“Vaccine Shedding”: Time Up For Another Vaccine Myth

One myth often pulled out by antivaccination lobbyists to malign vaccine safety is the senseless term “Vaccine Shedding”.

Whilst in context we all know what is meant, it’s worth pausing to consider that the term is a byproduct, if you will, of the antivaccination movement’s skill at sowing misinformation. The unrivaled ability to scan a headline and regurgitate some ghastly tale about vaccines. To squeeze another fallacious vaccine “danger” onto the shelf, content in the knowledge it will soon have a life of it’s own.

The colloquial use of this nonsensical term seeks to convey that an individual who has been vaccinated can readily shed part of the vaccine and cause infection in the unvaccinated. Which by definition demands them to have shed not a vaccine but an infectious agent. Indeed a virus. Which by extension demands the vaccine to be a live virus vaccine. This then opens the door to viral shedding the vast complexities of vaccine induced immunity and viable modes of excretion – aka shedding. That won’t stop your garden variety anti-vaxxer claiming any vaccine can lead to infection of the unvaccinated via this ghastly “vaccine shedding”.

But that’s only part of the story. “Vaccine shedding” is a double barrelled myth in that transmission is assumed to occur ipso facto. Shedding is not transmission. Period. Yet denial of vaccine efficacy requires internalisation of some whacky stuff. Including the erroneous belief that viral shedding follows MMR vaccination. Yet worse is the myth that inactivated vaccines pose the risk of infection due to “vaccine shedding”.  Pertussis often brings out the malicious side of anti-vaxxers. DTaP is inactivated. Indeed the pertussis component is acellular. So, you may wonder at the nature of Cynthia Janak who writes in Will the vaccinated infect the unvaccinated? That is the question with Whooping cough:

Before I continue I want to tell you about a fact that is known by the CDC, etc. That is called vaccine shedding. This is the transmission of the virus from a vaccinated person to an unvaccinated person. [….] I want you to understand that this is true for vaccines including the Whooping Cough. What you could have happen is that all these parents and child care workers are going to get the vaccine and then take care of children. [….] The vaccinated have the potential to infect the unvaccinated child. This could cause the next epidemic of disease like what happened with the small pox epidemic.

So, in Cynthia’s mind “vaccine shedding” is, “…transmission of the virus from a vaccinated person to an unvaccinated person”. Wrong. And it’s true for whooping cough. Impossible. Yet Cynthia Janak asserts there’s potential for an epidemic like smallpox? Pure fiction. Contracting pertussis because an unvaccinated and infected child or adult who ignores boosters has breathed on someone is, however, a simple fact. Aiming to inflate the danger of her misguided concern about “vaccine shedding” as “known by the CDC”, Cynthia uses references to FluMist.

FluMist a live attenuated influenza vaccine (LAIV) sprayed into the nostrils and well understood regarding shedding. Concerns about administering a live virus this way should be respected. So should the facts about any risks. It sheds in low concentration for short periods via nasal discharge. It is not associated with person to person transmission. Given that wild type influenza sheds at far higher concentration, is found on fixtures, objects, skin and is strongly associated with transmission, severe illness and complications it seems Cynthia has been selective about what’s “known by the CDC”.

“Vaccine shedding” is better suited to mid 19th century notions like the infectious miasma, wafting about in terrifying unseen clouds held aloft by our lack of knowledge. Nor does the rare instance of shedding suddenly turn any agent into a virus with the infectious capability of Ebola. But anti-vax voices are often raised in triumph that the crime of “vaccine shedding” places the community at greater risk than the rising numbers of unvaccinated.

The scale of error associated with this belief is akin to the myth of potential vaccine injuries outweighing the benefits of vaccination. Serious injuries that do occur are primarily in populations genetically predisposed to latent complications and manifestation is extremely rare. Injuries, disability and death from vaccine preventable disease would occur at magnitudes many hundreds or thousands of times greater and can manifest in anyone. Vaccine injuries are artificially inflated by confusing correlation (sometimes years apart) with causation, and by including red marks, crying, sleep disturbance or omitting that event X was a serious allergic reaction to latex syringe components. Similarly, arguing ones unvaccinated child is at risk from, or has been infected by, a recently vaccinated child is quite a claim.

Viral shedding itself is by no means ignored by the medical community. It’s of primary concern in the management of immune compromised patients, pregnant women and newborns. Varicella is an excellent example in that a.) viral shedding is well understood and b.) the risk from shedding can be discerned from precautions taken. Following varicella vaccination, viral shedding can be detected in the stools for six weeks.

In the case of immunodeficiency disorders or immune suppression from drugs, transfusions, stem cell transplant, chemotherapy etc, the recommendations are to avoid contact with fecal matter of vaccinated subjects and to observe good hygiene. To put this in context, unvaccinated children who spend one hour in a room with an infected child (shedding varicella) stand a 95% chance of contracting varicella (chicken pox). This is why vaccination against varicella is vital and choosing to not vaccinate your child places him or her and by extension countless others at risk of serious complication.

