Pfizer says it is discussing a clinical trial for a booster for new Covid-19 variants with the Food and Drug Administration, after the US regulator urged vaccine makers to prepare to adapt their shots. In testimony to a congressional committee, John Young, chief business officer of Pfizer, said the company was “laser focused” on the potential impact the emergent variants could have on its vaccine’s ability to protect against Covid-19.

“We’re discussing clinical trial designs with the FDA to investigate the safety and immunogenicity of an updated vaccine that involves a change to the mRNA construct to target an emerging variant,” Young said. “We’re preparing to respond quickly and hope to initiate a study to investigate the effectiveness of a third booster of our vaccine and trial participants who have already received two doses,” he added.

The BioNTech/Pfizer vaccine still works just as well against the B.1.1.7 variant from the UK but is less effective against the 501.V2, that was first discovered in South Africa. Many other vaccine makers including Moderna, AstraZeneca and Novavax have already started preparing for clinical studies on vaccines targeted to the new variants, or boosters.

Stephen Hoge, Moderna’s President, said the company would have the capacity to expand production in the second half of the year to make vaccines for variants, if necessary. He added that Moderna would need a few month’s worth of clinical studies before it could scale up a second-generation vaccine.