Shedding Doubt: Are Live Vaccines Contagious?

A common concern about vaccines that I’ve been hearing recently is a fear about “viral shedding” after receiving immunizations. I’ve even seen claims that this is the driving force behind various disease outbreaks, and that we should stop using vaccines because recently-vaccinated individuals are potentially infectious.

The argument generally goes like this: Healthy people frequently receive live virus vaccines (e.g.; immunizations against chicken pox, MMR or rotavirus). Because these vaccines are weakened (“attenuated”) —  but still living — versions of their “wild type” (disease causing) relatives, they can replicate in a limited but similar manner. Therefore these attenuated vaccine strains of the virus could hypothetically be spread from a recently-vaccinated person to a susceptible individual or group.

Like many claims, on first glance this argument makes some logical sense: if a virus can make you sick, and someone who recently received a vaccine containing this virus is nearby, why couldn’t you become sick from them? Let’s examine this claim and see if there’s any truth to it. First we’ll look at viral shedding in general.

Viral Shedding

So what are we talking about when discussing viral shedding? Take the example of the common cold. When you have a runny nose and a cough from a cold, the virus is replicating in your upper airway. When you cough or sneeze, small droplets containing the virus are placed in the environment. If someone else breathes them in or touches a surface they’ve landed on, they can contract your cold (hence the “cover your cough” and “wash your hands” advice for cold prevention). This is how most illnesses spread. How does this relate to vaccines?

Live vaccines

“Live” vaccines contain active viruses which have been selectively bred to be profoundly weak. This is referred to as an “attenuated” virus strain. These attenuated viruses are too weak to cause serious illness, but still offer our immune systems a chance to create protective antibodies. The following is a list of live vaccines recommended for routine use in the United States:

  • MMR (measles, mumps and rubella)
  • Rotavirus
  • Varicella (chicken pox)
  • Influenza (only the intranasal “flumist”; injectable flu vaccines are not live viruses)
  • Herpes Simplex Zoster (shingles – not a childhood vaccine)

There are others too, such as the yellow fever vaccine, but they are not a routinely used. There was an oral polio vaccine containing a live virus used in the United States previously, but it is no longer available here. Instead we use a version with no active virus.

Are Live Vaccines Dangerous?

These live viruses are able to infect our cells similar to their wild-type cousins, and some have argued that because they can replicate, then these viruses can be shed and are therefore dangerous

The questions we must ask in this case are:
1) Do live vaccine shed in enough to be infectious?
2) Are the attenuated viruses able to cause disease?

Let’s tackle each question individually.

Which vaccines can shed?

For routinely administered childhood vaccines:

  • Measles: does not appear to shed [1, 9]. There have been questionable cases of vaccine-strain associated illness [2] but these are exceedingly rare, if they occur at all. A recently vaccinated person having a rash and a fever (which occurs in approximately 5% of vaccine recipients) is indistinguishable from a case of wild-type measles, but there are no documented cases of harm from the vaccine strain virus.
  • Mumps: does not appear to shed [1].
  • Rubella: can be shed for 7-28 days after vaccination. Can be present in breast milk, but there are no cases of harm from vaccine strain rubella in infants [3].  
  • Rotavirus: between 10 and 50% of recipients will have detectable levels of attenuated rotavirus in stool, present in low amounts. Hand hygiene is recommended for everyone (if you’re changing diapers I hope you’re washing your hands anyways), and those with immunocompromised status (HIV, chemotherapy, etc.) should avoiding changing diapers of recently vaccinated infants [4].
  • Varicella: can be shed between 10 and 21 days after vaccination. Between 1995 and 2005 over 50 million doses of vaccine were administered and there were a total of 11 cases of transmission of the vaccine strain of chicken pox, none of these individuals were seriously ill [5].
  • Intranasal influenza: The vaccine strain of influenza can be detected in low amounts for approximately 3 days after vaccination [6]. Even in immunosuppressed patients with HIV, the viral shedding was minimal [7]. In children receiving the intranasal vaccine, shedding seemed to peak on day 2 [8].

So, several of the childhood vaccines can shed attenuated virus, but is this clinically significant?

Does viral shedding matter?

