Inactivated

Because influenza in some countries can circulate year-round, there’s no particular flu season as in more temperate climates. The women were therefore vaccinated at various times with a shot that contained three inactivated flu strains. Each group was followed for up to 180 days to see whether they developed fevers and body aches.

This was the case not just with the flu vaccine, but also with an inactivated polio vaccine. The state of intestinal bacteria appeared to make no difference in the effectiveness of other vaccines, though, including the yellow fever vaccine and vaccines containing adjuvants, which are added to boost immunity.