The highly infectious variant has caused near-vertical case growth across U.S. cities, with positive test rates doubling every two to three days over the last week. Reports of mild symptoms and South Africa’s relatively benign experience with Omicron have offered hopeful indications of a quick wave—but scientists have also warned that record high caseloads, high transmissibility, and COVID complacency might lead to greater hospitalization rates.

U.S. chief medical adviser Dr. Anthony Fauci, speaking on This Week on Sunday morning, warned of complacency around the new variant, noting the volume of new infections “might override a real diminution in severity.” With many more people becoming infected, the net positive effect of the new variant being mildly symptomatic or asymptomatic may be overridden by the spike in cases, Fauci noted.

Fauci added that the unvaccinated cohort, which includes 38% of the total population in the U.S., are especially vulnerable. According to the Centers for Disease Control and Prevention, unvaccinated people are five times as likely as vaccinated people to test positive for COVID-19, and 14 times as likely to die of COVID-19 as vaccinated patients.

And despite repeated urgings to get the vaccine, nearly 90% of unvaccinated adults said the Omicron variant would not spur them to get shots, according to a recent study by the Kaiser Family Foundation. This phenomenon could prove especially worrisome if Omicron cases peak across the country simultaneously.

“Those are the most vulnerable ones when you have a virus that is extraordinarily effective in getting to people and infecting them, the way Omicron is,” Fauci said.

While some countries have imposed tighter restrictions on public life in response to Omicron, others like the U.S., England, Italy, and Spain are taking Omicron as the beginning of the end-emic, implementing looser restrictions and letting the new variant run its course.

The relaxed guidance is based on preliminary scientific evidence that the new variant causes less hospitalization than previous variants. The latest study from Imperial College London found that the Omicron variant causes less severe disease when compared with Delta, the previously dominant variant, with far less likelihood of being hospitalized. In the 325,000-person study of people who tested positive for COVID-19 in England in the first two weeks of December, the risk of needing hospital treatment with an Omicron infection was down by 20% to 25% compared with Delta, and the need for overnight hospitalizations was lower by 40% to 45%.

Another study by the University of Edinburgh in Scotland offered further evidence, finding that people infected with Omicron were almost 60% less likely to be hospitalized than those infected with Delta.

The same numbers were reflected in South Africa, where only 1.7% of COVID-19 cases resulted in hospitalization during the Omicron-backed fourth wave. In the third wave, which was dominated by the Delta variant, 800 people were hospitalized on an average day, with 19% of those infected being hospitalized.

But all those hopeful figures come with a caveat. “Given the high transmissibility of the Omicron virus, there remains the potential for health services to face increasing demand if Omicron cases continue to grow at the rate that has been seen in recent weeks,” said Neil Ferguson, director of Imperial’s MRC Centre for Global Infectious Disease Analysis.

And with wildly varying news, there is little guidance on whether double-vaccinated and boosted people can keep their New Year’s Eve plans intact.