For one thing, the recombinant is extremely rare. Although it has existed since at least January, it has not yet shown the ability to grow exponentially.

Dr. Simon-Loriere said that the genome of the recombinant variant also suggested that it wouldn’t represent a new phase of the pandemic. The gene that encodes the virus’s surface protein — known as spike — comes almost entirely from Omicron. The rest of the genome is Delta.

The spike protein is the most important part of the virus when it comes to invading cells. It is also the main target of antibodies produced through infections and vaccines. So the defenses that people have acquired against Omicron — through infections, vaccines or both — should work just as well against the new recombinant.