Can the weakened viruses cause illness? To answer this question I think it is helpful to take a 10,000-foot view of the situation. Let’s remember that the MMR vaccine is typically administered twice. Once at 12 months of age and again between 4 and 6 years. Children older than 6 months but under 12 months old (as well as unvaccianted children of any age) are vulnerable to wild-type measles (which is one of the most contagious viruses known). If their recently vaccinated peers were shedding a dangerous, highly contagious virus, we would expect to see non-stop outbreaks of vaccine strain measles in this population. There is simply no evidence that this occurs. Not with measles nor any other live virus vaccines. Furthermore, even transmission were regularly occurring, the viruses in vaccines are weakened and unable to cause serious disease. For anyone exposed, the worst possible outcome would be the same as if they’d received a vaccine themselves. It is impossible to be more ill from a vaccine strain virus than the wild-type.

That is the contradiction at the core of this argument against vaccines: you cannot argue that measles, mumps, rubella, varicella, rotavirus and influenza are benign infections that are no risk to healthy individuals while also claiming that weakened versions of these viruses are somehow dangerous.

Take home points

Yes, some vaccines are able to shed virus at low levels. This is not dangerous to healthy individuals and poses limited risk to immunocompromised persons. None of the risks of an attenuated virus are larger than that of the highly infectious wild type viruses. Vaccines prevent disease outbreaks of these illnesses, and outside of rare circumstances there’s no reason to skip vaccination for fear of shedding these weakened viruses.


  1. M-M-R II live vaccine package insert.
  2. Murti, M, et al. “Case of Vaccine-Associated Measles Five Weeks Post-Immunisation, British Columbia, Canada, October 2013.” Eurosurveillance, vol. 18, no. 49, 2013, doi:10.2807/1560-7917.es2013.18.49.20649.
  3. “Rubella Prevention — Recommendations of the Immunization Practices Advisory Committee (ACIP).” Centers for Disease Control and Prevention, Centers for Disease Control and Prevention,
  4. 4. Anderson, Evan J. “Rotavirus Vaccines: Viral Shedding and Risk of Transmission.” The Lancet Infectious Diseases, vol. 8, no. 10, 2008, pp. 642–649., doi:10.1016/s1473-3099(08)70231-7.
  5. “Manual for the Surveillance of Vaccine-Preventable Diseases.” Centers for Disease Control and Prevention, Centers for Disease Control and Prevention, 15 May 2018,
  6. Talbot TR, Crocker DD, Peters J, et al. Duration of virus shedding after trivalent intranasal live attenuated influenza vaccination in adults. Infect Control Hosp Epidemiol 2005;26:494–500.
  7. James C. King, John Treanor, Patricia E. Fast, Mark Wolff, Lihan Yan, Dominic Iacuzio, Bernard Readmond, Diane O’Brien, Kenneth Mallon, William E. Highsmith, John S. Lambert, Robert B. Belshe; Comparison of the Safety, Vaccine Virus Shedding, and Immunogenicity of Influenza Virus Vaccine, Trivalent, Types A and B, Live Cold-Adapted, Administered to Human Immunodeficiency Virus (HIV)-Infected and Non-HIV-Infected Adults, The Journal of Infectious Diseases, Volume 181, Issue 2, 1 February 2000, Pages 725–728,
  8. Mallory, Raburn M., et al. “Shedding of Ann Arbor Strain Live Attenuated Influenza Vaccine Virus in Children 6–59 Months of Age.” Vaccine, vol. 29, no. 26, 2011, pp. 4322–4327., doi:10.1016/j.vaccine.2011.04.022.
  9. Katz, Samuel L., et al. “Studies on an Attenuated Measles-Virus Vaccine.” New England Journal of Medicine, vol. 263, no. 4, 1960, pp. 159–161., doi:10.1056/nejm196007282630402.
Maxwell Cohen, ND

Max Cohen, ND is a naturopathic physician in Portland, Oregon. He currently works for a Federally Qualified Health Center (FQHC) providing primary care for a wide variety of patients. He is a member of the Board of Directors of the Naturopathic Academy of Primary Care Providers (NAPCP), as well as the Scientific Advisory Board for Boost Oregon.He completed his medical training and residency at the National University of Natural Medicine. Prior to medical school he worked as a microbiologist in a tuberculosis vaccine development lab. Twitter @MaxwellCohenND

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