Covid-19 cases are surging upward again in the United States, and public health experts are warning the fast-spreading omicron variant may push the number of infections to their highest level yet. Whether this surge will be followed by an unprecedented level of hospitalization and death is uncertain, but researchers say it’s possible the most devastating phase of the pandemic is yet to come.

Already, countries like South Africa, the United Kingdom, and Denmark have seen sharp spikes in new Covid-19 cases, with some areas reaching record highs. South Africa has reported far fewer hospitalizations from omicron compared to previous waves, but the UK is is in the midst of a sharp rise in hospitalizations, about 30 percent higher week over week.

The big reason is that omicron appears to spread far more readily than the delta variant that has been dominant worldwide since the summer — omicron is 25 to 50 percent more transmissible, according to some UK estimates.

The current moment is an eerie echo of December 2020, when the first major variant of Covid-19 began infecting people around the world. But a key difference now is that there are effective vaccines that have been widely deployed in some countries. In the US, more than 70 percent of the population have had at least one dose of a vaccine and 30 percent of those vaccinated have received two doses and a booster, which should absorb some of the impact of omicron.

Yet epidemiologists and health officials are sounding the alarm about another tsunami of infections — in hopes people will take more precautions, and to help hospitals and health workers prepare to care for the sickest patients. “Omicron could be just as deadly as delta even if it causes milder disease,” Celine Gounder, an infectious disease specialist who has advised the White House, told reporters this week. Considering the potential impact on the health care system is also crucial as decision-makers weigh another round of restrictions — closing schools, banning large gatherings, reimposing mask mandates.

In one of the most comprehensive forecasts to date, researchers from the Covid-19 Modeling Consortium at the University of Texas at Austin on Friday chalked out 18 different scenarios for omicron. Their study was not peer-reviewed, but the findings show that the US is facing yet another dangerous variant while the conditions for spreading it — the holiday season — are at their most favorable.

The most optimistic pathway in the study would lead to more than 50 percent fewer deaths compared to last year — the six-month period spanning December 1, 2020, to May 1, 2021 — while the most pessimistic route would end with 20 percent more fatalities than that grim period last winter and spring.

“Everything we’ve seen so far — growth in Denmark, growth we’re seeing in the United States, in Canada, in the UK — suggests that these scenarios are actually very plausible … for our country,” said Spencer Fox, associate director of the UT Covid-19 Modeling Consortium.

A lot depends on the mutated virus itself, particularly how badly it sickens unvaccinated (and vaccinated) people. While some early reports have hinted that omicron causes a lower rate of severe Covid-19 illness compared to prior variants, there’s still not enough data to be sure. “It’s too uncertain right now to say that,” Fox said. (The UT model currently assumes that omicron’s severity is the same as with delta in unvaccinated people with no prior infection, and that protection against severe illness from prior infection and vaccines may be similar or reduced.)

Getting a booster dose of a Covid-19 vaccine is the most effective action an already-vaccinated individual can take to protect against the variant, and if enough people get an extra shot, thousands of deaths could be averted this winter, according to the models.

However, communities around the US have so far responded in drastically different ways to the Covid-19 pandemic — some imposing policies that have slowed transmission, and others rejecting those policies and suffering terrible consequences in the form of overwhelmed hospitals and thousands of preventable deaths. So it’s likely that both the best and worst scenarios could play out in parallel over the coming months, in different places.

Omicron is set to become the dominant Covid-19 variant in the US

As people head indoors to warm up from the cold and celebrate the winter holidays, omicron will find ample opportunities to jump from lung to lung. In addition to its greater transmissibility, early results also show omicron can better evade the shielding provided by the immune system that’s built up from vaccines or from prior infections. One recent study suggests that antibodies produced to counter past versions of the virus are far less effective at curbing omicron, which could make the variant more likely to cause a breakthrough infection or reinfection.

At the same time, the delta variant of SARS-CoV-2 is continuing to wreak havoc, having recently pushed the US death toll above 800,000. Flu could come roaring back this winter, as well, generating a fresh wave of hospitalizations among the most vulnerable to that infection, too. The combined threats of all these respiratory illnesses could push some hospitals to horrific new levels of overcapacity — especially those already stretched thin from staffing shortages and other strains after two years of crisis.

Specifically, experts worry that hospitals will have to ration care or turn patients who need life-saving care away — something many hospitals had to do in earlier Covid-19 surges — if a a lot of new severely ill patients come flooding through the doors.

“Besides the toll of suffering and death which will inevitably go up if, in fact, we have that convergence in the winter months of flu and omicron and delta, we could get our hospital systems overwhelmed,” Anthony Fauci, the director of the National Institutes of Allergy and Infectious Disease, said Thursday.

But how bad, exactly, could it get?

Fox and his colleagues modeled Covid-19 cases, hospitalizations, and deaths through the winter and into May 2022. For their 18 scenarios, they tweaked factors like the transmissibility of omicron, differing degrees of severity of the virus, the level of immunity in the US population, the likelihood of reinfection among Covid-19 survivors and breakthrough infections among those vaccinated, and the number of people who topped up their vaccine doses with a booster.

