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Pfizer's COVID-19 vaccine effective after 6 months and works against problem variant

Vials of the Pfizer-BioNTech Covid-19 vaccine.
(Image credit: PATRICK T. FALLON/AFP via Getty Images)

The COVID-19 vaccine developed by Pfizer and BioNTech remains highly protective against the novel coronavirus for at least six months after the second dose, the companies announced Thursday (April 1).

This analysis, based on an ongoing trial of more than 44,000 volunteers, included 800 participants in South Africa, where a coronavirus variant with some ability to evade antibodies, called B.1.351, is causing the majority of new cases. Nevertheless, the vaccine still appeared highly protective against this variant, according to a statement from Pfizer

Supporting this trial data, laboratory studies of vaccinated individuals' blood also showed that the shot induces a robust neutralizing antibody response against B.1.351, according to a report published March 8 in The New England Journal of Medicine.

Related: Quick guide: COVID-19 vaccines in use and how they work 

"The high vaccine efficacy observed through up to six months following a second dose and against the variant prevalent in South Africa provides further confidence in our vaccine’s overall effectiveness," Pfizer chairman and chief executive officer Albert Bourla said in the statement. 

The vaccine has now been evaluated in more than 44,000 participants ages 16 years and older, and more than 12,000 vaccinated participants have now had at least six months of follow-up after their second dose. The companies plan to submit these data for publication in a scientific journal "in the near future." 

Based on the company statement, the vaccine shows 91.3% efficacy against symptomatic COVID-19 overall, and 95.3% efficacy against "severe COVID-19 as defined by the U.S. Food and Drug Administration (FDA)," the company reported. The FDA definition of severe COVID-19 includes a variety of symptoms, such as an elevated heart rate, raised breathing rate indicating respiratory distress, blood oxygen saturation of 93% or lower, liver dysfunction, or the need for supplemental oxygen or admission to an intensive care unit, according to CNN

The U.S. Centers for Disease Control and Prevention (CDC) has a slightly different definition of severe COVID-19, namely requiring a blood oxygen level of 94% or lower and evidence of pneumonia in the lungs, as detected by X-ray. By the CDC standard, "the vaccine was 100% effective against severe disease," Pfizer said.

In total, the company recorded 927 confirmed symptomatic cases of COVID-19 in its late-stage trial. 850 of those cases occurred in people who received the placebo shots rather than the vaccine. Of the 77 cases that occurred in the vaccinated people, no cases were severe by CDC standards, and only one was severe by FDA standards, Pfzier reported. 

Nine cases of COVID-19 were reported among the 800 volunteers in South Africa, all of which occurred in the placebo group, "indicating vaccine efficacy of 100%," according to the statement. Given that this analysis includes a relatively small subset of participants compared with the overall trial, this estimate of the vaccine's efficacy is less certain than the overall estimate of 91.3%.  

"In an exploratory analysis, the nine strains were sequenced and six of the nine were confirmed to be of the B.1.351 lineage," which adds to the evidence that the shot protects against the variant, the company noted. 

In addition to these promising efficacy data, "no serious safety concerns were observed in trial participants up to six months after the second dose," according to the statement. "Side effects were generally consistent with previously reported results."

Originally published on Live Science. 

Nicoletta Lanese
Nicoletta Lanese is a science journalist and dancer who aims to bring science to new audiences, whether in print or on stage. She holds degrees in neuroscience and dance from the University of Florida and a graduate certificate in science communication from the University of California, Santa Cruz. Brains are her beat. Follow her on Twitter @NicolettaML.