Pfizer and BioNTech announced Tuesday that they have started a clinical trial testing a modified Covid-19 vaccine to protect against the supercontagious omicron variant of the coronavirus.

The drugmakers said they aim to enroll up to 1,420 healthy adults ages 18 to 55 in the trial, which will evaluate whether the omicron-specific vaccine is safe and generates a strong immune response.

The participants will be divided into three groups — fully vaccinated, fully vaccinated and boosted, and unvaccinated — which will determine whether they receive one, two or three doses of the modified vaccine.

The companies also said they expect to launch a similar study in the U.S. testing the shot in those older than 55.

The Centers for Disease Control and Prevention released new research Friday that found that a booster shot of the current vaccines provides robust protection against severe illness from the rapidly spreading variant, which accounts for virtually all new cases in the U.S.