Science is mixed on this, Favini explains. “There’s some evidence that getting a flu shot too soon in the season (in August or September, for example) might leave you with waning immunity to flu in January and February,” he says. “This is probably especially true for people over 65 who don’t tend to have as strong of an immune response to vaccines.” This is in line with research on when to get a flu shot published in the American Journal of Preventive Medicine out of the University of Pittsburgh School of Medicine. Their work found that tens of thousands of cases, as well as hundreds of deaths, could likely be avoided if older adults waited to get immunized.