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 A new Covid-19 virus, known in the medical community as, JN.1, close relative to BA.2.86, is the fastest growing variant in the US, according to estimates from the Centre for Disease Control and Prevention (CDC).

JN.1 was responsible for more than one in five new Coronavirus cases in recent weeks.

"We continue to see this Covid virus change," CDC Director Mandy Cohen said during an event recently.

"In August of this year, we probably saw one of the larger changes to the Covid virus at that time, and what we've seen lately in the last couple of weeks is an offshoot of that August variant."

The August variant Cohen referred to is BA.2.86, also known as "Pirola". When the strain emerged, organisations like the CDC and the WHO were warning that its high number of mutations made it one to watch. But now, it appears that JN.1 could be more of a problem, media reports said.

JN.1 is very similar to Pirola. "Even though BA.2.86 and JN.1 sound very different because of the way variants are named, there is only a single change between JN.1 and BA.2.86 in the spike protein," the CDC said in a recent update on the strain.

The new strain was first detected in the US in September. Since then, it has grown to represent an estimated 15 to 29 per cent of new infections, according to CDC data. The agency expects that JN.1's prevalence in the US will continue to increase, US News & World Report observed.

Covid-19 tests and treatments are expected to work on JN.1. So far, it doesn't appear to cause more severe disease, but it does seem to have advantages over the other strains.

"The continued growth of JN.1 suggests that it is either more transmissible or better at evading our immune systems," the CDC said.

Despite this, the CDC assesses that "at this time, there is no evidence that JN.1 presents an increased risk to public health relative to other currently circulating variants".

As of now, it's not known if JN.1 causes different symptoms from other strains.

"In general, symptoms of Covid-19 tend to be similar across variants," the CDC said.

"The types of symptoms and how severe they are usually depend more on a person's immunity and overall health rather than which variant causes the infection."

The updated Covid-19 shot is expected to offer protection against JN.1.

"The good news is, even with the changes that have come in the last several weeks, the updated vaccine is still good coverage for that based on lab studies," Cohen said about the latest strains.

Vaccine expert Peter Hotez called JN.1 a "bad" strain, noting its high transmissibility and immune escape capabilities.

"This new XBB annual immunisation that became available this September should offer some cross protection to keep you out of the hospital," Hotez said on social media.

Uptake of the shot, however, has been low as only 17 per cent of adults and nearly eight per cent of children have rolled up their sleeves for it, according to national survey data.

And Covid-19 metrics are on the rise. New hospital admissions have been increasing for a month, surpassing the peak numbers during the late summer wave. The CDC warned that JN.1's "rapid growth" compared to other strains raises a question: Could it drive an increase in infections?

"Right now, we do not know to what extent JN.1 may be contributing to these increases or possible increases through the rest of December like those seen in previous years," the agency said.

"CDC will closely monitor Covid-19 activity and the spread of JN.1."