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Is the MMR Vaccine Spreading the Measles Virus?: The Question of Shedding

Measles VirusIn light of the current measles outbreak, the Centers for Disease Control and Prevention (CDC) and other health care professionals have recommended that all eligible individuals receive the combination measles-mumps-rubella vaccine to help prevent the spread of the highly contagious respiratory illness. According to the CDC, the measles portion of the MMR vaccine is 93 percent effective after one dose and 97 percent after two doses in preventing the disease. Unfortunately, a growing anti-vaccination movement continues to argue against the use of disease-preventing vaccines.

One common argument against the MMR vaccine is the claim that the vaccine spreads the measles virus through a phenomenon called “vaccine shedding.” Although the CDC refers to “shed vaccine viruses” and the World Health Organization (WHO) uses the term “vaccine shedding,” neither organization offers a clear definition of the concept. However, the anti-vaccination movement claims that an individual vaccinated against a disease can spread the disease when the virus or other microbe in the vaccine “sheds,” similar to the transmission of the wild illness.

Among the currently recommended vaccines, the CDC and the WHO acknowledge that viral shedding has been observed with two vaccines, the live attenuated influenza vaccine (nasal spray flu vaccine) and the rotavirus vaccine. Both the nasal spray flu vaccine and the rotavirus vaccine contain live attenuated viruses, meaning that the vaccines are derived from disease-causing pathogens that have been weakened under laboratory conditions. Live attenuated vaccines provide protection against a disease when the weakened pathogen grows inside the body, stimulating an immune response in a vaccinated individual. The MMR vaccine is a live attenuated vaccine. However, because of the weakened state, the pathogen cannot cause a full-blown illness, only a mild form, if any.

Because the pathogen is alive, albeit weakened, the body can theoretically shed the weakened microbe. However, shedding of viruses in vaccines typically occurs in lower amounts than during shedding of wild-type viruses. In other words, weakened viruses in live attenuated vaccines can shed, but in weakened amounts. Thus, because weakened viruses in vaccines cause mild or no disease, shed weakened viruses also cause mild or no disease. As the CDC states in response to the shedding of the influenza virus in the nasal flu spray vaccine, “serious illnesses have not been reported among unvaccinated persons who have been.”

Additionally, although many live attenuated vaccines are contraindicated for individuals with weakened immune systems, most individuals who have contact with other individuals with weakened immune systems can and should receive live attenuated vaccines. For some live attenuated vaccines such as the nasal spray flu vaccine, the CDC does recommend that individuals with severely weakened immune systems who are being cared for in a protective environment avoid contact with vaccinated individuals for seven days because of the theoretical potential for viral shedding. Again, however, no serious illnesses have been reported among unvaccinated individuals inadvertently infected with vaccine viruses.

Furthermore, although both the CDC and the WHO acknowledge that viral shedding may occur after the administration of the nasal spray flu vaccine and the rotavirus vaccine, neither organization warns that shedding of the measles virus can occur after administration with the MMR vaccine. Unlike the influenza and rotavirus vaccines, shedding is not listed as a potential side effect of the measles vaccine. A review of the literature on viral shedding and the measles vaccine also fails to reveal conclusive evidence. Only one case study from 2013 could find a single case of vaccine-associated measles, and even then the authors caution that the symptoms might not be measles-vaccine-related but rather an inter-current illness that confounded the presentation.

Despite claims from the anti-vaccination movement, current research fails to find evidence that individuals who receive the MMR vaccine shed the measles virus, subsequently spreading the disease prevented by the vaccine. On the contrary, widespread use of the measles vaccine has led to a greater than 99 percent reduction in measles cases compared with the pre-vaccine era in the United States. In 2000, measles was declared eliminated from the country. Only as vaccination rates have begun to decline has the highly-preventable illness made a resurgence. Vaccinated individuals do not shed the measles virus, but 90 percent of unvaccinated individuals who come into someone infected with the virus will contract the disease. The MMR vaccine prevents, and only prevents, the measles.

The CDC currently recommends all children receive two doses of the MMR vaccine, with the first dose given between 12 and 15 months and the second dose between 4 and 6 years. Children under the age of 1 generally cannot receive the MMR vaccine, meaning that infants are extremely vulnerable to measles infection. Caregivers who are not already immune are therefore urged to receive the MMR vaccine.

Update: Unlike with the nasal spray influenza vaccine, only severely immunocompromised people should not be given MMR vaccine, but ¬†individuals who have received the MMR vaccine do not need to avoid contact with other individuals with weak or compromised immune systems and “[h]ousehold contacts of immunocompromised people should be vaccinated according to the recommended schedule.”


Case of vaccine-associated measles five weeks post-immunisation, British Columbia, Canada, October 2013:
Live Attenuated Influenza Vaccine [LAIV] (The Nasal Spray Flu Vaccine):
Live Attenuated Vaccines (LAV):
Measles – Q&A about Disease & Vaccine:
Measles: Questions and Answers:
Measles Vaccination:
Rotarix WHO leaflet – tube:
Transmission of Measles:

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