The MMR Vaccine Does Not Cause Measles Outbreaks

This post is written by Dr. Max Cohen, N.D. in participation with Dr. Joel Admundson, M.D. and Dr. Jay Rosenblum, M.D., Ph.D.

As the measles outbreaks continue to spread around the United States, particularly in the Pacific Northwest, I have been getting frequent questions about the vaccine. One topic I’ve seen over and over again is the argument that the MMR (measles, mumps, rubella) vaccine is actually causing outbreaks of measles due to “viral shedding.” This is an inaccurate statement, but let’s look at why.

First, what is viral shedding?

When we get sick with a virus, the virus will replicate in our cells and — depending on the location of the virus — can be spread to others.

For example, if you have one of the rhinoviruses (the common cold), it is likely replicating in your respiratory tract and when you cough or sneeze, small droplets containing the virus make their way into the local environment. If someone near you breathes in these particles, they can catch the cold.

Other viruses, like rotavirus (a diarrheal illness) are spread by the “fecal-oral” route, which is exactly what is sounds like (so wash your hands often!).

What does this have to do with vaccines?

Because some immunizations are made using weakened versions of the illnesses they prevent, you can sometimes shed this weakened (“attenuated”) virus after vaccinations. It’s important to remember that this is a weakened virus, and that the shedding is well below the levels of someone who had the normal-strength (“wild type”) virus.

This doesn’t happen with every live virus, and if it does, these weak viruses cannot make you sick unless you are immunocompromised (such as people with untreated HIV/AIDS, leukemia, or after bone marrow transplants).

Cases of a vaccine strain of a virus causing illness are exceedingly rare. For example, with the live varicella (chicken pox) vaccine, there have been 11 cases of someone getting a full case of chicken pox from a vaccinated person, from over 50 million people vaccinated. Less than a one in 1,000,000 chance. Compare that to the four million people who had chicken pox per year before the vaccine was developed.

Only a live attenuated virus vaccine even has the possibility of shedding. The live vaccines recommended for routine use in the US are:

  • MMR (measles, mumps, and rubella)

  • Varicella (chicken pox)

  • Influenza (only the intranasal “flumist;” injectable flu vaccines are not live viruses)

  • Zoster (shingles)

  • Rotavirus

There are others such as the yellow fever vaccine, but they are not routine vaccinations. Also of note, the oral polio vaccine which used to be administered as a live virus, is no longer available in the US. Instead we use a version with no active virus.

So, the MMR vaccine is a live, weakened virus. But does it cause viral shedding, and if so, does it cause outbreaks? The quick answer is “no,” but let’s dig deeper.

There are really two questions we want to ask ourselves here:

  1. Are we seeing evidence of vaccine shedding/transmission?

  2. If shedding did occur, what would be the consequence/result of that?

Is there evidence of MMR vaccine and viral shedding?

We have not seen evidence of measles viral shedding after MMR vaccination.

I’ve seen a lot of discussion about a single reported case of “vaccine associated measles” in a two-year-old girl in Canada who received the vaccine and 37 days later had a fever and a rash. Her illness resolved without incident, but because there had been a local outbreak of measles at the same time this patient was seen, her doctor tested her for measles. Of course, she tested positive for the vaccine strain of the illness — because she’d had the vaccine!

It is worth noting here that the average incubation period for measles is 7-21 days, so contracting the vaccine version of measles 37 days after receiving the vaccine would be quite odd, but testing positive for the virus would not be surprising.

But let’s assume that you could shed the virus for 37 days after getting the vaccine. Could this version of the virus cause outbreaks?

Wild-type measles is one of the most contagious viruses we know of: Nine out of 10 people exposed will contract the illness unless they’ve been immunized.

MMR vaccine is routinely administered at age one and again between ages four and six. So if the vaccine strain of measles being shed by vaccinated individuals were a problem, we would expect to see large numbers of unvaccinated children under age one with vaccine-type measles being shed from others who had been recently immunized. We simply haven’t seen this.

Even if shedding did occur, it wouldn’t cause an outbreak.

Since the measles vaccine is a weakened strain of the virus, even if shedding did occur and was somehow transmitted to someone else, they would only contract the weakened strain, not the wild type strain.

In other words, their experience would be no different from if they went and got the measles vaccine themselves. The weakened virus would not somehow revert to the more serious wild type - such changes require thousands of generations of adaptation, and don’t occur in a single transmission from one person to the next.

It’s worth noting that although we haven’t seen evidence of measles viral shedding after the MMR vaccine, there is evidence that the attenuated (weakened) mumps virus can be shed after the vaccine. However, since it’s the weakened strain and not the wild type, it similarly would not cause a mumps outbreak.

So, how do outbreaks occur?

Outbreaks happen when vaccination rates in a community drop below the level necessary for herd immunity.

When vaccination rates are high enough for herd immunity, we don’t see any disease regardless of where it might be coming from, including in unvaccinated children. Unvaccinated children in this setting would theoretically be exposed to plenty of vaccine shedding if this did occur, and yet they only contract disease when vaccination rates drop below the threshold for herd immunity, allowing wild type virus to return.

The bottom line

We are not seeing evidence of measles virus shedding from the MMR vaccine, which means it is either not occurring or occurs so rarely and without consequence that it is undetectable in the population, poses no threat to healthy individuals, and is not the cause of measles outbreaks.

The best protection from measles (as well as mumps and rubella) is the MMR vaccine.

While the number of cases of measles continues to climb, it’s not too late to get the vaccine. In fact, it can be given within 72 hours of a potential exposure as a post-exposure prophylactic treatment, though it is more effective if given before exposure.

If you have questions about getting the vaccine for you or your children, talk to your doctor.

Learn more

For more information about the measles in Oregon and Washington, and to see a list of places that have been identified as potential exposure sites, visit the following websites:


About the author

Max Cohen, N.D. is a naturopathic physician and lifelong Oregonian whose clinical interests include diet and lifestyle medicine, bedside ultrasound, and evidence-based integrative medicine. When not seeing patients, he spends time with his family, and adventuring in nature.