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.