See all our coverage of the coronavirus outbreak

Omicron seems more likely to cause reinfections than delta.

Immunity against infection wanes in the months afterward. That combined with the immune evasiveness of omicron has raised the odds that previously infected people might get infected again.

Data from the United Kingdom, for instance, suggest that recovered people are five times as likely to be infected by omicron as by delta, researchers from Imperial College London report December 19.

The same appears to be true in South Africa. People who caught the virus in South Africa’s first wave, which was driven by a variant called D614G, are 73 percent as likely to get reinfected with omicron as people without known prior infections. That’s higher than the 29 percent higher risk for recovered people in September and October 2021, when delta was prevalent there. Individuals infected in the second wave caused by the beta variant face a 60 percent higher risk, up from 27 percent in September and October.