The BA.2 subvariant of omicron was estimated to be 11.6% of the coronavirus variants circulating in the United States as of March 5, the U.S. Centers for Disease Control and Prevention (CDC) said on Tuesday.

Scientists are tracking a rise in cases caused by BA.2, the dominant variant in South Africa, which is spreading rapidly in parts of Asia and Europe.

The World Health Organization said last month that the BA.2 variant appears to be more transmissible than the original BA.1 subvariant, based on initial data. Africa's top public health body last month said BA.2 does not cause more severe disease than BA. 1.

U.S. daily cases have started to decline in recent weeks after touching record levels in January, with the CDC dramatically easing its COVID-19 guidelines for masks, including in schools.

Other omicron subvariants that have been circulating since December - called BA.1.1 and B.1.1.529 - now make up around 73.7% and 14.7% of circulating variants, respectively.

The CDC estimates that BA.2 made up 6.6%, revised down from 8%, of circulating variants in the country the previous week, according to a CDC model that estimates proportions of circulating variants.

The CDC has in the past revised its estimates as it gets more data.