While a three- or four-week gap between shots is ideal, the U.S. Centers for Disease Control and Prevention (CDC) says you can get your second shot within 42 days of the first one and still mount a full immune response. “Beyond that, we start to operate in an area where there’s simply less data,” Ratner says. That doesn’t mean your second shot will be ineffective if it’s given more than six weeks after the first. It only means that studies have not specifically measured how much protection the two-dose vaccines offer when the shots are given more than 42 days apart. Still, the CDC says you don’t have to start over if you can’t get a second vaccine within 42 days.