The second dose of Pfizer–BioNTech’s coronavirus disease (COVID-19) vaccine should ideally be given three weeks after the first. What should you do if you miss your second appointment? Schedule another one as soon as you can, says Adam J. Ratner, MD, a pediatric infectious disease doctor at NYU Langone Health and a vaccine researcher.

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,” Dr. Ratner says.

“I would say get the second dose now and consider yourself fully vaccinated,” he tells TIME. Just make sure you get a second dose of the same vaccine because the CDC does not recommend mixing and matching with different shots.

In order to get the full protection from your vaccine, getting the second dose is crucial.

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