Can vaccines spread measles?

The old myth about measles outbreaks is once again endemic, with anti-vaxxers claiming that the measles vaccine itself transmits the virus through “vaccine shedding,” spreading infection to vulnerable individuals.

This tweet falsely says that the measles vaccine spreads measles through something called shedding. It’s true that the MMR vaccine (which protects against measles, mumps, and rubella) has live viruses in it—but these viruses are weakened, so they can’t make people with healthy immune systems sick.

The MMR vaccine has live viruses in it, but they are special versions that are different from the real ones that cause disease. Scientists can tell them apart by looking at their genotype, kind of like how you can recognize someone by their fingerprint.

When someone has a real infection, their body can release viruses through a process called shedding. If someone else catches the virus and gets sick, that’s called transmission. Since the vaccine uses a virus, a tiny bit of shedding might happen after you get the shot. But that doesn’t mean it spreads disease.

Here’s what’s important to know: shedding is not the same as spreading illness. The virus in the vaccine is so weak (attenuated) that it can’t cause measles. Even though the vaccine inserts mention shedding, it’s just a possible risk, not something that’s ever actually happened. In fact, there’s never been a real case of anyone getting measles from someone who got the vaccine.

In fact, the Infectious Disease Society of America (IDSA) recommends people living with immunocompromised still receive the MMR vaccine.