Both the Covid-19 vaccine from Pfizer Inc. and Germany’s BioNTech SE and the one from Moderna Inc. require two doses, administered three or four weeks apart. In the U.S., President-elect Joe Biden plans to release all available doses right away, rather than holding back supplies for a second shot. The proposed departure from the Trump administration’s policy comes as U.K. authorities have decided to delay the second shot.
Here is what we know and don’t know about deviating from the recommended schedule.
Why do the vaccines require two doses a specified interval apart?
It takes two doses of the Pfizer-BioNTech and Moderna vaccines to fully prepare the immune system to guard against Covid-19, study results showed. The first shot helps the body recognize the virus and primes the immune system to defend against it, while the second shot strengthens that immune response, scientists say.
Researchers assessed the effectiveness of that two-dose regimen, spaced three or four weeks apart depending on the vaccine, in trials with more than 30,000 subjects. In Moderna’s late-stage trials, the first dose provided around 80% to 90% efficacy after 28 days. Two weeks after the booster shot, the efficacy had reached 94.1%, according to study results.
Pfizer said the late-stage trial found its shot was about 52% effective after the first dose, with study results showing protection starting as early as 12 days after receiving it. A week after the second dose, the shot’s effectiveness rose to 95%, Pfizer said its study found.