As of March 8, well over 90 million doses of the three COVID-19 vaccines currently approved for emergency use by the Food and Drug Administration (FDA) have been administered in the United States. 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. (A third option from Johnson and Johnson is a one-dose vaccine.)

Some people have expressed concern over missing a second appointment within that window as a result of distribution issues or logistical problems. 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?

While it’s not ideal, many health experts are advising that it’s nothing to panic over. 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. That extra time should allow any distribution issues to be worked out and for people to receive a second dose, which are often prioritized by providers in order to complete the proper vaccination schedule.

The Pfizer and Moderna vaccines work by instructing the body to create a spike protein unique to the virus, which will then trigger an immune response. The second shot bolsters the antibody response and should be given while the immune system “remembers” the initial dose, both to increase immunity and the duration of protection. A delay of a few weeks isn’t likely to matter, but skipping the second dose voluntarily is a bad idea. Protection from just one dose of the Pfizer or Moderna vaccines is limited and will not provide as strong or as long of an immune response as the two doses mandated in clinical trials. While some studies have demonstrated up to 85 percent effectiveness against symptomatic cases after one dose of Pfizer after four weeks, the duration of such protection is unclear, and there are currently no recommendations to stray from the two-dose regimen.

The best strategy is to do what you can not to miss a second dose, but not worry if it winds up being a few weeks late. Passing on it entirely, however, would be highly unadvisable.

[h/t 10 News San Diego]