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Delay the Second Covid Vaccine Shot? What Experts Say

U.K. Ramps Up Covid Vaccine Rollout With Hospitals Under Strain

Photographer: Dominic Lipinski/PA Wire/Bloomberg

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As countries contend with limited supplies of Covid-19 vaccine, governments are grappling with whether to prioritize getting a first shot into as many arms as possible, or to take a slower approach that reserves some vials to deliver a second dose at the interval prescribed for attaining maximum protection. The U.K. has adopted the approach, calculating that vaccinating twice as many people is preferable to vaccinating half as many in the same time even if a smaller group’s protection would be stronger. Some scientists have cautioned that untested timing regimens have the potential to undermine confidence in vaccines and, worse, facilitate the emergence of dangerous new variants.

Covid-19 vaccines made by Moderna Inc., Pfizer Inc.-BioNTech SE, and AstraZeneca Plc are administered in two doses. The recommended timing of the second dose varies depending on the vaccine, and is based on clinical trial data regulators used in their review process. In studies, the second Pfizer-BioNTech dose was given 21 days after the initial shot; Moderna’s was given 28 days later. Doses of AstraZeneca’s vaccine were scheduled 4 to 12 weeks apart across four trials.