The holidays have come and gone, and once again Americans are riding a tide of respiratory ailments, including Covid. But so far, this winters Covid uptick seems less deadly than last years, and much less so than in 2022, when the Omicron surge ground the nation to a halt.

Were not seeing the signs that would make me think that were heading into another severe wave, said Caitlin Rivers, an epidemiologist at the Johns Hopkins Center for Health Security. So far, were in relatively good shape.

Still, there are few masks in sight, and just a fraction of the most vulnerable people have received the latest Covid shots, she noted.

Its not too late, Dr. Rivers added. We have not even reached peak yet for Covid, and once you reach peak, you still have to get down the other side. That leaves plenty of time for the vaccine to provide some protection.

Federal officials are relying on limited data to measure this years spread. After the end of the public health emergency in May, the Centers for Disease Control and Prevention stopped tracking the number of Covid infections. The agency now has only partial access to information from states about vaccination rates.

But trends in wastewater data, positive tests, emergency department visits, hospitalization rates and deaths point to a rise in infections in all regions of the nation, according to the C.D.C. These patterns have prompted many hospitals to reinstate mask policies, after initially resisting a return to them this fall.

As in previous years, the numbers have steadily been rising all winter, and are expected to increase further after holiday travel and get-togethers.

Many of the infections are caused by a new variant, JN.1, which has rapidly spread across the world in recent weeks. I think that theres no doubt its helping drive, pretty substantially, this winter wave, said Katelyn Jetelina, a public health expert and author of a widely read newsletter, Your Local Epidemiologist.

Unfortunately, its coming at the same exact time as us opening up our social networks due to the holidays, she said, so theres kind of a perfect storm going on right now.

Some scientists have pointed to rising levels of the virus in sewage samples as an indicator that infections are at least as high this year as they were at this time last year. But Dr. Rivers urged caution in interpreting wastewater data as a proxy for infections and said hospitalizations were a more reliable metric.

In the week that ended on Dec. 23, hospitalizations rose by nearly 17 percent from the previous week. There were about 29,000 new hospital admissions, compared with 39,000 the same week last year and 61,000 in 2021.

And weekly hospitalizations are increasing more slowly than in previous years, Dr. Rivers said.

Covid is still claiming at least 1,200 lives per week. But that number is about one-third the toll this time last year and one-eighth that in 2021.

We are in this pretty big infection surge right now, but whats really interesting is how hugely hospitalizations have and continue to decouple from infections, Dr. Jetelina said.

She said she worried most about hospitals buckling under the weight of multiple epidemics at once. Even in years before the pandemic, outbreaks of just influenza and respiratory syncytial virus could strain hospitals; rising Covid rates now overlap both illnesses, adding to the burden.

The C.D.C. estimates that so far this season, there have been at least 7.1 million illnesses, 73,000 hospitalizations and 4,500 deaths from the flu.

While Covid tends to be mild in children and young adults, influenza and R.S.V. are most risky for young children and older adults. All three diseases are particularly dangerous for infants.

Emergency department visits for Covid are highest among infants and older adults. While R.S.V. has leveled off in some parts of the country, hospitalization rates remain high among young children and older adults.

The JN.1 variant accounts for nearly half of all Covid cases in the United States, nearly six times the prevalence just a month ago. The variant has one mutation that gives it a greater ability to sidestep immunity than its parent, BA.2.86, which was limited in its spread.

JN.1 may in fact be less transmissible than previous variants. But its immune evasiveness, coupled with the disappearance of preventive measures like masks, may explain its exponential growth worldwide, said Dr. Abraar Karan, an infectious disease physician and postdoctoral researcher at Stanford University.

Still, JN.1 does not appear to cause more severe illness than previous variants, and the current vaccines, tests and treatments work well against all of the current variants.

Experts urged all Americans including those not at high risk of severe illness to opt for vaccines against both Covid and flu, to use masks and air purifiers to prevent infections, to be tested and treated and to stay home if they become ill.

Even those who do not become severely sick run the risk of long-term complications with every new viral infection, researchers noted.

Im not at high risk, to be honest Im young and vaccinated, Dr. Rivers said. But I continue to take precautions in my own life because I do not want to deal with that disruption, and the risk that I could develop a longer-term illness.

But few Americans are following that advice. As of Dec. 23, only 19 percent of adults had received the latest Covid vaccine, and about 44 percent had opted for the annual flu shot. Just over 17 percent of adults aged 60 and older had received the vaccine for R.S.V.

Even among those 75 and older, who are at highest risk from Covid, only about one in three have received the latest shot, according to the C.D.C.

Many people dont realize that shots that protect against the newest variants are available, or that they should be vaccinated even if they are not at high risk, said Gigi Gronvall, a biosecurity expert at the Johns Hopkins Center for Health Security.

Even if the Covid vaccine does not prevent infection, it can shorten the duration and severity of illness, and minimize the risk of long-term symptoms, including brain fog, fatigue, movement problems and dizziness collectively known as long Covid.

Im sure also there are plenty of people who are actively hostile to the idea, but most of the people I encounter, they just dont even know about it, Dr. Gronvall said.

Poor availability of the shots, particularly for children and older adults, has also limited the vaccination rates.

Dr. Gronvall struggled to find a Covid vaccine for her teenage son. Dr. Jetelina has yet to find any for her young children. She said her grandparents, who are both in their mid-90s, also had an incredibly challenging time.

One of them is in a nursing home and still hasnt been immunized because she happened to be sick the one day the vaccines were offered.

Many nursing home residents and staff members remain unvaccinated, because the staff doesnt understand the benefits, said Dr. Karan, who worked with nursing facilities in Los Angeles County.

Financial incentives can improve vaccine coverage, but the lack of awareness about the benefits is a major problem, he said.

Experts also urged people who develop symptoms to take a test and ask for antiviral drugs Tamiflu for influenza, Paxlovid for Covid especially if they are at high risk of complications.

Paxlovid is still available free of charge to most people, but many patients and even doctors avoid it out of a mistaken belief that it causes Covid symptoms to rebound, experts said. Recent studies did not find a relationship between antiviral drugs and symptom rebound.

For many viruses, including the flu, we know that earlier use of antivirals is going to be beneficial, Dr. Karan said. You stop viral replication quickly, you have less of an immune dysregulation thereafter.

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Covid Has Resurged, but Scientists See a Diminished Threat - The New York Times