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Pfizer-BioNTech Announces They Will Test a Third Dose of Their COVID-19 Vaccine

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Pfizer-BioNTech has begun testing a third dose of its COVID-19 vaccine in a small group of people. The vaccine is currently authorized in several countries in a two-dose regimen, given 21 days apart, and has proven in studies to be about 95% effective in protecting against COVID-19 disease. But as new (and more infectious) genetic variants of the COVID-19 virus start circulating—so far, scientists have identified three major strains, first found in the U.K., South Africa, and Brazil, respectively—questions about how well authorized vaccines can still thwart the virus have become more urgent.

Pfizer-BioNTech have already done studies testing blood serum from people vaccinated with their original shot against some of these variants, and found that the immune response triggered by the two doses is still quite protective. In fact, in the case of the U.K. variant (referred to as B.1.1.7), levels of antibodies produced by the vaccine regimen, an important immune defense against the virus, were similar to levels against the original, non-mutated virus. However, the researchers found that the level of antibodies produced by the original vaccine were lower against the South African variant (referred to as B.1.351). Pfizer-BioNTech hasn’t yet completed the same level of rigorous testing against the Brazilian variant (referred to as P1).

“So far we haven’t heard any reports of breakthrough cases [among vaccinated people] related to any of the new variants,” says Mikael Dolsten, chief scientific officer at Pfizer, referring to serious infections with the variant viruses among vaccinated people. “But we always want to be one step ahead of the virus. That made us think about generating data on a third boost. We are going to give people who were part of our Phase 1-2 study from last summer a boost with another dose of the current vaccine. We expect that the antibody levels may rise against current variants to strengthen protection.”

Only a clinical trial will prove that. Pfizer-BioNTech plans to vaccinate 144 people who were enrolled in the companies’ early phase studies, who received their second dose between six and 12 months ago.

In the event that the additional dose does not enhance people’s protection, Dolsten says Pfizer has also developed a new vaccine targeting the South African variant, which has shown the most resistance to vaccine-induced immunity so far. Because the mRNA technology on which the original vaccine was built only requires the right genetic sequence to build a new version, Dolsten says developing a new vaccine targeting the South African variant only took six to eight weeks. In about a month or so, he says, Pfizer plans to begin a trial in which researchers will give participants who got two doses of the original vaccine a third dose—of the new variant vaccine.

It’s all part of a dry run for what might become a routine process in coming years: shifting to new versions of the vaccine after a few months, or a year or so, similar to the way seasonal flu vaccines change depending on which strains are circulating each influenza season. Producing the new COVFID-19 vaccine and testing it, Dolsten says, “paves the way for more rapid changes in the future. If the virus mutates beyond what we see today, we will already have data on how to do a strain shift and rapidly change production. We could just feed the production process with a different mRNA and everything else would be the same.”

He stresses that the original vaccine that received U.S. Food and Drug Administration emergency use authorization continues to protect well against all variants of the virus, but that all of these measures are ways to “constantly have options to stay ahead of the virus and move very fast if we see less protection against any new [mutant] strains.”

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