In December 2021, researchers discovered that the original omicron variant had split into two subvariants: BA.1 and BA.2.
The two subvariants arose around the same time and come from the same ancestor strain. They have many mutations in common, but around 20 mutations differ between the two subvariants. The differences between the subvariants can be seen in the spike protein of the virus.
Some researchers started calling BA.2 a “stealth” variant because they believed it might be more difficult to track than the original omicron variant.
Between Jan. 30 and Feb. 5, BA.2 cases only made up about 1% of the new coronavirus cases in the United States. But BA.2 cases increased to about 7% at the end of February and then doubled to about 14% by the first week in March.
By mid-March 2022, the CDC estimated that BA.2 caused 23.1% of all cases in the United States while BA.1 caused 10.8% of cases. Another subvariant (BA.1.1) was the cause of the remaining 66.1% of cases.