As of April 2025, measles outbreaks had been confirmed in 25 jurisdictions of the U.S. — with most cases linked to low vaccination rates. The Centers for Disease Control and Prevention (CDC) is urging healthcare providers to talk with travelers about vaccination. Health experts warn that if vaccination rates keep going down, measles could become common again in the U.S., leading to millions of cases over the next 25 years.

While the overall risk is low, measles remains highly contagious. People living with graft-versus-host disease (GVHD) may be more vulnerable if their immune system is weakened — especially if they are taking immunosuppressive medications like prednisone, tacrolimus, or ruxolitinib. After a person gets donor cells, those cells need time to grow, get stronger, and learn how to fight infections like measles. It’s also still uncertain how much the donor’s immune defense against some diseases is shared with the person who gets the transplant.

Although the measles, mumps, rubella (MMR) vaccine is highly effective, immunity can wane over time, which means even previously vaccinated adults may still be at risk.

What Is Measles?

Measles is a very contagious virus. It spreads through the air when someone who is sick coughs, sneezes, or talks — and it can stay in the air for up to two hours after they leave. If you’re exposed to the measles virus and not protected, there’s about a 90 percent chance you’ll get sick.

Symptoms usually begin 10 to 14 days after exposure. Early signs include fever, cough, runny nose, and red, watery eyes. Small white spots may also appear inside the mouth. A few days later, a rash spreads from the face down to the rest of the body. This rash appears red on light skin tones and purple or brown on darker skin tones.

Most people recover in a few weeks, but measles can cause serious complications such as pneumonia, severe diarrhea, and, in rare cases, encephalitis (brain swelling). These risks are higher among babies, older adults, pregnant people, and those with weakened immune systems, including many people living with GVHD, particularly those who just got a transplant or are on immunosuppressive medications.

Who’s at Risk for Getting Measles?

People at risk include:

• Those who are unvaccinated or have only had one dose of the MMR vaccine
• Adults who don’t know their vaccination history or were vaccinated before 1968, when less-effective vaccines were used
• Travelers, especially those visiting countries with measles outbreaks or regions in the U.S. where vaccination rates are low
• People living in communities where measles is spreading
• Pregnant individuals, who face a higher risk of complications like preterm birth or fetal loss
• Babies under 1 year old, who are too young to be fully vaccinated
• People in close-contact settings, like schools or shelters, where the virus can spread quickly
• People living with GVHD, especially those taking immunosuppressive medications

Also, a recent study found that measles could come back and spread more easily across the U.S. if fewer people get vaccinated — especially in areas where many people are not protected.

How Effective Are MMR Vaccines in Preventing Measles?

The measles vaccine is highly effective. After two doses, about 99 percent of people develop lasting immunity. The first dose protects most people, and the second dose helps those who didn’t respond the first time.

Most people remain protected for life. However, rare cases of measles can still occur in vaccinated individuals — especially if their immune system was weak when they got the vaccine or if their immunity has waned over time. This can happen in people treated with medications that suppress the immune system, such as those used to manage GVHD.

That’s why people living with GVHD or other conditions that affect the immune system should talk to their doctor about whether they need immunity testing. In some cases, revaccination may be considered — but only when it’s safe, such as after immune recovery and under medical guidance.

Are MMR Vaccines Safe for People With GVHD?

As with any vaccine, side effects can happen. Common side effects include:

• Mild fever
• Rash
• Temporary swelling of lymph nodes

Some adults may also experience joint pain or minor allergic reactions. Serious side effects are rare. Also, multiple studies have found no link between the MMR vaccine and autism.

However, because the MMR vaccine is a live virus vaccine, it is usually not recommended for people with GVHD who are currently receiving immunosuppressive medications or within the first two years after transplant. Even after two years, revaccination is only considered if:

• You’re no longer taking immunosuppressive medications.
• You don’t have active GVHD.
• Your immune system has recovered enough to respond safely.

Your doctor may recommend a blood test to check whether you still have immunity to measles, mumps, and rubella. If not, and your immune system is ready, they may advise vaccination. If it’s not safe for you to receive the MMR vaccine, they can suggest other ways to protect yourself.

In very rare cases — such as during a community measles outbreak — doctors may consider giving the MMR vaccine earlier, but only under close supervision and only if your immune risk is low.

How Can You Tell if You Need an MMR Vaccination or Booster?

Everyone’s health situation is unique — and that’s especially true after a stem cell transplant. If you’re living with GVHD, talk with your doctor or transplant team to understand whether and when the MMR vaccine is safe for you. Depending on your history and current health, they may recommend:

• Undergoing a blood test to check your immunity
• Postponing vaccination and taking extra precautions if your immune system is still weakened — especially if you’re taking immunosuppressive medications or have active GVHD
• Considering the MMR vaccine only after your immune system has recovered, usually at least 24 months posttransplant and 12 months off immunosuppressants

How Else Can You Protect Yourself and Others From Measles?

The CDC recommends additional precautions:

• Practice good hygiene — Wash hands frequently, avoid touching your face, and clean shared surfaces.
• Be mindful of travel — Before traveling internationally, confirm your vaccine status.
• Limit exposure during outbreaks — Avoid crowded indoor spaces in communities with low vaccination rates.

What Should You Do if You’ve Been Exposed to Measles?

If you’ve been exposed to measles and aren’t vaccinated:

• Getting an MMR vaccine within 72 hours can prevent or reduce illness severity — if you’re eligible. Most people living with GVHD are not eligible for the MMR vaccine right after exposure due to immune system suppression.
• If you’re not eligible for vaccination — such as if you’re immunocompromised or taking immunosuppressive medications — immune globulin (IG) can offer protection. It must be given within six days of exposure to be effective.

Check with your doctor to see if you are eligible for any of the above, if you’ve been exposed to measles and aren’t immune.

Stay Protected, Stay Connected

On myGVHDteam, the site for people with graft-versus-host disease and their loved ones, people come together to gain a new understanding of GVHD and share their stories with others who understand their experience.

Have you talked to your doctor about your measles immunity or vaccination status? How do you stay informed and prepared? Share your experiences in the comments below.