Until recently it was the ‘stealth’ Omicron: Here’s what we know about the BA.2 variant.

As the Omicron coronavirus surge subsides, researchers are keeping an eye on a highly transmissible subvariant known as BA.2. Although it doesn’t appear to have the capacity to drive a large new wave of infections, the variant could potentially slow the current decline of Covid cases and make treatments more difficult.

Here’s what we know so far about BA.2.

It’s not really new. Scientists first discovered the Omicron variant in November, and it quickly became clear that the viral lineage already existed as three genetically distinct varieties. Each branch of Omicron had its own set of unique mutations. At the time, the most common was BA.1, which quickly spread across the world. BA.1 was almost entirely responsible for the record-shattering spike in cases this winter. At first, BA.1 was a thousand times as common as BA.2. But in early 2022, BA.2 started to be found in a larger proportion of new infections.

It seems to be easier to catch. In Denmark, for example, scientists examined the spread of both subvariants in households. They found that people infected with BA.2 were substantially more likely to infect people they shared a house with than those with BA.1.

It is not yet causing a new surge in the United States, and probably won’t. Existing vaccines work against the BA.2 variant, and it’s vulnerable to antibodies made by the immune system after an earlier Omicron infection.

BA.2 does not appear to be more severe than the previous version of Omicron. British researchers have found that BA.2 infection does not carry a higher risk of hospitalization than BA.1.

BA.2’s ‘stealth variant’ nickname is outdated. BA.2 was nicknamed the “stealth variant” when it did not tip off its presence in positive P.C.R. test samples, making it a challenge for researchers to distinguish Omicron cases from those of Delta and other variants. BA.2 carried a mutation that concealed one of the three telltale coronavirus genes that the tests detect. Now that a vast majority of positive tests involve Omicron, the missing mutation doesn’t matter: Nearly all viruses picked up by P.C.R. are BA.1, and those that are not are BA.2.

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