When your body is infected with a disease-causing virus such as influenza, the virus is “shed” into the environment, via your saliva and other bodily fluids and skin lesions. If someone comes in contact with that shed virus, it’s possible that they, too, will become infected with the virus.

There’s still a lot of unknowns when it comes to viral shedding. How long a person sheds a virus, when shedding occurs and whether it occurs at the onset of symptoms or prior to symptoms, varies by virus and are influenced by a person’s age, health status and even weight.1

That viruses are shed from their hosts is common knowledge. However, the topic of viral shedding isn’t one you’ll hear about often when it comes to live attenuated viral vaccines, examples of which include measles, mumps, rubella (MMR), vaccinia (smallpox), varicella, zoster (which contains the same virus as varicella vaccine but in much higher amount), yellow fever, rotavirus and influenza (intranasal).2

Live viral vaccines use a weakened (or attenuated) version of the virus, which is typically passed through a living cell culture or other host, such as chicken embryo, many times over until it becomes weakened to a point that it’s not likely to make you sick when it’s injected, swallowed or inhaled.

That being said, it’s still a live vaccine-strain virus — one that can be shed like any virus — and research suggests that vaccination may increase viral shedding in the case of influenza.3

Flu Vaccine Recipients Shed 6.3 Times More Virus Into the Air

In a study published in PNAS, University of Maryland researchers revealed not only that influenza virus may be spread via simple breathing (i.e., no sneezing or coughing required) but also that repeated vaccination increases the amount of influenza virus released into the air.4

“Little is known about the amount and infectiousness of influenza virus shed into exhaled breath. This contributes to uncertainty about the importance of airborne influenza transmission,” the researchers noted. “We show that sneezing is rare and not important for — and that coughing is not required for — influenza virus aerosolization.”5

This is important, as it means that even someone who’s not actively sneezing or coughing can still potentially transmit the influenza virus to others.

Further, someone who’s recently received the live attenuated influenza vaccine (LAIV) may also potentially actively shed and transmit the virus. According to the study, in fact, people who were vaccinated for influenza shed more than six times more virus into the air than those who were not:6

“Self-reported vaccination for the current season was associated with a trend toward higher viral shedding in fine-aerosol samples; vaccination with both the current and previous year’s seasonal vaccines, however, was significantly associated with greater fine-aerosol shedding in unadjusted and adjusted models.

In adjusted models, we observed 6.3 times more aerosol shedding among cases with vaccination in the current and previous season compared with having no vaccination in those two seasons.”

Annual Vaccination May Actually Reduce Your Protection

Receiving a flu vaccination to prevent type A or B influenza is not nearly as cut-and-dry as health officials would have you believe. There are many unanswered questions that deserve further research, like the possibility that getting repeated annual flu shots may increase your susceptibility to influenza. In a 2013 study, it was found that those who received the influenza vaccine two years in a row got no significant protection during the current flu season.7,8

Another finding revealed by the study was that vaccination failed to prevent household transmission once influenza was introduced, with adults being particularly at risk despite vaccination.9 Again in 2019, researchers found that vaccine effectiveness was lower against certain types of flu (H3N2 and B) for those vaccinated during two consecutive flu seasons compared to those vaccinated in the current season only.10

The possibility that repeated vaccination may reduce effectiveness, and that annual vaccination may increase aerosol viral shedding, demands more investigation. According to the featured study researchers:11

“The association of current and prior year vaccination with increased shedding of influenza A might lead one to speculate that certain types of prior immunity promote lung inflammation, airway closure, and aerosol generation. This first observation of the phenomenon needs confirmation.

If confirmed, this observation, together with recent literature suggesting reduced protection with annual vaccination, would have implications for influenza vaccination recommendations and policies.”

It’s possible, for instance, that after vaccination, you may become a contagious silent carrier of disease. A person with influenza who fully expresses symptoms of fever, body aches, cough and other signs of respiratory illness would likely stay at home.

However, a vaccinated individual, who is silently contagious, would go to work and into stores and other public places and be unaware they are spreading infection, which can be done even via regular breathing.

This is an especially important fact for vaccinated health care workers, who move freely among patients in hospitals and other medical facilities because everyone assumes vaccinated medical personnel are “immune” to influenza if they get a flu shot every year, even though they could potentially be transmitting influenza to hospital patients, including those in the ICU.

