Moderna hopes to offer updated COVID-19 boosters in the fall that combine its original vaccine with protection against the omicron variant. On Tuesday, it reported a preliminary hint that such an approach might work.

Today's COVID-19 vaccines all are based on the original version of the coronavirus. But the virus continues to mutate, with the super-contagious omicron variant — and its siblings — the latest threat.

Before omicron came along, Moderna was studying a combination shot that added protection against an earlier variant named beta. Tuesday, the company said people given that beta-origi...