NEW YORK (NYTIMES) - Like it or not, the choose-your-own-adventure period of the pandemic is upon us.

Mask mandates have fallen. Some free testing sites have closed. Whatever parts of the United States were still trying to collectively quell the pandemic have largely turned their focus away from community-wide advice.

Now, even as case numbers begin to climb again and more infections go unreported, the onus has fallen on individual Americans to decide how much risk they and their neighbours face from the coronavirus - and what, if anything, to do about it.

For many people, the threats posed by Covid have eased dramatically over the two years of the pandemic. Vaccines slash the risk of being hospitalised or dying. Powerful new antiviral pills can help keep vulnerable people from deteriorating.

But not all Americans can count on the same protection. Millions of people with weakened immune systems do not benefit fully from vaccines.

Two-thirds of Americans, and more than one-third of those 65 and older, have not received the critical security of a booster shot, with the most worrisome rates among Black and Hispanic people. And patients who are poorer or live farther from doctors and pharmacies face steep barriers to getting antiviral pills.

These vulnerabilities have made calculating the risks posed by the virus a fraught exercise. Federal health officials' recent suggestion that most Americans could stop wearing masks because hospitalisation numbers were low has created confusion in some quarters about whether the likelihood of being infected had changed, scientists said.

"We're doing a really terrible job of communicating risk," said Dr Katelyn Jetelina, a public health researcher at the University of Texas Health Science Centre at Houston. "I think that's also why people are throwing their hands up in the air and saying, 'Screw it.' They're desperate for some sort of guidance."

To fill that void, scientists are thinking anew about how to discuss Covid risks. Some have studied when people could unmask indoors if the goal was not only to keep hospitals from being overrun but also to protect immunocompromised people.

Others are working on tools to compare infection risks to the dangers of a wide range of activities, finding, for instance, that an average unvaccinated person 65 and older is roughly as likely to die from an Omicron infection as someone is to die from using heroin for 18 months.

But how people perceive risk is subjective; no two people have the same sense of the chances of dying from a year and a half of heroin use (about 3 per cent, by one estimate).

And beyond that, many scientists said they also worried about this latest phase of the pandemic heaping too much of the burden on individuals to make choices about keeping themselves and others safe, especially while the tools for fighting Covid remained beyond some Americans' reach.

"As much as we wouldn't like to believe it," said Dr Anne Sosin, who studies health equity at Dartmouth College, "we still need a society-wide approach to the pandemic, especially to protect those who can't benefit fully from vaccination."

Collective metrics

While Covid is far from America's only health threat, it remains one of its most significant. In March, even as deaths from the first Omicron surge plummeted, the virus was still the third leading cause of death in the United States, behind only heart disease and cancer.

More Americans overall have been dying than would have in normal times, a sign of the virus's broad toll. As of late February, 7 per cent more Americans were dying than would have been expected based on previous years - a contrast with Western European nations like Britain, where overall deaths have lately been lower than expected.

How much virus is circulating in the population is one of the most important measures for people trying to gauge their risks, scientists said. That remains true even though case numbers are now undercounting true infections by a large margin because so many Americans are testing at home or not testing at all, they said.

Even with many cases being missed, the Centres for Disease Control and Prevention (CDC) now places most of the North-east at "high" levels of viral transmission. In parts of the region, case numbers, while far lower than during the winter, are nearing the peak rates of autumn's delta variant surge.

Much of the rest of the country has what the CDC describes as "moderate" levels of transmission.

Covid-19 versus driving

Even two years into the pandemic, the coronavirus remains new enough, and its long-term effects unpredictable enough, that measuring the threat posed by an infection is a thorny problem, scientists said.

Some unknown number of people infected will develop long Covid, leaving them severely debilitated. And the risks of getting Covid extend to others, potentially in poor health, who may consequently be exposed.

Still, with far more immunity in the population than there once was, some public health researchers have sought to make risk calculations more accessible by comparing the virus with everyday dangers.

The comparisons are particularly knotty in the United States: The country does not conduct the random swabbing studies necessary to estimate infection levels, making it difficult to know what share of infected people are dying.

Dr Cameron Byerley, an assistant professor in mathematics education at the University of Georgia, built an online tool called Covid-Taser, allowing people to adjust age, vaccine status and health background to predict the risks of the virus. Her team used estimates from earlier in the pandemic of the proportion of infections that led to bad outcomes.

Her research has shown that people have trouble interpreting percentages, Dr Byerley said. She recalled her 69-year-old mother-in-law being unsure whether to worry earlier in the pandemic after a news programme said people her age had a 10 per cent risk of dying from an infection.

Dr Byerley suggested her mother-in-law imagine if, once out of every 10 times she used the restroom in a given day, she died. "Oh, 10 per cent is terrible," she recalled her mother-in-law saying.

Dr Byerley's estimates showed, for instance, that an average 40-year-old vaccinated over six months ago faced roughly the same chance of being hospitalised after an infection as someone did of dying in a car crash in the course of 170 cross-country road trips. (More recent vaccine shots provide better protection than older ones, complicating these predictions.)

For immunocompromised people, the risks are higher. An unvaccinated 61-year-old with an organ transplant, Dr Byerley estimated, is three times as likely to die after an infection as someone is to die within five years of receiving a diagnosis of stage 1 breast cancer. And that transplant recipient is twice as likely to die from Covid as someone is to die while scaling Mount Everest.

With the most vulnerable people in mind, Dr Jeremy Faust, an emergency physician at Brigham and Women's Hospital in Boston, set out last month to determine how low cases would have to fall for people to stop indoor masking without endangering those with extremely weakened immune systems.

He imagined a hypothetical person who derived no benefit from vaccines, wore a good mask, took hard-to-get prophylactic medication, attended occasional gatherings and shopped but did not work in person. He set his sights on keeping vulnerable people's chances of being infected below 1 per cent over a four-month period.

To achieve that threshold, he found, the country would have to keep masking indoors until transmission fell below 50 weekly cases per 100,000 people - a stricter limit than the CDC is currently using but one that he said nevertheless offered a benchmark to aim for.

"If you just say, 'We'll take masks off when things get better' - that's true I hope - but it's not really helpful because people don't know what 'better' means," Dr Faust said.