Guest post: “Viral Shedding” is not something to worry about

The Pediatric Insider

© 2015 Roy Benaroch, MD

Today features the first-ever guest post at The Pediatric Insider, from a pediatrician buddy of mine. He wrote this as an open letter to Bob Sears, the most well-known vaccine doubting physician in the USA.  Thanks to the author for helping to fight unnecessary fear!


Hello, Bob Sears, M.D., FAAP.  Before I continue, I’d like to disclose that my sole source of income is from my employer* and that I am not selling any books (I haven’t written any).  I do not have any financial or business relationships with any vaccine manufacturer, nor have I ever accepted any gift from a vaccine manufacturer.  My only financial incentive with respect to vaccines is that I get a small (about 1.5% of my annual income) “quality of care” bonus from my employer at the end of the year if a certain percent of my patients are fully vaccinated according to CDC/ACIP guidelines.**  I’m writing and researching this post in my off time and I’m not expecting any compensation for it.

I just read this post of yours in which you raise concern that recently vaccinated patients might shed their vaccine viruses.  Certainly, I’m not familiar with this as a major danger to public health, so I decided to look into it.

sears bob shedding

As a fellow pediatrician, you are doubtlessly aware that only five types of live attenuated vaccines are currently use in the United States.  Those are the Live Attenuated Influenza Vaccine (LAIV), the two competing rotavirus vaccines RV1 (“ROTARIX”, GlaxoSmithKline) and RV5 (“ROTATEQ”, Merck), MMR, and the Varicella Zoster Vaccines.  These latter two are often combined into a single injection called MMRV.  As you are aware, the remaining routine vaccines (DTaP/TdaP, IPV, HBV, HAV, PCV13, HiB, MCV4, HPV4/9, IM/ID flu) are inactivated and incapable of shedding.(1)

We can ignore LAIV given that it’s June and I doubt anyone here in the Northern Hemisphere is currently using it.  That said, while 98% of recent vaccinees do shed LAIV, the rate of actual transmission is less than 1% (2).

As far as MMR is concerned, we know that the wild viruses are spread by respiratory secretions and that infection occurs by mucosal contact with the virus, but I was not able to find any cases in the medical literature documenting that the vaccine-strain Measles, Mumps, or Rubella viruses are shed in respiratory secretions after receipt of the MMR vaccine used in the United States or Canada.  I could find only a single report of possible vaccine-strain rubella (HPV-77 [unrelated to Human Papilloma Virus]) horizontal transmission from 1968 (3) and that did not result in symptomatic infection.  It is also possible that the seroconverted contact in that study actually had subclinical wild infection.  But I was also able to find a study in 1972 in which 67 rubella-nonimmune teachers were monitored after their classes of children were vaccinated and none showed evidence of seroconversion.(4)  Clearly, the risk of horizontal transmission of HPV-77 vaccine strain rubella virus is very low if it is not zero.

For the mumps component, I was able to find that the Leningrad-Zagreb mumps vaccine strain commonly used in Russia and India can rarely spread horizontally (5), and one case of the now disused Urabe mumps vaccine strain transmitted (6) but I was not able to find this for the Jeryl-Lynn strain used in the United States MMR vaccine.  Perhaps you have a reference that this has occurred?  If so I’d appreciate if you shared it.

It certainly is true that the vaccine measles strain may be shed in the urine of recently vaccinated individuals (7), but given that transmission requires mucosal contact this should not be cause for concern.  I certainly hope that nobody at your gathering will be drinking the urine of recently vaccinated infants!  In all of medical history since the time MMR has been in use there has only been one case report of horizontal transmission of the measles vaccine strain (8) and that was between siblings.  I cannot find any others, so I would say that this risk is so vanishingly small that it would be unreasonable to worry about it at a gathering like yours.

The other two live attenuated vaccines used in this country carry a bit more risk of horizontal transmission.  Cases of transmission of varicella zoster vaccine virus have been described (9) but in all of these cases, the individuals who transmitted the vaccine virus had a visible varicella-like rash and the recipients either had no symptoms or mild clinical disease.  There is, to my knowledge, one case of varicella vaccine transmission without a rash, but that was from a new mother to her infant(10).  It’s possible that it might have been transmitted in breast milk.  For this reason, both Merck and the CDC recommend that the <3% of children who do develop this rash be kept away from susceptible contacts until the rash resolves (10).  I’ll note than in ten years of clinical practice, I estimate that I have given over 2,000 doses of VZV either as a single-component vaccine or as the MMRV combination product and I am only aware of six such rashes in my patients (actually, one was in my medical student’s recently vaccinated wife who was not my patient, but I count her among the cases).  Thus, my own clinical experience has been about ten times better than Merck would claim.

