• Omicron cases could be peaking roughly a month after they started rising in the US, models suggest.
• Disease experts predict that cases could fall back down as quickly as they rose.
• By the end of the wave, Omicron may have infected “a substantial fraction,” if not a majority, of the US.

Omicron appears to take a swifter, sharper course than its predecessors: Cases of the variant tend to rise within a population for about a month, then fall back down just as quickly, early observations suggest.

South Africa’s daily COVID-19 cases began to drop off in late December, roughly one month after scientists first spotted Omicron there. The United Kingdom seems to have followed suit: Daily COVID-19 cases started rising there in early December, but in the last week they’ve fallen 14%, on average, from around 181,000 to 156,000 cases per day. 

New models predict the US could be headed for a similar trajectory.

“It’s certainly possible that we could see a rapid fall just as much as we’ve seen a rapid rise,” David Dowdy, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health, told Insider.

According to projections from the University of Washington’s Institute for Health Metrics and Evaluation (IHME), daily coronavirus infections may have already peaked at more than 6.2 million on January 6. A recent report from the University of Texas suggests that daily COVID-19 cases could peak around Thursday. The model assumes, based on the current science, that Omicron is more transmissible and better at evading the immune system than Delta, but results in less severe disease.

Those same models, however, predict that Omicron will continue to drive up COVID-19 hospitalizations and deaths in the US through the end of January. The IHME model estimates that COVID-19 deaths will peak at around 2,100 per day on January 28.

Omicron’s rise and fall could be even more dramatic on the local level, Dowdy said — though many cities and states haven’t seen the worst of their outbreaks yet.

“What we’re seeing on a national level is a smoothing over of all of these local effects,” Dowdy said, adding: “The West Coast is just taking off. The South is just taking off. You’re going to see these cases continue to rise because there are more places where it’s taking off than where it’s peaking.”

Winter weather and indoor gatherings may have fueled Omicron’s rapid rise 

A highly transmissible virus tends to tear quickly through a population until it runs out of people to infect, and Omicron may be close to reaching that point. The US already reported its highest-ever number of daily cases — more than 1.4 million — on Monday, according to data from Johns Hopkins University. That’s nearly five times the number of daily cases reported during the last peak in January 2021.

But Dowdy said there are other explanations for why Omicron cases took off so quickly. Winter months often facilitate the spread of respiratory viruses, he said, and more people are congregating indoors this winter than last.

“It’s not only about, or even primarily about, the transmissibility of the virus,” Dowdy said. “Last winter, schools were closed, you couldn’t go into a restaurant at full capacity, and you surely couldn’t go to an indoor gathering of more than 50 people. Now you can go to a concert, you can easily go to a full restaurant, and all our schools are open. Had you had this behavior last year, you might have seen a much more rapid rise.”

With Omicron, there’s also a shorter window of time between getting infected and infecting someone else. A recent study from Norway found that Omicron symptoms usually appear around three days after exposure, whereas Delta symptoms develop around four to five days after someone is infected, on average.

“It may only be shorter by a day, but when you’re talking about how long it takes to double your numbers, a day makes a big difference,” Dowdy said.

Omicron will have infected a ‘substantial fraction’ of the US by the end of the wave

Even if Omicron cases fall dramatically soon, many Americans will have gotten sick during the current wave.

“Omicron, with its extraordinary, unprecedented degree of efficiency of transmissibility, will ultimately find just about everybody,” Dr. Anthony Fauci said during a virtual chat with the Center for Strategic and International Studies Commission on Tuesday.

“Those who have been vaccinated, and vaccinated and boosted, would get exposed,” Fauci said. “Some, maybe a lot of them, will get infected but will very likely, with some exceptions, do reasonably well in the sense of not having hospitalization and death.”

Dowdy is bit more optimistic. The current Omicron wave will likely infect “a substantial fraction of people,” he said, but probably not the majority of US residents.

“This virus is burning through certain networks of people who are having more contact with each other [or have] lower levels of vaccination or recent infection,” he said. “Once it makes it through those networks, it doesn’t have other places to go, and then you start to see it fall.”