A new study in 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. This is the first demonstration of the effects of broad spectrum antibiotics on the immune response in human in this case, our response to vaccination directly induced through the disturbance of our gut bacteria.
The idea that the trillions of bacteria inhabiting our guts play a role in human 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 new study was inspired by a mouse study that Pulendran and his colleagues conducted in 2011.
Effectiveness of the flu
Those investigators found that mice raised from birth to have germ-free intestinal tracts failed to mount as strong an immune response to vaccination as their normal counterparts. So did mice given antibiotics or bioengineered to lack an immune sensor for flagellin, the chief protein constituent of the threadlike flipper that bacteria use for swimming around.
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 taking. 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. But this time, they selected only individuals; whose low level of flu antibodies indicated low prior exposure to the virus or to the vaccine itself.
None of the new recruits had getting flu vaccinations for at least the past three years. Five individuals got broad-spectrum antibiotics, as in the previous year. The other six served as controls. All 11 getting vaccinating. Again, gut bacteria counts in individuals who received antibiotics plummeted, as in the previous year. But this time there was a big change in levels of an antibody subtype; most responsible for countering the influenza virus; This subtype failed to burgeon in the blood in response to the vaccine.