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Pfizer and BioNTech have started testing their COVID-19 vaccine that specifically targets the highly contagious Omicron variant. The companies said Tuesday they’ve begun a clinical trial using volunteers in the U.S.

The study is divided into three groups. For unvaccinated people, the study tests the vaccine as a three-shot regimen. For those who already have gotten two doses of the original vaccine, the new version will be given as a booster shot. And for those who’ve already gotten three doses of the Pfizer/BioNTech vaccine, the study tests a fourth dose of the current vaccine against a fourth dose of the Omicron-based vaccine.

The U.S. Centers for Disease Control and Prevention says a third dose of an mRNA vaccine like Pfizer’s provides 90% protection against hospitalization, but Pfizer has said two doses may not be enough to guard against the Omicron variant, which has eluded some of the protection provided by the two-dose vaccine regimen.

BioNTech said it may not be possible to realize a current plan to launch an Omicron-targeting vaccine by the end of March.

Investors drove shares of Pfizer and BioNTech higher in early trading amid a weak open for the U.S. markets.