WASHINGTON (BLOOMBERG) - The Omicron subvariant BA.2 is continuing to gain ground in the US, according to Covid-19 tests sequenced over the last two weeks.

Helix, a San Diego-based genomics firm, has been watching the BA.2 variant since it first popped up in the US in early January.

Although it was initially slow to take hold, Helix now estimates that 50 per cent to 70 per cent of all Covid cases nationwide are BA.2.

Dr Will Lee, Helix's chief science officer, said this type of surveillance is essential and can help arm the US healthcare system against future variants.

Data from the UK - where BA.2 has already caused cases to spike - show the country's rise in infections began around the same time that BA.2 surpassed the 50 per cent mark of overall cases.

So far, the variant appears to be no more severe than the initial Omicron strain, but there's concern about its ability to reinfect people and its links to long Covid.

Cases are still dropping overall in the US, but some health experts are worried that decline won't last for long.

Already, cases are beginning to climb in New York City, where BA.2 accounts for a larger share of cases than in other parts of the country, according to data from the Centres for Disease Control and Prevention (CDC).

The CDC has yet to update its variant surveillance data for this past week, but the agency has reported BA.2 nearly doubling in prevalence each week since early February.

The CDC most recently said BA.2 made up 23.1 per cent of cases for the week ending March 12.

Variant surveillance at Helix is funded by the CDC and its sequencing data is one of many the agency takes into account when creating its Nowcast estimates each week.

According to Helix, the firm is able to process up to 150,000 Covid-19 tests per day across the country.

Dr Lee, the Helix scientist, said that although he expects cases to rise in coming weeks, he does not anticipate as significant a spike as was caused by the first omicron strain.

That's due to the immunity people have already built up through vaccines and prior infections.