The gut microbiome influences the effectiveness of many viral vaccines, including the seasonal flu vaccine, researchers from Emory University in Atlanta said recently in a new publication in the journal Immunity.  Through a toll-like receptor 5 (TLR5)-mediated mechanism, the bacteria enhance the immune response to produce better protection for the host.


Though the vaccines used in the study were developed to induce a response to viral pathogens, and therefore are not known to induce a bacteria-based immune response, the researchers showed that the microbiome is highly activated after injection.  This activation of the microbiome induces upregulation of TLR5, an innate immune response the bacterial protein flagellin.  TLR5 then turns on B-cell production of antibodies, which attack the components of the vaccine, developing host immunity towards the particular virus in the vaccine.

Importantly, each of the vaccines in the study had little to no adjuvant present in them.  Adjuvants are used to increase the immune response, targeting the immune system to the injection site.  Once there, the immune system begins a coordinated attack on the parts of the vaccine, including the viral component.  Without adjuvants, the immune system usually has a hard time reacting to the small amount of virus in the vaccines, which is one reason why the effectiveness of the seasonal flu vaccine varies from person to person.

Through coordinated efforts with microbe-free and fully colonized mice, the researchers showed that the microbiome induces the immune response against viral components of the vaccine when the vaccines contained no adjuvant.  When the vaccines contained an adjuvant, as is the case with the polio vaccine, the microbiome was not upregulated and the immune system developed a response separate from TLR5.  The effects “may be more relevant to inactivated viral vaccines that contain weak or no adjuvants more than others,” said lead author Bali Pulendran.

“[The results are] paradigm-shifting [in] how they make us think about the interactions between vaccines and the microbiome,” said immunologist Bonnie Blomberg of the University of Miami Miller School of Medicine who was not involved with the work.  Indeed, this research shows how the coevolution of man and his microbiome work together to ward of invading pathogens like viruses.  Future studies will work to show how to develop the “best” immune system in order to develop the most efficient vaccines, perhaps based on the complexities of an individual’s particular microbiome. For more infectious disease news and information, visit and “like” the Infectious Disease News Facebook page