Single Pfizer, Moderna vaccine dose nearly as effective as two

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Single doses of the Pfizer and Moderna vaccines are more than 92 percent effective in preventing COVID-19 illness after two weeks, Canadian researchers are now saying.

The FDA’s own data show that a single shot of the BioNTech-Pfizer vaccine is 92.6 percent effective after two weeks, and a single Moderna jab is 92.1 percent effective, the researchers note in the New England Journal of Medicine.

Getting that second shot of Pfizer’s vaccine hikes the efficacy only marginally, to 94 percent, according to a separate study based on real-world data from Israel’s vaccination program.

And so the prescribed second doses should be given instead to those in priority groups who are still waiting for their first shot, “given the current vaccine shortage,” the researchers urge.

“With such a highly protective first dose, the benefits derived from a scarce supply of vaccine could be maximized by deferring second doses until all priority group members are offered at least one dose,” the researchers say in a letter to the NEJM editors.

“There may be uncertainty about the duration of protection with a single dose,” the researchers said.

“But the administration of a second dose within 1 month after the first, as recommended, provides little added benefit in the short term, while high-risk persons who could have received a first dose with that vaccine supply are left completely unprotected.”

A health worker receives a dose of the Pfizer-BioNTech vaccine against COVID-19 on February 18, 2021.
A health worker receives a dose of the Pfizer-BioNTech vaccine against COVID-19 on Feb. 18, 2021.
RAUL ARBOLEDA/AFP via Getty Images

The letter was written by Dr. Danuta M. Skowronski of the British Columbia Centre for Disease Control in Vancouver and Dr. Gaston De Serres of the Institut National de Sante Publique du Quebec in Quebec City.

In a letter to NEJM responding to the two researchers, Pfizer stressed that “alternative dosing regimens” still need to be evaluated.

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