Pfizer and BioNTech are beginning a new study testing out a vaccine specifically aimed at going after the Omicron variant of SARS-CoV-2, the virus that causes COVID-19. 

On Tuesday, the companies reported that they have started taking in participants for the trial, noting that the clinical study is being done to “evaluate the safety, tolerability and immunogenicity of an Omicron-based vaccine candidate in healthy adults 18 through 55 years of age.”

The trial will include three groups of up to 1,420 people, and will look at “different regimens of the current Pfizer-BioNTech COVID-19 vaccine or an Omicron-based vaccine.” It will also use some people who were involved in the Phase 3 COVID-19 booster study.