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It’s still unclear if omicron causes milder disease.

The Hong Kong results may be a sign that omicron might be less likely than variants like delta to invade the lungs of infected people and cause severe illness, but that’s far from definitive. “I don’t think you can really say that the virus is going to be less virulent based on that data alone,” Chandran says. “We’re going to have to wait and see what happens to people.”   

There are some hopeful hints from South Africa that omicron might cause less severe disease than what delta causes. But experts caution that it’s far too early to make solid conclusions.

“We should not be lulled into any type of complacency,” Ryan Noach, CEO of Discovery Health, a health insurance provider based in South Africa, said December 14 in a news conference.

The optimism comes because hospitalizations in South Africa aren’t rising as fast as they did in previous surges. What’s more, anecdotal reports from the country suggest that fewer hospitalized patients in the current wave rely on medical interventions to breathe, such as supplemental oxygen.

That information, however, comes with a massive caveat: More than 70 percent of people there have been exposed to the coronavirus in the last 18 months, Noach said. Protection provided by previous infections, or vaccinations, could be the reason people there tend to have milder symptoms.

Experts need to see what happens in other parts of the world before concluding that omicron is a less virulent virus than other variants, Glenda Gray, president and CEO of the South African Medical Research Council, said at the December 14 news conference. Monitoring the severity of COVID-19 cases in places with low vaccination rates and low infection rates will be particularly informative, she said.

A study from the United Kingdom found that so far there is no indication that omicron might cause milder, or more severe, COVID-19 than delta, researchers report December 17. Data on hospitalizations there, however, are still limited.

Even if omicron is ultimately linked to milder disease, that doesn’t mean it isn’t dangerous. More infections overall mean more hospitalizations and more deaths, even if the severe outcomes are a smaller proportion of overall omicron cases than with delta.

In the United States, where delta dominated until recently, the two variants are colliding — and that’s raising fears of this latest wave turning into a tsunami in some places.

“Our delta surge is ongoing and, in fact, accelerating,” Jacob Lemieux, an infectious diseases physician at Massachusetts General Hospital and Harvard Medical School in Boston, said in a December 14 call with reporters. “On top of that, we’re going to add an omicron surge. That’s alarming because our hospitals are already filling up. Staff are fatigued. We’re almost two years into the pandemic, and there may be limits on capacity to handle the kinds of caseloads that we see from an omicron wave superimposed on a delta surge.”