Pfizer Inc and BioNTech SE said on Tuesday they started a clinical trial to test a new version of their vaccine specifically designed to target the Covid-19 Omicron variant, which has eluded some of the protection provided by the original two-dose vaccine regimen.

Banking on volunteers in the United States, the companies plan to test the immune response generated by the Omicron-based vaccine both as a three-shot regimen in unvaccinated people and as a booster shot for people who already received two doses of their original vaccine.

They are also testing a fourth dose of the current vaccine against a fourth dose of the Omicron-based vaccine in people who received a third dose of the Pfizer/BioNTech vaccine three to six months earlier.