Similarly, for rotavirus vaccine, the transmission rate to unvaccinated twins was 18.8% for the “ROTARIX” product in one placebo-controlled study in twins (11). But in these cases, these were twins living in the same household and having their diapers changed by the same parents.  It’s important to note that none of the transmission cases caused clinical gastroenteritis symptoms.  There is also a single case report of a recently vaccinated child with “ROTATEQ” transmitting a symptomatic infection to an unvaccinated older sibling (12), which resulted in an emergency department visit but no long-term sequelae.  Because transmission of rotavirus vaccine would only occur in very young infants (the vaccine should only be given <7mo) (13)(14), and the transmission is fecal-oral, simple good hand washing after changing a recently vaccinated infant’s diaper should reduce the risk of transmission to unvaccinated non-household contacts at a public gathering like the one you are holding.  Either way, the risk of transmission of a symptomatic case from a recently vaccinated child to an unvaccinated child appears to be vanishingly small, based on the available evidence.

But I also wonder why you would be so concerned about transmission of vaccine viruses at your event.  Perhaps you feel that people should consent to being exposed to infectious agents.  That would be nice, but that has never been the case.  My parents did not consent to my exposure to varicella at age 6 that caused me to spend a week in misery and left me with two scars on my face to this day.  Similarly, my parents did not consent to the exposure to (likely) rotavirus that hospitalized me as an 8mo infant.  Moreover, I have never consented to any of the symptomatic infectious diseases from which I have suffered as an adolescent or adult.  But certainly, as I have demonstrated, the risk of poor health outcomes after accidental exposure to attenuated vaccine-strain viruses is much lower than the risk for the wild-type viruses.  Yet you propose to have a gathering with a very high rate of unvaccinated children present.  To me, your risk-benefit calculation just doesn’t work out.

Perhaps you are concerned for any immunocompromised members of your audience, but we do not isolate recently vaccinated children and adults from contact with the general public, even though immunocompromised individuals live and walk among us.  They are as much at risk traveling to your gathering as they are at the gathering.

To further confuse matters, you have no way of knowing if any of the unvaccinated children attending your gathering might be in the asymptomatic but contagious prodromal phases the precede most symptomatic viral infections.  Diseases like measles and chicken pox are so contagious that virtually all children contracted them before the vaccines were available, even though it has always been policy to immediately exclude children with symptoms of these illnesses from attendance at school.

So perhaps you could explain your reasoning for this unusual restriction.  I’d appreciate it if you’d demonstrate that you have done your research as I have.


*I am technically an independent contractor.

**We get other “quality of care” bonuses for other “best practices,” like not treating viral infections with antibiotics, testing sexually active patients for STIs, and keeping problem lists in the medical record up to date.



(2) Vesikari T, Karvonen A, A randomized, double-blind study of the safety, transmissibility and phenotypic and genotypic stability of cold-adapted influenza virus vaccine. The Pediatric Infectious Disease Journal 08/2006; 25(7):590-5.

(3) Lefkowitz LB, Rafajko RR, et al. A Controlled Family Study of Live, Attenuated Rubella-Virus Vaccine — Seroconversion of a Susceptible Contact. N Engl J Med 1970; 283:229-232

(4) Fleet WF, Shaffner W, et al. Exposure of Susceptible Teachers to Rubella Vaccinees Am J Dis Child. 1972;123(1):28-30

(5) Atrasheuskaya A1, Kulak M, et al. Horizontal transmission of the Leningrad-Zagreb mumps vaccine strain: a report of six symptomatic cases of parotitis and one case of meningitis. Vaccine. 2012 Aug 3;30(36):5324-6

(6) Sawada H, Yano S, Oka Y, Togashi T. Transmission of Urabe mumps vaccine between siblings. Lancet.1993;342:371. doi: 10.1016/0140-6736(93)91515-N.

(7) Rota AS, Kahn AS et al. Detectin of Measles Virus RNA in Urine Specimens From Vaccine Recipients J Clin Microbiol. 1995 Sep;33(9):2485-8.

(8) Millson D. Brother-to-sister transmission of measles after measles, mumps, and rubella immunisation. Lancet. 1989; 1(8632):271.

