Two of them—one from Pfizer and one from Moderna—require a two-dose regimen, with a second shot coming 21 to 28 days after the initial injection. Providers are also worried some people may willingly skip a second shot owing to having minor side effects like fever or body aches from the initial dose or scheduling conflicts. So what happens if the second shot is delayed? According to the Centers for Disease Control and Prevention (CDC), the second shot can be delayed up to six weeks following the first shot without affecting the vaccine’s overall efficacy. A delay of a few weeks isn’t likely to matter, but skipping the second dose voluntarily is a bad idea.