For nursing mothers post natal varicella vaccination need not be delayed if they are varicella-susceptible as varicella hasn’t been found in breast milk post maternal vaccination. There is no problematic risk of viral shedding to newborns provided hand washing and other hygiene measures are followed.

Whilst rare, a post-varicella immunisation vesicular rash can form. Again whilst quite rare, viral shedding can occur at this site. Plainly stated it’s incredibly rare for an unvaccinated child to be infected with varicella from a vaccinated subject and a series of events, including transmission, must occur within a small window of opportunity. Greatest precautions must be taken in the case of immune suppression. Writing in Vaccines in immunocompromised patients, Janet R. Serwint, MD Consulting Editor notes:

Because the varicella virus rarely can be shed through a postimmunization vesicular rash that may develop, recommendations include avoiding contact until the rash resolves.

In March this year there was an interesting case of viral shedding. The antivaccination lobby bellowed that Varicella zoster virus DNA had been found in the saliva of people over 60 vaccinated with the live Zostavax vaccine manufactured by Merck. In this age group Herpes zoster (shingles) is the target. Shingles is the result of infection with VZV earlier in life which may reactivate as immunity declines or from novel infection. Despite blog headings like Vaccinated people SHED LIVE HERPES for up to a month AFTER vaccination, be aware it was 2 of 36 “vaccinated people” who made the grade.

There was no indication of infection risk at the time. Today transmission is considered rare. Packet inserts carried the standard warnings found in varicella immunisations to avoid contact with infants, nursing mothers and immunocompromised individuals. “Doctors never tell you this”, lied the anti-vax lobby. The end result is that, fortuitously, it appears a saliva test could be developed allowing for detection and antiviral therapy before the painful rash appears. All up with rare potential for transmission from about 5% of recipients of a vaccine that’s not widely used it was a non event.

With MMR the lack of viral shedding renders any risk of horizontal transmission in this manner null and void. If challenged with the claim of “vaccine shedding” specific to Measles, Mumps, Rubella vaccination you’re being misled.

Peak shedding of Rotavirus occurs on “post-vaccination days 6 through 8”. Published in The Lancet Rotavirus vaccines: viral shedding and risk of transmission, notes:

Immunocompromised contacts should be advised to avoid contact with stool from the immunised child if possible, particularly after the first vaccine dose for at least 14 days. Since the risk of vaccine transmission and subsequent vaccine-derived disease with the current vaccines is much less than the risk of wild type rotavirus disease in immunocompromised contacts, vaccination should be encouraged.

The “vaccine shedding” bogeyman got a free kick with the FluMist LAIV vaccine. You may remember the hype. The spraying of “living influenza virus” straight into children’s brains was going to lead to mutation and death on an unprecedented scale. It would genetically revert to the wild type. Transmission would thus be uncontrolled. It would quickly prove useless against changing seasonal strains. ADR’s would rise…. and so on. Ultimately the cost proved to be a deterrent. Mayo Clinic have produced a welcome article on LAIV Myths.

In a comprehensive 2008 study with a sample aged 2 – 49 years, shedding “of short duration and at low titers” was detected in nasal swabs on days 1 – 11. LAIV recipients “should only avoid contact with severely immunocompromised persons for 7 days after vaccination”.

On Shedding and Transmission of Vaccine Viruses, in a larger piece on influenza vaccination of HCP, the CDC write:

One concern regarding use of LAIV among HCP has been the potential for transmitting vaccine virus from persons receiving vaccine to nonimmune patients at high risk. Available data indicate that children and adults vaccinated with LAIV can shed vaccine viruses for >2 days after vaccination, although in lower titers than typically occur with shedding of wild-type influenza viruses. Shedding should not be equated with person-to-person transmission of vaccine viruses, although transmission of shed vaccine viruses from vaccinated persons to nonvaccinated persons has been documented in rare instances among children in a day care center.

One study conducted in a child care center assessed transmissibility of vaccine viruses from 98 vaccinated persons to 99 unvaccinated controls aged 8–36 months; 80% of vaccine recipients shed one or more virus strains (mean duration: 7.6 days). [….] The estimated probability of acquiring vaccine virus after close contact with a single LAIV recipient in this child care population was 0.6%–2.4%.

It was also documented that should HIV positive children be exposed to LAIV shedding, “… serious adverse outcomes would not be expected to occur frequently”. So the combination of live virus shedding and immune deficiency in the case of LAIV presents low risk. Certainly the overall risk associated with the rare transmission following shedding after LAIV is insignificant given the risk of regular influenza virus transmission.

