Moderna has begun testing an omicron-specific COVID-19 booster in healthy adults.

The company announced Wednesday that the first participant had received a dose. Earlier this week, competitor Pfizer began a similar study of its own reformulated shots.

It’s not clear whether global health authorities will order a change to the vaccine recipe in the wake of the hugely contagious omicron variant. The original vaccines still offer good protection against death and severe illness. Studies in the US and elsewhere show a booster dose strengthens that protection and improves the chances of avoiding even a milder infection.

Moderna pointed to a small study published in the New England Journal of Medicine on Wednesday that showed antibodies able to target omicron persisted for six months after a booster dose, although the levels were dropping.

Moderna’s new study will enroll about 600 people who already have received either two doses of the company’s original shots or two plus a booster dose. All the volunteers will receive a dose of the experimental omicron-matched version.