The CDC recently released estimates showing the proportion of coronavirus strains circulating in the United States. During the two weeks ending January 2, variants that increase transmissibility, severity, treatments, or impact of vaccine effectiveness made up an estimated 4% of cases.
Preliminary estimates on the two weeks ending March 13 show that those strains made up 39% of the cases during that period. The B.1.1.7 variant, first detected in the UK, accounted for 27% of the cases during the period, the most of any strain. Other common variants — B.1.351, first detected in South Africa, and P.1, first detected in Brazil — each account for less than 1% of cases. According to the CDC, the UK variant is notable for increased transmissibility and increased risk of death. There is no evidence suggesting that it increases the severity of symptoms or decreases the effectiveness of the vaccine.
