There is a growing debate over a controversial plan to stretch the limited number of available vaccine doses.
Some experts now say that it may be worthwhile to prioritize vaccinating as many people as possible with their first shot of either the Pfizer or Moderna vaccine and delay giving people their second dose.
The Pfizer and Moderna vaccines both require two shots several weeks apart in order to achieve about 95% effectiveness against COVID-19.
But officials in the UK are stretching that protocol to as much as 12 weeks, in order to spread the vaccine among more people.
Dr. Bob Wachter, Chair of the Department of Medicine at UC San Francisco, tells KCBS Radio that he now embraces that idea.
"The slow rollout and the presence of this new variant, which is more contagious than the old COVID virus we’ve been battling for the last year, has convinced me that we would be better off collectively if we give people the first shot, which seems to get you to close to 90 percent effectiveness, probably between 80 and 90%. The second shot gets you to 95%."
FDA analysis of the Pfizer vaccine found that the first shot is about 50% effective on its own, with efficacy increasing to nearly 95% after the second dose. However, the vaccine takes at least a week to begin working, and Dr. Wachter says that it is possible that the 50% efficacy found in the three-week period between the first and second shots is artificially low because of that lag.
"If you looked at the curves from those studies, you saw that about 10 days after the first shot it began to work and by the time the day before you got your second shot it was already working 85 or 90%."
Pfizer is pushing back against the proposal, emphasizing that its trials only studied the effectiveness of giving patients two shots.
"There is no data to demonstrate that protection after the first dose is sustained after 21 days," the company warned.
Dr. Wachter said patients should still get the second dose, calling the delay a necessary tradeoff.
"We want people to get two shots, but in terms of the rollout, we think at this point it makes more sense for people to get the first shot and get as many people, particularly high risk people, to get the first shot as quickly as possible and then start doing the second shots after a delay of a month or two."
The country’s top infectious disease expert Dr. Anthony Fauci has confirmed that some U.S. officials are contemplating the idea but warned against it, telling CNN: "I would not be in favor of that."
Dr. Eric Topol, founder and director of the Scripps Research Translational Institute has been one of the loudest voices arguing against a single shot protocol, saying the only hard data available is on the two-dose protocol.
Dr. Topol said switching to a single shot protocol does not solve the underlying issues of the slow rollout, and that governments should instead focus on increasing the number of people vaccinated per day.
The second shot not only increases efficacy, but likely extends the length of protection as well.