CDC: Virus Shedding ‘Common’ After Receipt of LAIV

Although you may be surprised to learn that virus shedding occurs after certain vaccinations, even the U.S. Centers for Disease Control and Prevention (CDC), in their “Safety of Influenza Vaccines” report for professionals,12 states that shedding of the live attenuated vaccine virus is “common” after receipt of LAIV. They cite numerous studies confirming as such, including:

  • Among 345 LAIV3 recipients, 29% had detectable virus in their nasal secretions, with maximal shedding occurring within two days of vaccination13
  • In a study of 200 children aged 6 months through 59 months, 79% shed at least one vaccine virus; shedding was most common among younger children, with 89% of 6- to 23-month-olds shedding at least one vaccine virus14

The live influenza vaccine is FluMist, which is approved for nonpregnant women as well as anyone aged 2 to 49 years. It’s administered in the form of a nasal spray.

While the CDC states that the live type A and B vaccine strain influenza viruses in FluMist are too weak to actually give recipients influenza, according to the CDC, “transmission of shed LAIV vaccine viruses from vaccine recipients to unvaccinated persons has been documented …”15

MedImmune, the company that developed FluMist, is also aware that the vaccine sheds vaccine-strain virus. In its prescribing information, they describe a study on the transmission of vaccine-strain influenza viruses from vaccinated children to nonvaccinated children in a day care setting.

In 80% of the FluMist recipients, at least one vaccine-strain influenza virus was isolated anywhere from one to 21 days following vaccination. They further noted, “One placebo subject had mild symptomatic Type B virus infection confirmed as a transmitted vaccine virus by a FluMist recipient in the same playgroup.”16

How Often Does Vaccine Virus Transmission Occur?

According to the CDC, “The estimated probability of transmission of vaccine virus within a contact group with a single LAIV recipient in this population [a child care center] was 0.58%,” however in a child care setting it’s likely that multiple children would have received LAIV at any given time.17

However, there’s no way to know for sure how often vaccine-strain live virus shedding and disease transmission actually occurs, since it’s not being actively monitored or tested for.

Barbara Loe Fisher, co-founder and president of the National Vaccine Information Center (NVIC), wrote a special report, “The Emerging Risks of Live Virus and Viral Vectored Vaccines: Vaccine Strain Virus Infection, Shedding and Transmission,” which contains over 200 references and delves into virus shedding and vaccine virus shedding. She noted:18

“There is no active surveillance and testing for evidence of vaccine strain live virus shedding, transmission and infection among populations routinely being given multiple doses of live virus vaccines, including measles vaccine. Therefore, it is unknown exactly how many vaccinated children and adults in the U.S. or other countries are shedding and transmitting vaccine strain live viruses.

Whether or not vaccine strain live virus shedding, transmission and infection is causing undiagnosed or misdiagnosed health problems, especially among people with severe immune deficiencies or autoimmune and other immune system disorders, is an open question.”

A Live Attenuated COVID-19 Vaccine Is Being Produced

The featured study has implications for COVID-19, which is still a mystery in terms of transmission and treatment. It’s unknown, for instance, if Sars-CoV-2, the virus that causes COVID-19, has airborne transmission potential in addition to being spread by more direct contact or droplets.

Airborne transmission appears likely,19 however, raising even more concerns considering the development of a live attenuated COVID-19 vaccine has begun.

In June 2020, biotechnology company Meissa Vaccines announced that it has initiated preclinical studies and manufacturing of a live attenuated COVID-19 vaccine,20 which was derived by modifying the company’s live attenuated RSV vaccine candidate.

Whether a live attenuated COVID-19 vaccine will end up causing recipients to shed vaccine-strain virus into the air remains to be seen, but there are inherent risks. Even Martin Moore, Ph.D., co-founder and CEO of Meissa Vaccines, told BioSpace about the potential for virus shedding. BioSpace reported:21

“Moore explained that if a virus is attenuated based on one or two gene mutations, the virus could revert to being infectious in the vaccine recipient and they can shed live virus, spreading it to others. This is the worst-case scenario and why there are such strict safety standards for vaccines, especially LAVs [live attenuated vaccines].

‘Safety is critical, there are no cutting corners with safety,’ explained Moore. ‘Coronaviruses, in particular, are prone to genetic recombination, so using a live attenuated coronavirus in a vaccine would run the risk of becoming infectious again.’”

In terms of both influenza and any novel COVID-19 vaccines licensed by the Food and Drug Administration (FDA) and recommended by the CDC, it’s important to be aware of the differences between attenuated live virus vaccines and inactivated vaccines, especially if you’re in a vulnerable population, such as very young children, the elderly, pregnant and breastfeeding women and people with acute or chronic health problems or a compromised immune system.

There remain many unanswered questions regarding live virus vaccines and their ultimate impact on public health.

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