(9) Tsolia M, Gershon AA, et al., National Institute of Allergy and Infectious Diseases Varicella Vaccine Collaborative Study Group Live attenuated varicella vaccine: evidence that the virus is attenuated and the importance of skin lesions in transmission of varicella-zoster virus. J Pediatr.1990;116:184–9. doi: 10.1016/S0022-3476(05)82872-0

(10) VARIVAX package insert, Merck.

(11) Rivera L, Peña LM, et al. Horizontal transmission of a human rotavirus vaccine strain–a randomized, placebo-controlled study in twins. Vaccine. 2011;29:9508–13. doi: 10.1016/j.vaccine.2011.10.015.

(12) Payne DC, Edwards KM, et al. Sibling transmission of vaccine-derived rotavirus (RotaTeq) associated with rotavirus gastroenteritis. Pediatrics. 2010;125:e438–41. doi: 10.1542/peds.2009-1901.

(13) ROTATEQ package insert, Merck

(14) ROTARIX package insert, GlaxoSmithKline

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8 Comments on “Guest post: “Viral Shedding” is not something to worry about”

  1. Sheri Says:

    Dr Roy: I’m sure you’ve already seen this but, just in case…

    Steve Silberman: The forgotten history of autism

    13:48 minutes · Filmed Mar 2015 · Posted Jun 2015 · TED2015
    One in every 68 people is on the autism spectrum — a condition that, even just a few decades ago, was rarely diagnosed. To understand the history of autism, says writer Steve Silberman, we have to go back even further, to an Austrian doctor by the name of Hans Asperger, who published a pioneering paper in 1944 on the “little professors” he was working with. Asperger’s paper was buried by war and time — and much crueler theories of autism prevailed. Follow Silberman’s journey as he uncovers the lost history of autism. (This talk was part of a TED2015 session curated by Pop-Up Magazine; @popupmag.)
    Watch now »


  2. Dr. Roy Says:

    Sheri, I haven’t seen it– I like TED talks, but honestly I often don’t have the patience to watch things (I don’t watch television, either. Yes. I am weird.) Here’s a link:


  3. The Skeptical OB has also recently addressed this on her own blog:

    My guess would be that Dr. Sears in fact knows that this restriction is preposterous, but he just wanted to discriminate against vaccinated kids in order to make his own vaccine-avoiding patients feel superior.


  4. wzrd1 Says:

    I recall that oral polio vaccine also sheds, helping build passive herd immunity. As I recall, one first questions if there are immunocompromised persons in the home before administering the OPV to a patient.

    I can think of one vaccine that would give some reason for concern, although it’s likely outside of your experience. Vaccinia is known to shed from the scab and infect those cohabitating with a recently vaccinated person, if due care is not exercised.
    That said, I find it highly unlikely that Dr Sears is concerned about new military recruits or newly hired public health emergency team members attending his group.


  5. Angela Hopkins Says:

    On his Facebook page he states that he was “kidding” about the shedding. It’s buried in the comments, and he only says this after people start debating it.

    However, more than one commenter took him seriously. It seems like a pretty inappropriate thing to joke about.


  6. catherinajtv Says:

    Can I point out that the “measles transmission” occurred during a period when measles were still endemic in the UK (over 26000 cases were reported for 1989 in England and Wales), the timeline for transmission was not correct, with symptoms in the vaccinated brother preceding symptoms in his infant sister only by “a few days” and symptoms in both children peaked at the same time, pointing to a common source/time of wild virus infection.
    Finally, the vaccine strain in the MMR in the UK was Schwarz, while the MMR currently used in the US is Enders’ attenuated Edmonston. Therefore, I would argue that even this “case” is not relevant to the risk of measles vaccine virus transmission in 2015 California.

    Liked by 1 person

  7. mikeginnymd Says:

    catherinajtv, can you provide me the detailed source for that information? I’m not disbelieving you; I’m gathering all the information I can.


  8. Kirstin Says:

    Is it possible to report this clown to the CA medical board? We had a similar case of vaccine lie-spreading in AZ (“paleo-cardiologist” [whatever that means] Wolfson), and locals complained enough that he was investigated by the medical board. I’m not sure whether the complaints came from the population at large or other doctors, but if the situation in medicine is similar to that in science, misconduct complaints from peers are taken more seriously than those from outside the community. If this is the case, I’d strongly encourage you and every other responsible member of the CA medical community to complain about his spreading of dangerous misinformation. Even if an investigation amounts to nothing, seeing their hero under suspicion of medical misconduct might impress upon the more impressionable (and less militant) of the anti-vaxers just how fringe and misinformed his view is.


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