Scientists saw the coronavirus Omicron variant sweep across the world at a pace that was unprecedented. On Tuesday, drugmakers Pfizer and BioNTech announced a clinical trial of a vaccine adapted to target the strain.

The scientists will enroll more than 1400 adults ages 18 to 55. They will evaluate how the vaccine performs in people who have already received two or three doses. They will also test the shots in those who have not been vaccinated.

Officials at the World Health Organization have said the current Pfizer vaccine protects people exposed to Omicron from severe disease and hospitalization.

Executives at the US pharmaceutical company said, over time, those protections could fade. They said they need to be prepared.