The increase, epidemiologists say, is likely caused by the changing of the seasons, with colder weather pushing people indoors and prompting them to close windows, raising the risk of transmission. Cold, drier air may also benefit the coronavirus by helping viral particles to survive and linger in the air for longer.
Holiday travel and gatherings could also accelerate transmission.
One disease model from a team at New York University Grossman School of Medicine, which predated Omicron, predicted about 10 deaths a day from Delta in the weeks ahead, according to a member of the team, Dr. Ronald Scott Braithwaite, a professor at N.Y.U. and an adviser to the city. At an earlier point this year, during the height of last winter’s wave, there were as many as 98 Covid-19 deaths in a single day.
In New York City, the Delta variant is expected to drive only a minor surge in cases. But it is creating a potentially dire situation in Western New York, where last week, the number of coronavirus patients admitted to hospitals on a single day — 120 on Nov. 26 — was the highest in the region since the start of the pandemic.
Surveillance and testing systems are strong
As new forms of the virus began to emerge across the globe early this year, New York City began cobbling together its own surveillance system. Turning to universities, hospitals and other laboratories, the city set out to analyze at least 10 percent of coronavirus cases in New York for mutations.
The Pandemic Response Lab, a private start-up that opened last year, quickly became a big player in this effort. By early this year, it was sequencing — examining genetic material for mutations — hundreds of coronavirus cases each week, and offering insights into how different variants were circulating across the city.
The Coronavirus Pandemic: Key Things to Know
Travel restrictions and lockdowns. As more Omicron cases emerge globally, countries are responding in varied ways. Japan joined Israel and Morocco in barring all foreign travelers, and Australia delayed reopening its borders for two weeks. The C.D.C plans to increase testing and screening of international fliers to the U.S.
A patchwork of regulations. As the new Omicron variant spread around the world, two KLM flights from South Africa became emblematic of the scattershot and lax global approach to coronavirus containment. Of the more than 60 people who tested positive for the virus, at least 14 had Omicron.
A new type of treatment. An expert panel voted to recommend that the F.D.A. authorize a Covid pill from Merck for high-risk adults, the first in a new class of antiviral drugs that could work against a wide range of variants, including Omicron. The pill could be authorized within days, and available by year’s end.
Vaccine hesitancy in Africa. The detection of the Omicron variant in Africa signals the next stage of the battle against Covid-19: getting more people inoculated in poorer nations. But though vaccine supplies are becoming sufficient, the new hurdle is overcoming local skepticism or outright hostility.
About 15 percent of confirmed Covid-19 cases in the city are sequenced, a number some epidemiologists hope will grow. In recent months, just about every virus sample run through genome sequencing machines has turned out to be the Delta variant, said Dr. Jon Laurent, who oversees the Pandemic Response Lab’s sequencing program.