We’re running out of dramatic scenarios for the antivaccination lobby to cling to. With polio the wild virus replicates in the intestine and is shed in stools for up to a month. Transmission in developed nations is thus faecal-oral like other stool shed viral components. It is of course so rare as to be unheard of. However, given that the IOM report into evidence and causality of vaccine adverse effects found a causal link between the oral polio vaccine (OPV) and vaccine associated paralytic polio (or Vaccine Derived Polio Virus), we should seriously consider shedding in areas where this is documented.

In fact the question has been asked if prolonged VDPV shedding could be a source of reintroduction following polio eradication. The more compromised the immune system the more likely the individual is to have problems with vaccine induced immunity. A study looking for VDPV shedding in immune deficient subjects in Abidjan, Cote d’Ivoire found no cases in a sample of 419, and therefore a “minimal risk of reintroduction [after eradication]”. In respect of general exposure to shedding in these environments transmission of the wild type polio virus eliminates any concern over post vaccination viral shedding. Crowding, sewerage, water quality etc all contribute to wild polio spread in ways that do not apply to the developed world.

Remembering that viral shedding is of paramount concern in the management of immune deficiency and immunocompromise, let’s revisit the Janet R. Serwint, MD of Vaccines in immunocompromised patients. Rather than warn against exposure to immunised children the recommendation is to ensure schedules are up to date and an annual inactivated influenza vaccine is on board. Pay attention to reference to MMR, varicella and rotavirus:

One strategy worth emphasizing is the immunization of household contacts, particularly other children and adolescents in the family. This procedure is essential to try to minimize exposure of the immunocompromised patient to household contacts who might contract vaccine-preventable illnesses. Pediatric health-care clinicians need to update and review the vaccine status of all siblings and pediatric-age household members. Annual influenza vaccination of all family members with inactivated influenza vaccine is recommended in addition to ensuring routine immunization of all other recommended vaccines.

MMR, varicella, and rotavirus vaccines, although live viral vaccines, are recommended for immunocompetent household contacts because transmission of the virus is rare. The lack of viral shedding with MMR eliminates concern regarding transmission. Because the varicella virus rarely can be shed through a postimmunization vesicular rash that may develop, recommendations include avoiding contact until the rash resolves. For the rotavirus vaccine, avoidance of contact with the stools by the immunocompromised patient and good hand hygiene measures by all family members for at least 1 week after vaccination should be implemented.

In conclusion it’s clear that “vaccine shedding” is a nonsense phrase. The lack of accounts of children transmitting viruses to younger siblings and friends after vaccination is a dead giveaway. Whilst viral shedding is a reality we can be confident that:

  • Viral shedding applies only to live virus vaccines and is significantly low, low risk
  • Post vaccination viral shedding of rotavirus and varicella is detected in the stools for 4-6 weeks respectively. It’s of such low risk as to be of cautionary interest regarding immunocompromised individuals
  • Genuine concern about viral shedding in these groups is managed with sound hygiene and avoiding contact with stools
  • In rare cases of post varicella immunisation vesicular rash shedding may occur. Transmission is still unlikely
  • The lack of viral shedding following MMR eliminates any concerns about transmission
  • Claims of DTaP shedding and transmission are bogus
  • Stories about whooping cough transmission from vaccine shedding are demonstrably false
  • Stories of polio infection being a risk due to shedding are designed to scare
  • Antivaccination lobbyists use false and incomplete information about shedding to create fear of vaccines/the vaccinated
  • Shedding of LAIV is at markedly low concentration, short duration and transmission is dwarfed by seasonal influenza transmission
  • Accurate information about the topic is drowned out by antivaccination sites and “mothering” forums making inaccurate claims

Update: April 13th 2015 – Added references;
Is the MMR vaccine spreading the measles virus?: The question of shedding

Case of vaccine-associated measles five weeks post-immunisation, British Columbia, Canada, October 2013: http://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=20649
Live Attenuated Influenza Vaccine [LAIV] (The Nasal Spray Flu Vaccine): http://www.cdc.gov/flu/about/qa/nasalspray.htm
Live Attenuated Vaccines (LAV): http://vaccine-safety-training.org/live-attenuated-vaccines.html
Measles – Q&A about Disease & Vaccine: http://www.cdc.gov/vaccines/vpd-vac/measles/faqs-dis-vac-risks.htm
Measles: Questions and Answers: http://www.immunize.org/catg.d/p4209.pdf?q=measles
Measles Vaccination: http://www.cdc.gov/measles/vaccination.html
Rotarix WHO leaflet – tube: http://www.who.int/immunization_standards/vaccine_quality/Rotarix_liquid_tube_product_insert_text_2009.pdf?ua=1
Rotavirus: http://www.cdc.gov/vaccines/pubs/pinkbook/downloads/rota.pdf
Transmission of Measles: http://www.cdc.gov/measles/about/transmission.html