Yes, but it's rare and usually mild

You can get measles even if you've been vaccinated, but the chance is small. The measles vaccine is about 97% effective after two doses. That means roughly 3 out of every 100 vaccinated people could catch measles if exposed to the virus. When vaccinated people do get measles, the illness is almost always milder than in unvaccinated people — fewer complications, shorter duration, and lower fever.

The vaccine works by training your immune system to recognize and fight the measles virus. In most people, this protection lasts a lifetime. But immunity can occasionally fade, or the vaccine may not have worked as well in a particular person. Age, overall health, and how your individual immune system responded to the vaccine all play a role in whether you stay protected.

Key Takeaways

  • The measles vaccine is about 97% effective after two doses, meaning vaccinated people have strong but not absolute protection.
  • Vaccinated people who catch measles typically have milder illness with fewer serious complications than unvaccinated people.
  • Immunity can occasionally weaken over time or fail to develop properly in some individuals, particularly those with certain immune system conditions.
  • A small number of vaccinated people may not respond well to the vaccine due to age at vaccination, immune system differences, or other medical factors.

Why vaccination doesn't may provide immunity

No vaccine is 100% effective. The measles vaccine is one of the most effective vaccines available, but it still has a small failure rate. Some people's immune systems simply don't respond as strongly to the vaccine as others do. This can happen for reasons that have nothing to do with how well the vaccine was made or how it was given.

Immunity can also fade over decades. Most people who received two doses of the measles vaccine as children maintain protection throughout their lives. However, studies show that a very small percentage of vaccinated people lose measurable immunity over time. This is one reason why healthcare workers and people planning to travel to countries with measles outbreaks sometimes get a booster dose — an extra vaccination to refresh immunity.

Certain medical conditions also affect how well the vaccine works. People with weakened immune systems — from HIV, chemotherapy, organ transplants, or certain medications — may not build strong immunity from the vaccine. Babies under 12 months old cannot receive the vaccine because their immune systems are too immature to respond to it effectively.

What happens when a vaccinated person gets measles

When someone who is vaccinated catches measles, their immune system recognizes the virus faster than an unvaccinated person's would. This means the infection is usually stopped before it spreads widely through the body. The result is a milder illness with fewer of the serious complications that measles can cause.

A vaccinated person with measles might experience fever, cough, and rash, but the fever is typically lower and the rash may be less extensive. Serious complications like pneumonia, brain inflammation (encephalitis), or hospitalization are much less common in vaccinated people. Some vaccinated people who catch measles have such mild symptoms that they don't realize they had measles at all — they may think they had a minor cold or flu.

Recovery is usually faster in vaccinated people. While unvaccinated measles can cause illness lasting one to two weeks or longer, vaccinated people often recover in a few days. They can still spread measles to others during the illness, so isolation is still necessary, but the personal risk of serious harm is much lower.

Who is most likely to get measles despite vaccination

People vaccinated as infants (before age 12 months) did not receive an effective vaccine because their immune systems were too young. If they were not revaccinated after their first birthday, they may not have strong immunity. This is why the standard measles vaccine schedule calls for the first dose at 12 to 15 months and a second dose at 4 to 6 years.

People with certain medical conditions have a higher risk of measles despite vaccination. This includes people with HIV, people undergoing cancer treatment, people who have had organ or bone marrow transplants, and people taking medications that suppress the immune system. Some people are born with rare immune system disorders that prevent them from building immunity to vaccines.

A very small number of people simply don't respond well to the vaccine for reasons doctors don't fully understand. Their immune system doesn't mount a strong response even though the vaccine was given correctly. Blood tests can sometimes detect whether someone has measles immunity, which is useful for healthcare workers or people planning to travel to areas with measles outbreaks.

How doctors tell the difference between vaccinated and unvaccinated measles

A doctor diagnoses measles the same way regardless of vaccination status — by recognizing the characteristic symptoms and sometimes confirming with a blood test or throat swab. The illness itself looks similar at first: fever, cough, runny nose, and red eyes. The rash appears a few days later, starting on the face and spreading downward.

The main difference is severity. In an unvaccinated person, the fever is usually higher (often above 104°F), the rash is more extensive, and complications are more likely. In a vaccinated person, symptoms tend to be milder overall. A doctor may suspect the person is vaccinated based on how mild the illness is, but the only way to know for certain is to ask about vaccination history.

Blood tests can show whether someone has measles immunity before they get sick. These tests measure antibodies — proteins your immune system makes to fight measles. Healthcare workers sometimes get these tests before working with patients, and people planning to travel to countries with measles outbreaks may want to know their immunity status.

What to do if you're vaccinated and exposed to measles

If you know you've been exposed to measles and you're vaccinated, your risk of getting sick is low. You don't need to do anything special beyond watching for symptoms. Measles symptoms appear 7 to 21 days after exposure, most commonly around 10 days. Watch for fever, cough, runny nose, and red eyes, followed by a rash that starts on the face.

If you develop symptoms, contact your doctor before going to the office or hospital. This prevents spreading measles to other patients and staff. Your doctor can confirm whether you have measles and advise you on isolation. Most vaccinated people recover at home without needing hospitalization.

If you're unsure whether you're vaccinated or if you received only one dose, you can get a second dose even if you're not sure about your immunity. Getting an extra dose of the measles vaccine is safe and won't cause problems. Some people choose to do this before traveling to countries where measles is still common.

Measles outbreaks and vaccinated people

During measles outbreaks, a small number of vaccinated people do get sick. This is expected and doesn't mean the vaccine failed — it means the vaccine is working exactly as designed for the vast majority of people, while a tiny percentage experience breakthrough infection. In large outbreaks, the people who get sick are still overwhelmingly unvaccinated.

When vaccinated people do get measles during an outbreak, they tend to have milder illness and recover faster. They also spread the virus less efficiently than unvaccinated people, meaning they infect fewer other people. This is one reason why high vaccination rates in a community protect everyone — even the small number of vaccinated people who might catch measles have less severe disease and spread it less widely.

Frequently Asked Questions

If I'm vaccinated and get measles, can I spread it to others?

Yes, you can spread measles to others even if you're vaccinated, though you're likely to spread it less efficiently than an unvaccinated person. You should isolate at home for at least four days after the rash appears to reduce transmission risk. Tell close contacts that you have measles so they can watch for symptoms and inform their healthcare provider if they get sick.

Should I get a booster dose of the measles vaccine?

Most people who received two doses of measles vaccine as children don't need a booster. However, healthcare workers, people planning to travel to countries with measles, and people with certain immune conditions may benefit from a booster dose. Talk to your doctor about whether a booster makes sense for your situation.

Can a blood test tell me if I'm immune to measles?

Yes. A blood test can measure measles antibodies and show whether you have immunity. This test is most useful for healthcare workers, people planning to travel, or people who are unsure about their vaccination history. If the test shows you don't have immunity, you can get vaccinated or revaccinated.

What if I was vaccinated but don't remember when or how many doses I got?

If you can't find your vaccination records, you have two options: get a blood test to check your immunity, or get revaccinated. Revaccination is safe even if you've already been vaccinated, and it's often the simpler choice. Your doctor can help you decide which approach makes sense for you.

Is measles more dangerous if you're vaccinated and still get it?

No. Measles is less dangerous if you're vaccinated and still get it. Vaccinated people have milder illness, fewer complications, and faster recovery than unvaccinated people. Serious complications like pneumonia and brain inflammation are much less common in vaccinated people with measles.