The best time to get a flu shot is when you haven't had antibiotics recently, a new study has found, because healthy gut bacteria protect immunity.

Planning on getting an influenza vaccination this year? A new study led by immunologist Bali Pulendran, PhD, and his Stanford colleagues, in collaboration with researchers at several other institutions, suggests you get that shot when you're not just coming off a course of antibiotics, right in the middle of such a regimen or just about to begin one.

Planning on not getting a seasonal flu shot this year? The same study hints that you're better off getting one, in case the real bad bug comes along just when when your gut-bacterial population happens to be depleted -- due, say, to your recent use of antibiotics.

The study, published in Cell, showed that decimating healthy adults' trillions-strong population of gut-resident bacteria by subjecting them to a round of antibiotic treatment reduced a standard measure of their immune systems' responsiveness to influenza vaccination. From my release:

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... "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.

The findings 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.

It also seems to make sense that infants and older people probably shouldn't be getting vaccinated just before, during or just after a course of antibiotics. Children in almost every country get a standard battery of childhood vaccines within the first 15 months of life -- and not rarely, rounds of antibiotics along the way. In adulthood, we receive occasional booster shots for diphtheria, tetanus and so forth and, often, annual influenza shots as well. Older people can now get vaccinated for shingles and pneumococcus.

Pulendran offered some advice for old, young and everyone in between. "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 just happen to be in short supply.

Photo by Heather Hazzan/ American Academy of Pediatrics and SELF Magazine