Pfizer and BioNTech have initiated a new trial to determine the efficacy of the third dose of their Covid-19 vaccine (BNT162b2) against emerging SARS-CoV-2 variants.

The new study will enrol up to 144 participants from the Phase I study in the US in two age cohorts of 18-55 and 65-85.

Under the trial, a 30µg booster of the current vaccine will be administered to participants who received the initial two-dose regimen around six to 12 months ago.

The study will assess the safety and immune response of the third dose of BNT162b2.

All the participants will be monitored after they received the third dose, and then a week and one month after. They will be monitored for up to two years as originally planned.

The Pfizer-BioNTech vaccine has been authorised for emergency use by the US Food and Drug Administration for use in individuals 16 years of age and older.

The first interim efficacy analysis data from the trial of BNT162b2 found the vaccine to be more than 90% effective in preventing Covid-19.

Separately, Pfizer and BioNTech are engaged in discussions with the regulatory authorities to undertake a registration-enabling clinical study to evaluate a variant-specific vaccine having a modified mRNA sequence.

The study plans to use a construct of the Pfizer-BioNTech vaccine based on the B.1.351 lineage, which was first identified in South Africa. This will support the companies’ preparations in addressing potential strain changes in future.

Pfizer chairman and CEO Albert Bourla said: “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.

“This booster study is critical to understanding the safety of a third dose and immunity against circulating strains.

“At the same time, we are making the right investments and engaging in the appropriate conversations with regulators to help position us to potentially develop and seek authorisation for an updated mRNA vaccine or booster if needed.”