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Antibiotics May Affect Some Immune Responses to Flu Vaccine

Disrupting Gut Microbiome Could Be Key

The normal human gut microbiome is a flourishing community of microorganisms, some of which can affect the immune system. In a study reported in Cell, researchers from Stanford University found that oral antibiotics can alter the human immune response to seasonal influenza vaccination.

The research team examined two groups of healthy volunteers. One group of 22 people had high pre-existing immunity to the influenza virus strains in the 2014–2015 seasonal influenza vaccine. The second group, of 11 volunteers, had low immunity to the 2015–2016 seasonal influenza vaccine’s virus strains.

First, half the participants in each group received a five-day course of a broad-spectrum antibiotic regimen that included neomycin, vancomycin, and metronidazole. Then all study participants received a seasonal influenza vaccine. By analyzing stool and blood serum samples taken at various times for a one year after vaccination, the researchers tracked the participants’ immune response to the vaccines as well as the diversity and abundance of the organisms in their gut microbiomes.

It wasn’t a surprise that most participants who received antibiotics had reduced levels of gut bacteria. In addition, among the 2015–2016 participants who had little prior immunity to the seasonal influenza virus vaccine strains, a course of antibiotics hindered their immune responses to one of the three influenza virus strains in the vaccine, an H1N1 A/California-specific virus. This indicates that were they to be exposed to this H1N1 virus after vaccination, these participants would be less protected against infection with that strain than people who had not received antibiotics, according to the authors.

The researchers also found that people who took antibiotics experienced changes to their immune systems that promoted a pro-inflammatory state, similar to a condition seen in older adults who have received influenza vaccines. The investigators believe this pro-inflammatory state is related to the process by which the microbiome regulates the metabolism of bile acid; this process is disrupted if antibiotics diminish the size or diversity of the microbiome. The human microbiomes changes naturally as people age, and the researchers suggest that further research on these pathways could provide insights into why older adults respond differently to influenza vaccination and why they have weaker immune systems overall.

Source: Cell, Sept. 5

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