When news of the novel coronavirus’s Omicron variant first broke over the Thanksgiving holiday in the U.S., the sense of dread and fatigue was palpable. Just when the COVID pandemic’s outlook had started to improve, we were faced with a new arrival that was clearly more transmissible than previous variants—and likely able to significantly evade immunity conferred by vaccination or prior infection. But very preliminary data offered a shred of hope that Omicron may cause milder illness than previous variants.
It is still too soon to relax our guard, however. These early data are based largely on reports from South Africa—a country with a relatively young population and high level of prior infection—and on reports involving international travelers, who tend to be younger and healthier. Additionally, COVID hospitalizations and deaths usually lag infections by weeks. It remains to be seen whether observations of milder disease will hold up over time and in countries such as the U.K. and U.S., whose populations are older and may have less prior immunity from infection (but more immunity from vaccines).
If Omicron does cause less severe disease than previous variants, the sheer number of infections could still result in a similar or greater number of severe cases. “The transmissibility alone is such a big factor, just in terms of how quickly outbreaks can grow,” says Natalie Dean, a biostatistician at Emory University. “Even if just a small fraction of [Omicron cases] go on to severe disease,” that’s still a lot of cases, she notes. Such a fraction could lead to a large absolute number of hospitalizations and deaths—overwhelming health care systems already stretched to the breaking point.
The assumption that Omicron may be milder is based in part on a report from a hospital in South Africa, one of the countries where the variant was first identified. This report found that many of the people infected with Omicron were already in the hospital for other reasons, and their infection was detected incidentally. It also found that fewer COVID patients were admitted to intensive care and that a smaller percentage of them needed supplemental oxygen, compared with patients in previous waves. Furthermore, many of the earliest reported Omicron cases in Europe and the U.S. were mild to moderate.
Still, more than 195 people in the U.K. with a confirmed Omicron infection have been hospitalized, and at least 18 have died. (Experts say the true number of Omicron hospitalizations may be 10 times higher and that deaths may be higher as well.) And a recent report from the country found that Omicron cases did not lead to fewer hospitalizations than Delta.
It is too early to know if Omicron truly is milder or only appears so because it has largely been infecting people with some prior immunity. A preprint (not yet peer-reviewed) study by South African researchers found that people who had previously been infected with the novel coronavirus and developed a “breakthrough” case were nearly 2.4 times more likely to be reinfected in the recent Omicron wave, compared with the country’s first wave of COVID.