Pharmaceutical companies Pfizer and BioNTech said that they have begun an evaluation of the safety and immunogenicity of a third dose of the Pfizer-BioNTech Covid-19 vaccine (BNT162b2).

It is to understand the effect of a booster on immunity against Covid-19 caused by the circulating and potential newly emerging SARS-CoV-2 variants, the companies said in a statement on February 26. "While we have not seen any evidence that the circulating variants result in a loss of protection provided by our vaccine, we are taking multiple steps to act decisively and be ready in case a strain becomes resistant to the protection afforded by the vaccine," said Albert Bourla, Chairman and Chief Executive Officer, Pfizer. "This booster study is critical to understanding the safety of a third dose and immunity against circulating strains," Bourla added.

The study will draw upon participants from the Phase 1 study in the US who will be offered the opportunity to receive a 30 µg booster of the current vaccine 6 to 12 months after receiving their initial two-dose regimen. The study is part of the companies' clinical development strategy to determine the effectiveness of a third dose against evolving variants.

Separately, in order to be prepared for any potential future strain changes, Pfizer-BioNTech are in ongoing discussions with regulatory authorities, including the US Food and Drug Administration (FDA) and the European Medicines Agency, regarding a registration-enabling clinical study to evaluate a variant-specific vaccine having a modified mRNA sequence.

This study would use a new construct of the Pfizer-BioNTech vaccine based on the B.1.351 lineage, first identified in South Africa. This could position the companies to update the current vaccine quickly if the need arises to protect against Covid-19 from circulating strains. The Pfizer-BioNTech Covid-19 vaccine has not been approved or licensed by the US Food and Drug Administration, but has been authorized for emergency use by FDA under an Emergency Use Authorization to prevent Covid-19 for use in individuals 16 years of age and older.