https://www.scientificamerican.com/article/is-it-safe-to-delay-a-second-covid-vaccine-dose/

Vaccine shortages and distribution delays are hampering efforts to curb the SARS-CoV-2 pandemic. So some scientists have suggested postponing the second shots of two-dose vaccines to make more available for people to get their first doses. The original recommended interval was 21 days between doses for the Pfizer vaccine and 28 days for the Moderna shots, the two currently authorized in the U.S. Now the U.S. Centers for Disease Control and Prevention has updated its guidance to say that people can wait up to 42 days between doses, though the agency still advises individuals to stick to the initial schedule. And developers of the University of Oxford–AstraZeneca vaccine—which is authorized for use in the U.K.—suggest even longer stretches are possible, saying their shot performs better when its doses are spaced 12 weeks apart. Their data is in a new preprint paper, released before peer review. So what gives? How long can you go on a single shot and still stay safe? And what happens if your second shot isn’t available on time? Scientific American explores the potential risks and benefits of delaying vaccine doses.

Why do you need two shots?

Vaccines are designed to create immunological memory, which gives our immune system the ability to recognize and fend off invading foes even if we have not encountered them before. Most COVID vaccines elicit this response by presenting the immune system with copies of the novel coronavirus’s spike proteins, which adorn its surface like a crown.