Under all the scenarios they modeled, omicron supplanted delta and became the main driver of Covid-19 infections, pushing case counts higher. “The first key finding is that unless significant transmission reduction happens in our communities, we’re likely to see an omicron surge that rivals the previous peak that we saw in January 2021,” Fox said.

The most optimistic scenario emerged when omicron was 50 percent more transmissible than delta and 10 percent better at eluding immunity from vaccines and previous infections, yet led to equally severe illnesses. That scenario also presumed many people will get boosters — at 80 percent uptake by March 2022 — but that no other policy or behavior changes are made to reduce transmission. It predicted a Covid-19 peak in mid-January 2022, but with 8 percent fewer cases and 43 percent fewer hospitalizations than the same six-month period the year before. It also led to 54 percent fewer deaths, totaling 152,000 (still a grim result).

The worst outcome arose when the model assumed omicron was just as transmissible as delta, but far more evasive of prior immunity and much more likely to cause severe disease. In this scenario, prior immunity was 85 percent less effective at preventing infection from omicron, and protection against death was 22 percent lower. In this worst case, vaccine booster uptake remained fairly low, reaching only 57 percent by the end of March 2022. This resulted in Covid-19 cases peaking in early February 2022 and 342,000 deaths over six months, a 20 percent increase from 2021.

That Covid-19’s devastation could be even worse in an era of vaccines and treatments is tragic, a stark consequence of failing to get the pandemic under control across the country.

However, if the booster uptake rate increased to 80 percent, Covid-19 cases in this scenario dropped by 5 percent, hospitalizations by 12 percent, and deaths by 13 percent. That translates to 1.3 million averted infections and 39,000 lives saved between December 2021 and May 2022.

The scenarios show that there is a bit of luck involved in how harsh the next few months will be, but specific actions like getting booster doses of vaccines rolled out can vastly improve the outlook.

The US is not united when it comes to Covid-19

For millions of Americans, now two years into the pandemic, omicron is triggering an exasperating episode of déjà vu.

However, the question of the variant’s severity is still unclear. And researchers warn that even if omicron turns out to be less dangerous for individuals, it could still cause widespread damage if it continues to spread out of control.

The context for the omicron surge also varies throughout the country, something not accounted for in the UT Austin simulation. “This analysis is really just looking at an average across the whole country,” said Fox.

There are things it doesn’t factor in: the rate of preexisting health conditions, access to health care, exposure to prior waves of infection, adherence to mask-wearing, and vaccine uptake — which can be radically different around the country. Around 72 percent of the US population have received at least one shot of a Covid-19 vaccine, but in states like Idaho and Mississippi, only half have gotten it. New Hampshire, meanwhile, is above 90 percent. There are around 90 million people who are unvaccinated against Covid-19 throughout the country, but many are concentrated in distinct regions, often aligned with political views.

That means omicron could play out quite differently in different parts of the country, with some places facing far more hospitalizations and deaths than others with high vaccination rates. And given how readily omicron can spread, the regions with lower vaccination rates that have so far lucked out of previous waves may now be vulnerable.

This is definitely not the time to let our guard down

People who have not been previously infected or immunized against Covid-19 face the greatest risk of omicron infection. But omicron has also shown that vaccinations are not an impermeable shield against infection.

Many omicron cases have been detected in people who completed their course of Covid-19 vaccines, even in some people who received booster doses. It’s a disheartening prospect for people who have rigorously followed public health advice throughout the pandemic, eager for it to be over.

“I share the frustration,” said Justin Feldman, a research fellow and social epidemiologist at Harvard University. “Unfortunately, I don’t think that [the coming omicron wave is] something that individuals can solve with their own personal behaviors.”

The most impactful measures for dealing with Covid-19 have to happen at the policy level, according to Feldman. That includes easily accessible widespread testing for Covid-19 to detect infections early so people can isolate from others and seek treatment, something the US is still struggling to do. It also includes mandates for vaccines, quarantine and isolation rules for workers, regulations for indoor ventilation, making high-quality masks widely available, and training a corps of pandemic responders to administer tests, treatments, and vaccines.

“These are things Biden should have been trying to build since January 20, but largely hasn’t because the administration went with a very vaccine-centric approach,” Feldman said.

While the White House has taken some steps to implement some of these policies, like purchasing and distributing millions of rapid Covid-19 tests, it’s becoming clear there is little political will to implement more restrictions even as another towering wave of Covid-19 infections crests. Mask mandates are lapsing, travel is increasing, businesses are reopening, and large indoor gatherings are resuming.

“But that doesn’t mean apathy is the solution,” wrote Vox’s Dylan Scott.

The pandemic playbook of maintaining social distancing, rigorous hand-washing, wearing face coverings, getting tested for Covid-19 after a possible exposure, and getting vaccinated remains useful, even if it doesn’t completely solve the problem.

With omicron, there are some tweaks to this; namely, for eligible vaccinated people to get boosters and for people to ditch cloth face masks for higher-quality options, like N95 respirators and KN95 masks.

“We are really recommending that people up their mask game, especially now with omicron,” Gounder said.

Regardless of what course omicron takes throughout the country, health officials are bracing for a situation that will get far worse before it gets better. “I think we really do need to anticipate there probably will be a surge and increase in hospitalizations over the coming months,” Gounder said. Just how dangerous it will be is partly in our hands.