Pfizer and BioNTech have begun assessing the safety and immunogenicity of a third dose of their COVID-19 vaccine against the coronavirus’ South African mutation and other potential viral variants.

The companies said Thursday that they have started a study that will offer participants in their phase 1 U.S. vaccine study a third booster shot of their currently authorized vaccine, BNT162b2, six months to a year after they receive their two-dose vaccination regimen. Pfizer and BioNTech explained they are playing it safe despite in-vitro studies and real-world evidence suggesting their two-dose vaccine, which is being employed globally, holds up well against current variants.

The pair are also in talks with regulators, including the FDA and the European Medicines Agency, about conducting a clinical study to evaluate a variant-specific vaccine with a modified mRNA sequence based on the South African strain.

“This could position [us] to update the current vaccine quickly if the need arises to protect against COVID-19 from circulating strains,” the companies said.

Like Moderna, which announced Wednesday it had shipped modified vaccine doses to the NIH for a variant trial and is conducting trials of its own (DID, Feb. 25), the companies said they will follow the guidance the FDA has issued on Emergency Use Authorizations (EUAs) for modified vaccines. For future modified mRNA vaccines, they said they plan to use a regulatory pathway “similar to what is currently in place for flu vaccines.”

Vaccine makers have been scrambling to determine the efficacy of their vaccines against concerning variants, including the South African strain, and prepare for future COVID-19 strains that could significantly impact or entirely evade inoculation.

Moderna is also evaluating whether a third dose of its original vaccine will provide adequate protection against mutations. Johnson & Johnson, whose one-dose vaccine will be voted on by the FDA’s vaccine advisory committee today and is likely to earn an EUA, submitted new data to the panel suggesting its vaccine is effective against the South African strain and a mutation in Latin America (DID, Feb. 25).

AstraZeneca and the University of Oxford also have a modified COVID-19 vaccine in the works meant to address new viral strains. They hope to have it ready by the fall. Recent studies have found that the vaccine may only provide “minimal protection” against mild-to-moderate infection from the South African variant, but Western governments have continued to champion the vaccine as a significant shield against severe COVID-19 and death (DID, Feb. 9). — James Miessler