Can MMR Shedding Start a Measles Outbreak?

Have you ever noticed that any time that there is a new outbreak of measles, folks tend to ask the same basic questions.

How did the outbreak start?

Why weren’t they vaccinated?

Was it from shedding?

Wait, what?

Shedding??? Really?

Can MMR Shedding Start a Measles Outbreak?

It seems that the idea that the MMR vaccine sheds and can lead to measles outbreaks is one of those anti-vaccine myths that just won’t go away.

It comes back with each new measles outbreak.

The myth that MMR shedding leads to measles outbreaks is commonly spread on Facebook, like this discussion on a recent outbreak at a Kansas City daycare center.
The myth that MMR shedding leads to measles outbreaks is commonly spread on Facebook, like this discussion on a recent outbreak at a Kansas City daycare center.

While the oral polio vaccine is indeed associated with shedding and vaccine associated disease, that doesn’t happen with MMR. Experts don’t even recommend any restrictions for use of the MMR vaccine for household contacts of people who are immunosuppressed. And yes, your kids can even visit a cancer patient if they just had their MMR, as long as they don’t have RSV, the flu, or some other contagious disease.

What about the fact that a study once found measles virus RNA in the urine of of kids who had recently been vaccinated? Doesn’t that mean that they were shedding the vaccine virus?

No. It doesn’t.

To be considered shedding, those measles virus RNA particles in their urine would have to be contagious. Now, measles is spread by respiratory secretions. So how are measles virus RNA particles in urine going to become airborne and get someone else sick?

They don’t.

Another Facebook post about MMR and shedding.
Most measles outbreaks in daycare centers have been started by an unvaccinated child or worker who traveled out of the country.

But what about that case in Canada? Anti-vaccine folks like to bring this up when they talk about shedding. In 2013, there was a case of vaccine-associated measles. That proves that the vaccine sheds, right?

Absolutely not!

“Of note, only one case report of transmission from vaccine-associated measles has been identified.”

Murti et al. on Case of vaccine-associated measles five weeks post-immunisation, British Columbia, Canada, October 2013

That child got measles about 5 weeks after she was vaccinated in the middle of a measles outbreak. Because she had no links to the other cases and she tested positive for vaccine-strain measles, it is thought that she had MMR vaccine-associated measles, which is extremely rare.

Shedding Light on Measles Outbreaks in Daycare

MMR shedding is not causing outbreaks of measles – or rubella and mumps, for that matter.

If shedding from the MMR, by any method, got kids sick, then why aren’t there even more cases of measles?

Daycare centers everywhere have a mix of infants and toddlers, including some who are intentionally unvaccinated, those who are too young to get their first MMR, and those who are just getting vaccinated.

When a case of measles does pop up though, it isn’t because of shedding, it is typically because someone who wasn’t vaccinated traveled out of the country, got measles, and brought it back home, exposing others.

What to Know About MMR and Shedding

Measles outbreaks are not caused by shedding from the MMR vaccine.

More on the MMR and Shedding

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