Maybe you’re traveling, there’s a natural disaster, or an emergency comes up when you’re supposed to get your precious second shot. What happens if you miss your appointment?

While the Johnson & Johnson vaccine is effective after only one dose, Pfizer and Moderna require two shots in order to gain the full protective benefit. And the timing between these doses is important.

The recommended interval between the first and second dose is:

• Three weeks (21 days) apart for Pfizer-BioNTech
• Four weeks (28 days) apart for Moderna

And though it's best to get your second dose on time, stuff happens. If a delay is unavoidable, missing this window doesn't mean you've missed your only chance of getting your second dose. It also doesn't mean you need to start your vaccination plan over completely. According to the CDC, when following the recommended guidelines isn't feasible, the second dose of either the Pfizer or Moderna vaccine can be administered up to six weeks after the first one.

And keep in mind that the Pfizer and Moderna vaccines are not interchangeable. When you’re at your first appointment, don't forget to ask which vaccine you're receiving. This information will help you stay on top of the timing for your second dose, especially if your vaccination site doesn't allow you to schedule your second dose immediately.