Taking your best shot: Antibiotics appear to reduce flu vaccine effectiveness

September 10, 2019

It’s almost that time of year again, but before you roll up your sleeve for that flu shot, pay attention to a warning released on September 5 by Stanford University School of Medicine: A study conducted among healthy adults suggests that antibiotics may reduce the effectiveness of the flu vaccine.

The depletion of gut bacteria by antibiotics appears to leave the immune system less able to respond to new challenges, such as exposure to previously unencountered germs or vaccines, according to Stanford’s Bali Pulendran, Ph.D., professor of Pathology and of Microbiology and Immunology at the school.

“To our knowledge, this is the first demonstration of the effects of broad-spectrum antibiotics on the immune response in humans — in this case, our response to vaccination—directly induced through the disturbance of our gut bacteria,” Pulendran said in a Stanford University news release.

The idea that the trillions of bacteria inhabiting the human gut play a role in our health is far from new, but it hasn’t been rigorously proved. Hard data in humans has been sparse, with causal evidence coming mainly from studies in mice.

The antibiotics lowered the gut-bacterial population by 10,000-fold. The resulting loss of overall diversity was detectable for up to one year after the antibiotics were taken. Still, 30 days after vaccination, vaccine-induced increases in antibodies capable of preventing influenza infection were comparable among the two groups.

But the participants in this experiment tended to have pretty high levels of those antibodies to begin with, suggesting they’d already had some exposure to the flu strains represented in the current or prior seasons’ vaccines.

 “The study indicates that when it comes to responding to vaccination against a previously encountered infectious pathogen, our immune systems are remarkably resilient even in the face of the most severe depletion of our intestinal bacteria,” Pulendran said. “But they seem to lose this resilience when confronted with a vaccine containing new pathogenic elements of which they have little or no prior memory.”

The findings, Pulendran said, imply that when next season’s flu strain comes along, you want your gut-resident microbes to be in full bloom in order for your immune system to rise to the occasion. Pulendran offered some advice. “Get your annual flu shot,” he said. “The greater your inventory of immune memory to influenza strains bearing any resemblance to the one that’s coming over the hill, the more likely you’ll be able to deal with it, even if your gut microbes are in short supply.”

The study findings were published earlier this month in the journal, Cell. Pulendran is the senior author. Lead authorship is shared by Stanford postdoctoral scholars Thomas Hagan, PhD, and Mario Cortese, PhD; and Nadine Rouphael, MD, PhD, associate professor of medicine and infectious disease at Emory University.

Research contact: @Stanford

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