The World Health Organization warned on Wednesday that the ongoing circulation of the Delta variant and the emergence and rapid spread of Omicron could create a “tsunami” of infections that could overwhelm health care systems, even as top American health officials emphasized that early data showed Omicron infections producing milder illness.
“Delta and Omicron are now twin threats driving up cases to record numbers, leading to spikes in hospitalization and deaths,” Dr. Tedros Adhanom Ghebreyesus, the W.H.O.’s director-general, said at a news conference in Geneva. “I am highly concerned that Omicron, being highly transmissible and spreading at the same time as Delta, is leading to a tsunami of cases.”
But along with the warnings, U.S. officials and the leading scientists at the U.N. agency said that the early data from places where Omicron was spreading offered some cautiously positive signs.
Dr. Rochelle P. Walensky, the Centers for Disease Control and Prevention director, said at a White House news conference that even as cases had increased by around 60 percent over the past week, to around 240,000 cases recorded each day, hospital admissions and deaths were hinting at a milder wave of the virus.
“While our cases have substantially increased from last week, hospitalizations and deaths remain comparatively low right now,” she said, pointing to a seven-day average of hospitalizations of 9,000 per day, an increase of about 14 percent from last week. The seven-day average of daily deaths stood at roughly 1,100 per day, she added, a decrease of about 7 percent.
“This could be due to the fact that hospitalizations tend to lag behind cases by about two weeks,” she said, “but may also be due to early indications that we’ve seen from other countries like South Africa and United Kingdom — have milder disease from Omicron, especially among the vaccinated and boosted.”
Citing a series of international studies showing milder Omicron outcomes, Dr. Anthony S. Fauci, President Biden’s chief medical adviser, said at the same news conference that “the pattern and disparity between cases and hospitalizations strongly suggest that there will be a lower hospitalization-to-case ratio when the situation becomes more clear.”
Dr. Soumya Swaminathan, the chief scientific officer for the W.H.O., said that early real-world data indicated that the link between infection numbers and hospitalizations had been “disrupted.”
She cautioned that the evidence on Omicron is just emerging. “We can still not predict what this virus will do,” she said.
While it was increasingly clear that vaccinated people are being infected with Omicron, meaning that there was a reduction in the capacity of vaccines to neutralize the virus, the early evidence on the protection vaccination might provide was positive.
Vaccines, she said, still “appeared to be protective” against severe illness and death. But it was a complicated equation that needed to take into account a host of factors — including the clinical vulnerability of those being infected — and there was simply not enough data.
Dr. Mike Ryan, the head of the W.H.O. emergencies program, said that Omicron had yet to work its way into all parts of society — including the most vulnerable populations and the unvaccinated.
The Omicron outbreaks around the world, he said, started in primarily younger age groups and the variant is only now moving into older populations.
“I think we will still see decoupling from cases and severe disease,” he said. But the sheer number of daily cases — the “force of infection” — could lead to surges of patients and increased pressure on health care systems.
He also noted that even in countries with plentiful vaccines, there were large pockets of unvaccinated people, and it was simply too early to know if Omicron itself is less virulent than the variants that have come before.
Dr. Tedros said the “narrative going on right now that it is milder or less severe” might be dangerous since the high transmission rates alone could lead to an increase in hospitalizations and death.
“We should not undermine the bad news with the good news,” he said. “There are both elements here.”