NEW YORK – As the holiday season winds down and Covid-19 cases start to pick up, a variant called JN.1 has now become the most common strain of the virus spreading across the United States.

JN.1, which emerged from the variant BA.2.86 and was first detected in the US in September, accounted for 44 per cent of Covid-19 cases nationwide by mid-December, up from about 7 per cent in late November, according to data from the US Centres for Disease Control and Prevention (CDC).

To some extent, this jump is to be expected.

“Variants take some time to get going,” said Dr William Schaffner, an infectious disease specialist at Vanderbilt University Medical Centre. “Then they speed up, they spread widely, and just when they’re doing that, after several months, a new variant crops up.”

JN.1’s momentum in December suggests that it may be more transmissible or better at evading human immune systems than other variants currently circulating, according to a CDC report published on Dec 22.

The agency said that Covid-19 remains a serious public health threat, especially for those who have always been at high risk of severe disease, such as older adults, infants, people with compromised immune systems or chronic medical conditions, and those who are pregnant.

Experts say JN.1 does not seem to be causing severe illness in most other people, though even a mild case can still make you feel “quite miserable for three or four days”, Dr Schaffner said. The symptoms of a JN.1 infection are similar to those caused by previous Covid-19 variants, including a cough, fever, body aches and fatigue.

To protect oneself against infection and severe disease, experts continue to recommend wearing masks, improving ventilation indoors when possible, staying home when sick and getting the latest Covid-19 vaccine.

Preliminary research shows that the updated Covid-19 vaccines released in September produce antibodies effective against JN.1, which is distantly related to the XBB.1.5 variant that the vaccines were designed to target.

People may not build up as many antibodies to JN.1 as they would to XBB.1.5, but the levels should still decrease the risk.

“For those who were recently infected or boosted, the cross-protection against JN.1 should be decent, based on our laboratory studies,” said Dr David Ho, a virologist at Columbia University who led the research on JN.1 and Covid-19 vaccines. The research was released as a pre-print paper in early December.

Rapid tests also continue to be a valuable tool, and the CDC has said tests already on the market work well at detecting JN.1.

There are signs that Covid-19 cases are once again creeping up.

There were just under 26,000 hospitalisations due to Covid-19 the week of Dec 10, a 10 per cent increase from about 23,000 hospitalisations the week prior.

But Covid-19 hospitalisations are still far lower than they were during the peak of the first Omicron wave in January 2022, and so far, only about half as high as they were during the peak of the tripledemic last winter, when Covid-19, flu and respiratory syncytial virus cases all surged at the same time.

It is too early to know whether JN.1 is responsible for the rise in hospitalisations, or whether cases are picking up partly because of an increase in travel and large get-togethers for Thanksgiving and the winter holidays.

“When people are gathered inside close to each other, having parties and travelling and the like, those are the kind of circumstances where all respiratory viruses, including JN.1, have opportunities to spread,” Dr Schaffner said.

Covid-19 generally also has some seasonality, he added. Countries in the Northern Hemisphere tend to see a lull in cases in autumn before infections and hospitalisations rise again in the winter.

JN.1 will most likely remain the dominant version of the coronavirus through spring, Dr Schaffner said. NYTIMES