The BA.2 subvariant of COVID-19's Omicron strain may be starting to make itself known within the United States, epidemiologists fear. Nevertheless, its spread has not followed the steep upward path there that it has in some other countries, nor that shown by the original Omicron,

BA.2 is a form of Omicron – different enough to be worth noting, but similar enough it hasn't been given its own letter. Virologists say it is more of a sibling to the original Omicron than a descendent, sharing a distant ancestor. It's now present in almost every country. In some, such as Denmark and South Africa, it has outpaced other strains, leading public health officials to brace for yet another wave of the virus.

In other places, however, its numbers have been modest or stalled entirely, but no one knows why.

"A lot of us were assuming that it was going to quickly take off in the United States just like it was doing in Europe and become the new dominant variant," Dr Nathan Grubaugh of the Yale School of Public Health told NPR.

First detected in the USA in early January and making up only 3.9 percent of new infections in mid-February according to the CDC, that hasn't happened. By contrast, Omicron's two earlier successful subvariants BA1.1 and BA1.1.529 went from a handful of cases to complete dominance in the space of December.

Nevertheless, that doesn't mean BA.2 is a non-threat, nor that America is somehow immune. After all, on the same CDC data, the strain doubled between the week ending February 5 and February 12. Another month at that rate, and it could unleash yet another wave on the continent's battered health care system.

The optimistic view, expressed by Grubaugh as a probability but not a certainty, is that recent infection with Omicron will prove protective against BA.2. With so many people in that category, the wave could fizzle out, despite its high transmissibility.

Some see signs of this happening in Australia, where BA.2 infection rates have been stable for weeks. Others are more worried, pointing to the dramatically higher rate of spread described in a pre-print describing BA.2's appearance in Denmark.

Decisions both at the government and individual level will play a part. If BA.2's advance has been delayed because the people most susceptible are wearing masks and avoiding crowded places, overconfidence could allow it to take off.

"There is this lurking threat of BA.2. And we need to make sure this isn't going to be a problem before we roll back all the mandates, before we tell everybody that it's safe," The Rockefeller Foundation's Dr Samuel Scarpino told NPR. 

The situation with BA.2 is complicated by the fact it's harder to differentiate from Omicron BA.1 than Omicron was from previous variants. PCR tests can still recognize the so-called “stealth sub-variant”, but take longer to do so, making progress harder to track.

Despite Omicron sometimes being presented as mild, it has now caused more deaths in the United States than the Delta wave.