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Pfizer said Saturday that using new versions of its Covid-19 vaccine as boosters led to a superior antibody response against the Omicron variant compared to its current shot.

The results in some ways mirror those released by Moderna earlier this month. Data from both companies will be evaluated on Tuesday by a panel of experts convened by the Food and Drug Administration in the hopes of deciding what strains of the SARS-Cov-2 virus should be included in booster shots for the fall. Companies will need lead time to manufacture doses of new vaccines if it is decided they are needed.


In briefing documents posted by the FDA ahead of the meeting, FDA researchers say that either of Pfizer’s Omicron-containing booster “improves the neutralizing antibody response to Omicron BA.1 compared to the prototype vaccine and does not negatively
affect the neutralizing antibody response against the ancestral strain of virus against which the prototype vaccine was designed.” The documents also say there are “caveats to be noted relevant to the interpretation of these data” that will be discussed at Tuesday’s meeting.

At the FDA meeting, experts will vote on whether boosters should include an Omicron strain and discuss exactly what viral lineages should be included and what the makeup of the vaccines should be, according to discussions questions the FDA posted for the meeting. The committee will also consider how a booster developed against the original strain by Novavax works as a booster.

The Pfizer data introduce a new question for the FDA panel to consider, however. Moderna released data from only a bivalent vaccine, meaning that the vaccine contains antigens for both the Omicron variant and the original strain that was prevalent earlier in the pandemic. Pfizer and its partner BioNTech provided data for both the bivalent vaccine and a monovalent one, which contained only antigen for the Omicron variant.


Pfizer and BioNTech provided figures for the efficacy of the vaccine that appeared to show the monovalent vaccine might boost antibody titers more.

By one measure, known as the geometric mean ratio (GMR), the monovalent 30 microgram booster compared to the current booster was 2.23; the 60 microgram monovalent booster had a GMR of 3.15 (higher is better). For the bivalent 30 and 60 microgram doses, the GMR values were 1.56 and 1.97. Moderna had previously said that the GMR for its bivalent booster was 1.75.

Expressed another way, Pfizer said that one month after administration, the Omicron-modified monovalent booster increased neutralizing geometric mean titers, a measure of antibodies that neutralize the virus, by 13.5-fold for the lower dose and 19.6-fold for the higher one. The bivalent candidates increased titers by 9.1-fold for the lower dose and 10.9-fold for the higher one. Moderna previously said that its bivalent booster increased antibody titers by 8-fold above baseline levels.

It’s not clear how or if those differences would translate into better clinical results from the vaccines.

There could be benefits of a bivalent vaccine that are not shown by these antibody figures, which capture only some of the immune response from a vaccine. It’s possible, for instance, that giving more types of antigen will help the vaccine be hardier against other variants. These will be the type of issues the FDA advisors will likely discuss on Tuesday.

In a prepared statement, Ugur Sahin, the CEO and co-founder of BioNTech, suggested that adapting these vaccines to new variants could become the new normal, noting that new Omicron sublineages are outcompeting the original Omicron strain.

“We will therefore remain vigilant and are prepared to rapidly adapt our Omicron-based vaccine to emerging sublineages if epidemiological and laboratory data suggest,” Sahin said.

Editor’s note: This article was updated to include documents the FDA posted for the meeting on Tuesday, June 28.

Correction: a previous version of this story misstated one geometric mean ratio for the Pfizer booster.

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