Things get squishier for vaccinated people with relatively healthy immune systems. They’ll already be flush with newly minted B and T cells, which lie in wait to produce antibodies and attack the coronavirus. Ali Ellebedy, an immunologist at Washington University in St. Louis, told me that the longer those cells mature in the body, the more prepared they are to fight off the invader. Delivered too early, another dose of the vaccine could end up “restarting something that was already working,” he said. Ellebedy recommended delaying any booster shots by at least six months from your initial course of vaccination. Eight months is better; even a year would be fine.

At the same time, booster shots do increase the measurable level of antibodies in the blood, pretty much whenever they’re received. The clinical benefits of this spike for fully vaccinated people remain unclear, though some preliminary evidence suggests that an antibody surge could reduce your chances of getting sick, or of transmitting the Delta variant to other people—at least until your antibody levels wane once more.

Most people’s antibody levels peak a few weeks after their initial COVID-vaccine shots. If that holds true for boosters, too, then you might be tempted to time your next injection for three-ish weeks before you’d most want to be protected. Maybe the virus surged in your county last December, and you’re afraid it’ll do the same this year—so you decide to get your booster around Veterans Day. Maybe you want to make sure you don’t infect Uncle Dave at Thanksgiving—so you make an appointment for Halloween.