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Single Pfizer Shot Staves Off Covid, but for How Long Unknown

Dec. 9, 2020, 7:58 PM

A single dose of Pfizer’s Covid-19 vaccine appears to be effective more than half the time, but doctors advise against skipping the second shot because of uncertainty over how long that will provide immunity.

A Food and Drug Administration advisory panel of doctors and scientists will meet Thursday to discuss whether the agency should grant an emergency use authorization for the Pfizer vaccine. That vaccine, which requires two shots, is effective roughly 95% of the time. However, a new FDA report shows just one dose appears to be 52.4% effective, said Matthew Laurens, a principal investigator for the phase 3 Moderna vaccine trial based out of the Center for Vaccine Development and Global Health at the University of Maryland School of Medicine.

Single dose vaccines, such as the one developed by Johnson & Johnson, may be preferable to one requiring two shots because historically nearly half of people skip the second inoculation. The 52% effectiveness from a single dose of Pfizer’s vaccine would pass the threshold the FDA initially set for considering coronavirus vaccines.

However, there’s no data on how long that single dose of Pfizer’s vaccine would provide immunity. That tamps down chances the FDA would recommend people just get one dose of Pfizer’s vaccine, versus two, said Craig Spencer, an assistant professor of emergency medicine at the Columbia University Medical Center who treats Covid-19 patients in New York City.

“Getting that booster dose is important for stimulating a more long lasting immunity with higher levels of neutralizing antibodies,” Spencer said. “So it may be that this number may look OK, but it’s a mirage. We need to be focusing on making sure we can implement the vaccination as initially intended and set up in this clinical trial.”

Federal health officials also suggested more research is necessary before signing off on a single Pfizer dose.

“I think there will be some data presented at FDA’s meeting and CDC’s meeting later this week about how much protection there is between those two first two doses but it’s a limited period of time,” Nancy Messonnier, director of the National Center for Immunization and Respiratory Diseases at the Centers for Disease Control and Prevention, said at an event this week.

“So if you get your first shot on day one and your second shot on either day 21 or 28, there may be some protection in those limited three or four weeks, but we have no idea if you got the first shot that if you have any protection three or four months later,” she said.

Misinterpreting Report

The FDA’s report says because of “the number of cases accumulated after Dose 1 and before Dose 2, there does seem to be some protection against COVID-19 disease following one dose; however, these data do not provide information about longer term protection beyond 21 days after a single dose.”

Many people are misinterpreting the report to say one dose is 82% effective, but a closer look at the data shows the first shot is just 52.4% effective, Laurens said.

“The primary endpoint for this Pfizer mRNA vaccine trial was to assess vaccine efficacy by counting cases of Covid-19 that occurred at least seven days after the second dose was given,” Laurens said.

There were other efficacy analyses the company looked at, including comparing all cases that occurred after the first dose, which showed the vaccine was 82% effective. But that “includes any cases occurring after dose 1, plus any cases occurring after dose 2,” he said. A better representation of how well the vaccine works in just one dose is when the company compared all Covid-19 cases that occurred between the first and second dose, which shows just one dose is 52.4% effective, he said.

“I’m absolutely not suggesting the vaccine should be a one-dose vaccine,” Moncef Slaoui, the chief adviser to Operation Warp Speed, told reporters Wednesday. Slaoui was referring to the Pfizer and Moderna vaccines, which are meant to be administered in two doses.

—With assistance from Jeannie Baumann

To contact the reporter on this story: Jacquie Lee in Washington at

To contact the editors responsible for this story: Fawn Johnson at; Andrew Childers at