You're sitting in a waiting room, or maybe scrolling through a news alert about a sudden outbreak in a nearby school district, and the question hits you. Can you get the measles if you've been vaccinated? It’s a fair thing to ask. We’re taught from a young age that vaccines are like a suit of armor. You put the armor on, the virus bounces off, and everyone goes home happy. But medicine is rarely that black and white.
Honestly, the short answer is yes. It's rare, but it happens.
If you’ve had both doses of the MMR (measles, mumps, and rubella) vaccine, you are about 97% protected. That sounds like a stellar grade on a math test, right? It is. But that lingering 3% is where things get interesting—and sometimes a little bit scary for parents and immunocompromised folks. In the medical world, we call this "breakthrough infection." It doesn't mean the vaccine "failed" in the way a car engine fails. It just means the virus found a tiny, microscopic crack in the door.
Why the "97% Effective" stat matters for measles
The CDC is pretty clear about the numbers. One dose of the MMR vaccine is about 93% effective. Two doses bump that up to 97%. Most of us in the U.S. got that second dose before starting kindergarten.
But let’s talk about that 3%.
If you put 100 vaccinated people in a room with someone who has an active, coughing, sneezing case of measles, about three of those vaccinated people might still catch it. Why? Biology is messy. Some people’s immune systems just don't "take" the vaccine as well as others. Maybe they were sick when they got the shot, or maybe their genetics just don't produce a robust enough antibody response. It’s not necessarily anyone's fault. It’s just how human bodies work.
Measles is incredibly contagious. Like, terrifyingly so.
If you walk into a room where someone with measles was standing two hours ago, you can still catch it. It hangs in the air like an invisible fog. This is why that 97% protection is so vital. Without it, the virus spreads like wildfire through a dry forest. With it, the fire usually hits a brick wall.
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What a breakthrough case actually looks like
Here is the good news: if you are one of the unlucky few who gets a breakthrough case, it’s usually a "measles-lite" version.
Most people think of measles and imagine the classic symptoms: a high fever that can spike to 105 degrees, the "three Cs" (cough, coryza/runny nose, and conjunctivitis/pink eye), and that head-to-toe red rash. For the vaccinated, it often doesn't look like that. You might get a mild fever. You might get a few spots that look more like a heat rash or bug bites than the full-blown measles "paint job."
Crucially, vaccinated people who get sick are significantly less likely to suffer from the really nasty complications like pneumonia or encephalitis (brain swelling).
According to Dr. David Sugerman and other researchers at the CDC who have tracked outbreaks in highly vaccinated populations, breakthrough cases also tend to be less infectious. Your body is already fighting the virus, so you aren't shedding as much of it into the air when you cough. You're less of a "superspreader" than someone who never had the shot.
The mystery of waning immunity
Does the vaccine wear off? This is a hot topic.
For the vast majority of people, the MMR vaccine provides lifelong protection. You don't need a booster every ten years like you do with tetanus. However, doctors have seen rare cases where immunity seems to dip as people hit their 40s or 50s. This isn't common enough for the CDC to recommend universal boosters, but if you’re traveling to a country where measles is rampant, your doctor might suggest a blood test called a "titer" to check your antibody levels.
Real-world examples of the 3%
Think back to the 2014-2015 Disneyland outbreak. Or more recently, the outbreaks in Ohio and Florida.
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In almost every major outbreak, there are a handful of cases among the vaccinated. Anti-vaccine groups often point to this and say, "See! It doesn't work!" But that's a massive misunderstanding of how statistics work. If 95% of a town is vaccinated and 5% isn't, and 10 people get sick—5 vaccinated and 5 unvaccinated—it might look like an even split.
But look closer.
The 5 unvaccinated cases came from a tiny pool of people. The 5 vaccinated cases came from a pool of thousands. The "attack rate" is massively higher for the unvaccinated. It's the difference between a minor rain leak and a dam bursting.
Secondary vaccine failure vs. primary failure
Scientists actually categorize these breakthrough cases into two buckets:
- Primary Vaccine Failure: This is when your body never developed the antibodies in the first place after the shot. This is why we give two doses; the second dose is basically a "safety net" for the people who didn't respond to the first one.
- Secondary Vaccine Failure: This is when you did have immunity, but it faded over decades. This is much rarer with measles than it is with something like whooping cough (pertussis).
How to know if you're actually protected
If you were born before 1957, you are generally considered immune. Why? Because measles was so common back then that you almost certainly caught it as a kid. Natural infection gives you "God-tier" immunity—it’s basically permanent.
For the rest of us, it depends on when you got your shots.
There was a version of the vaccine used briefly in the 1960s (the "killed" virus vaccine) that wasn't very effective. If you got that one, you aren't fully protected. Most people who received the modern "live" attenuated vaccine are good to go.
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If you’re worried, don’t just guess. Ask your doctor for a measles IgG titer test. It’s a simple blood draw. They check for the presence of antibodies. If it comes back positive, you can stop worrying. If it’s negative or "equivocal," you just get another dose of the MMR. Simple.
The "Herd Immunity" factor
You’ve heard the term. It’s become a bit of a buzzword lately.
But for measles, herd immunity is everything. Because the vaccine is 97% effective and not 100%, we rely on our neighbors to keep us safe. Measles needs a "chain" of people to jump from person to person. If 95% of the population is immune, the virus keeps hitting dead ends. It can’t find that 3% of vaccinated people or the people who can't get vaccinated (like infants or chemo patients).
When vaccination rates drop to 90% or 88%, those chains start forming. That’s when the "can you get the measles if you've been vaccinated" question becomes a reality instead of a theoretical risk.
Actionable steps to take right now
Stop wondering and start verifying. If you aren't sure of your status, here is exactly what you should do:
- Dig up your "yellow card" or digital health records. Look for two doses of MMR. If you only see one, call your clinic.
- Check your travel plans. If you are heading to Europe, Southeast Asia, or Africa, measles is much more common there than in the U.S. Get that titer test before you buy your plane tickets.
- Watch for the "Prodrome." If you know you've been exposed, don't wait for the rash. The "prodrome" phase (fever, cough, runny nose) is when you are most contagious. Stay home.
- Trust the science, but respect the virus. Acknowledge that while you're likely safe, the 3% risk exists. If you develop a weird fever and a cough after being around a known case, tell your doctor before you walk into the clinic so they can isolate you.
Measles isn't just a "childhood rash." It’s a serious respiratory virus that can have long-term effects on the immune system, sometimes even "wiping" your immune memory of other diseases (a phenomenon called immune amnesia). Staying updated on your vaccination status isn't just about avoiding a few spots; it’s about keeping your entire immune system’s history intact.