Is Flu Shot Effectiveness Determined in Your Gut?

By Lillian So Chan

Studies have revealed an unappreciated but significant role of the gut microbes in modulating vaccine immunity. In particular, data show that the effectiveness of the flu shot in inducing antibody response and protection is controlled by the gut microbiome.

Emerging scientific evidence shows that the gut microbiome plays diverse roles in influencing our health. One of the most notable of these roles involves the development and function of the immune system. 

Researchers have reported that immunity to infection can be impacted by the gut microbiome. Specifically, the depletion of the gut microbiome has been shown to enhance susceptibility to viral infections both systemically and in the lung.

Influenza affects millions of people globally, and despite it being the most widely targeted viruses through annual vaccination programs, considerable rates of morbidity and mortality persist. Neither protection nor effectiveness is complete, and a significant proportion of those vaccinated, especially among children and elderly, remain susceptible to flu infection.

In addition to the age factor, another important factor affecting flu shot effectiveness for a given flu season is the status of pre-existing immune memory – whether the flu shot recipient’s body recognizes and remembers the circulating virus from previous exposure.

A team of international researchers from the US, France, and Brazil therefore conducted a series of studies to examine immune responses induced by TIV, one of the two flu shots approved by the US FDA. (TIV, Trivalent inactivated flu vaccine, is derived from three different strains of the flu virus.)

Using both germ-free mice and antibiotics, they demonstrated that:
  • Immune response induced by the flu shot depends on the gene expression of a specific cell surface receptor
  • But the flu shot did not directly signal to induce immunity through this receptor
  • Gut microbiome is crucial in induction of rapid antibody responses after the flu shot
  • Gut microbiome is necessary to mediate immunity
  • Antibiotics deplete gut microbes and impair antibody responses induced by the flu shot
  • On the other hand, re-establishment of gut microbiome promotes antibody responses to flu shot
  • Multiple types of gut microbial communities, rather than specific bacterial species, may be required to support flu shot effectiveness

The team suggested that an important implication of their findings for global public health is that the status of an individual’s gut microbiome may be a critical determining factor in flu shot effectiveness. Also, alteration of gut microbiome through antibiotic use could negatively impact flu shot protection.

The protective effects of gut microbiome against flu infection were first identified more than a decade ago. Since then it has gradually become known that these effects depend on immunoregulatory cells and factors circulating between the gut and the lung.

More recent studies have confirmed that beneficial gut bacteria can indirectly protect against flu infection by interacting with the host’s immune system.

It has also been shown that mice treated with a product present on the exterior surface and shed by bacteria called bacterial lipopolysaccharide can trigger antiviral response to provide direct protection against flu infection.

Together, data to date show that a healthy gut microbiome is crucial in protecting against flu infection and impact the effectiveness of flu shots. 

The gut microbiome forms a dynamic environment that can be altered by flu infection, or the administration of antibiotics and probiotics. The use of probiotics is currently being investigated not only as an adjunct to boost immune protection against flu infection, but also as probiotic vaccine potentially capable of protection against different strains of the flu viruses.

Link: WellnessOptions Website

About the Author

Lillian So Chan is the founding editor of WellnessOptions, a print magazine and website, and author of the book WellnessOptions Guide to Health published by Penguin Books. With over thirty years of experience in journalism and editing, Lillian has established unique editorial directions for several award-winning publications. She has worked for Maclean’s, Canada's largest news magazine, and served as a Governor and Deputy Chairperson of the Board of Governors at the Simon Fraser University, British Columbia, Canada.

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