Here’s why the WHO is urging caution:

South Africa: Age and antibodies

Dr. Abdi Mahamud, the WHO’s incident manager for COVID-19, said that while data from South Africa suggesting Omicron causes milder illness was encouraging, the population in South Africa is mostly younger. “How it behaves in the elderly population, the vulnerable—we don’t know yet,” Mahamud said of Omicron during a press briefing in Geneva on Wednesday.

He said people should not “overinterpret” the South African data, saying the world still needed more information on how Omicron affects people over 60 years old, those with underlying medical conditions that weaken their immune response, and the unvaccinated.

Mike Ryan, executive director of the WHO health emergencies program, also said in the same press briefing that not only was the South African population generally younger than in other places, but that a large number of South Africans had also carried antibodies from prior COVID-19 infections. That might confound attempts to determine whether the Omicron strain is innately less likely to cause severe disease than prior variants of SARS-CoV-2, the virus that causes COVID-19.

Europe: High vaccination

Ryan said the data emerging out of Europe was also difficult to interpret at this stage. Many European countries have very high vaccination rates, which also makes it hard to tease out whether the fact hospitalizations have not soared in step with skyrocketing infection rates is largely due to protection against severe illness conferred by the vaccines or because Omicron is inherently less potent.

What’s more, the majority of those infected with Omicron in Europe so far have been younger adults. The virus has yet to make major inroads into the over-60-year-old population, so it remains difficult to know how dangerous Omicron is to those in this age group, he said.

“What we haven’t seen is the Omicron wave fully established in the broader population,” Ryan said. “I’m a little nervous to make positive predictions until we see how well the vaccine protection is going to work in those older and more vulnerable populations.”

Also confounding the hospitalization data from Europe, Ryan said, is the fact that the Delta variant, which is known to be more likely to cause serious illness than some earlier strains of the virus, is still spreading there.

U.S. view

Some national health authorities have been more willing to say Omicron appears less dangerous than previous strains, based on the South African data. In the U.S., Anthony Fauci, the White House chief medical adviser, said Wednesday that so far all indications from South Africa suggested that Omicron causes less severe disease than the Delta variant. But he did say it was important not to become complacent, saying that U.S. demographics differed from South Africa’s and it was unclear exactly what impact Omicron would have.