window, not the routine schedule. The HSE says babies under 6 months cannot get MMR, and babies 6 to 11 months can get it before travelling. It adds that a baby given MMR before 12 months does not change the regular schedule. Ask your child’s clinician about any dose before travel.
Is the MMR vaccine safe?
The FDA says “FDA-approved vaccines to prevent measles are safe and effective,” and that their known side effects are “generally mild and short-lived, such as rash and fever.” The HSE calls the MMR vaccine “very safe.”
The FDA says the autism concern “has been refuted,” citing studies by the World Health Organization, the Institute of Medicine (now the National Academy of Medicine) and the American Academy of Pediatrics. In the 2020 Cochrane review, two studies with 1,194,764 children found diagnosed autism was similar in vaccinated and unvaccinated children. Two further studies, with 1,071,088 children, found no evidence of an association with conditions including encephalitis, inflammatory bowel disease, type 1 diabetes, asthma and multiple sclerosis. The review’s lead author said the risks posed by these diseases “far outweigh those of the vaccines.”
What are the side effects, including neurological ones?
| Effect | What the source says | Source |
|---|---|---|
| Sore arm, redness where the shot was given, fever, mild rash | “can happen after MMR vaccination” | CDC |
| Swollen glands in the cheeks or neck; temporary joint pain and stiffness | Sometimes occur, mostly in teenage or adult women for joints | CDC |
| Mild measles-like rash, high temperature, loss of appetite | Around 6 to 10 days after the injection, for about 2 or 3 days | HSE |
| Seizures, often linked to fever | Rare, about 1 in every 1,000 doses, 6 to 11 days after | HSE |
| Temporary low platelet count, bruise-like spots | About 1 in 24,000 doses; “usually gets better without treatment” | CDC, HSE |
| Severe allergic reaction | A “very remote chance”; call 9-1-1 for hives, facial or throat swelling, or trouble breathing | CDC |
The main neurological side effect is seizures. The HSE says they are rare, and that “You are more likely to have a seizure because of a measles infection than after the MMR vaccine.” The Cochrane review found no association between MMR and encephalitis.
Who should not get the MMR vaccine, or should wait?
| Situation | What the source says | Source |
|---|---|---|
| Pregnant, or might be | “pregnant women should not get MMR vaccine”; avoid becoming pregnant for 1 month after | CDC, HSE |
| Serious immune system problem, such as chemotherapy | “should not get MMR vaccine” | CDC, HSE |
| Severe allergic reaction to a previous MMR dose, neomycin or gelatine | Should not get the vaccine | HSE |
| Moderately or severely ill, or high fever | Wait until you recover; a minor cold may not stop vaccination | CDC, HSE |
| Varicella (chickenpox) vaccine in the past 4 weeks | Delay the MMR vaccine | HSE |
Your clinician makes the call in these cases, so bring your history to that visit.
What is not known yet?
Three things are still open. First, the percentages are study results, not a check on your own immunity. Second, the sources give different figures: about 95% to 97% for two doses in the review and the UCLA column, and around 99% in the Irish guidance. Third, the Cochrane review rated its certainty as moderate, because its studies were not randomized.
What should you check before you get a dose?
Start with your vaccination records. The CDC suggests asking your health care provider, calling your local or state health department, or visiting the FDA website for vaccine package inserts. If your records are missing, ask about a titer test. You can report any reaction to VAERS at 1-800-822-7967.
The CDC’s statement cites 42 U.S.C. § 300aa-26 as its legal basis. It also describes the National Vaccine Injury Compensation Program, a federal program created to compensate people who may have been injured by certain vaccines, and says claims have a time limit for filing that may be as short as two years. This is information, not legal advice. For other health topics, see our guides on intermittent fasting and erectile dysfunction 101.
By SterlingMedicalCenter.org Editorial Team
This article is for general information purposes only and does not constitute medical advice. Consult your doctor or qualified healthcare provider before making changes to your health routine.