US drugmaker Pfizer and its German partner BioNTech announced Thursday that they have started “an evaluation of the safety and immunogenicity” of a third dose of their coronavirus vaccine.

The study aims 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.

Currently, the Pfizer-BioNTech vaccine is administered as two doses of 30 micrograms each given 21 days apart.

The Phase 1 study in the US will evaluate 144 participants in two age cohorts – 18-55 and 65-85 – who will receive a 30-microgram booster of the current vaccine “6 to 12 months after receiving their initial two-dose regimen.”

“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 of Pfizer.

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

He said the firms are “making the right investments and engaging in the appropriate conversations with regulators to… potentially develop and seek authorization for an updated mRNA vaccine or booster if needed.”

Ugur Sahin, chief executive officer and co-founder of BioNTech, underlined that these efforts aim to ensure preparation for different scenarios to “address the challenges of tomorrow.”

“Therefore, we will be evaluating a second booster in the current regimen as well as preparing for a potential rapid adaption of the vaccine to address new variants which might escape the current version of our mRNA-based vaccine,” he said.

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