New research is bolstering the case for delaying second doses of coronavirus vaccines.

Why it matters: Most vulnerable Americans remain unvaccinated heading into March, when experts predict the more infectious virus variant first found in the U.K. could become dominant in the U.S.

By the numbers: Around 41% of adults 65 and older have been vaccinated, per KFF. Millions more Americans with underlying health conditions also remain vulnerable to severe disease.

Driving the news: An analysis of the real-world outcomes of the Pfizer vaccine in Israel, released earlier this week in the New England Journal of Medicine, found that the vaccine was 60% effective at preventing documented coronavirus infection 21 to 27 days after the first dose, and 92% effective 7 or more days after the second dose.

• But one shot was much more effective at preventing severe disease and hospitalization during the same time period, with respective efficacies of 80% and 78%. Two doses were 92% effective at preventing severe disease and 87% effective at preventing hospitalization.
• An analysis published in The Lancet of the AstraZeneca vaccine, which isn't yet authorized for use in the U.S., found that a longer interval between doses actually gives more protection than a shorter interval.
• And a growing body of research suggests that people who have previously been infected with COVID are sufficiently protected by only one dose, per the NYT.

The Food and Drug Administration this week released its findings on the efficacy of the one-shot Johnson and Johnson vaccine, which increased over time.

• After 28 days, the shot was 85.9% effective at preventing severe disease in the U.S.

What they're saying: There's now enough data to support delaying second doses of the Pfizer and Moderna vaccines, in order to give more people more protection faster.

• And the variant makes doing so urgent, a group of public health experts argue in a white paper, released this week, calling for the FDA and the CDC to review the data.
• "There is a narrow and rapidly closing window of opportunity to more effectively use vaccines and potentially prevent thousands of severe cases, hospitalizations, and deaths in the next weeks and months," the paper, published by the University of Minnesota’s Center for Infectious Disease Research and Policy, argues.
• The writers suggest giving people 65 and older vaccine priority, deferring second doses until after the predicted virus surge, deferring second doses for people with confirmed previous COVID infections and authorizing the use of half-doses of the Moderna vaccine.

The other side: Scottish researchers released a preprint of a study that suggests protection from a single dose may decline after five weeks, Insider reports.

• "I think delaying the second dose for a considerable period of time is a mistake," Paul Offit, the director of the Vaccine Education Center at Children's Hospital of Philadelphia, told Insider.