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% percent 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 severity of symptoms or decreases effectiveness of vaccine.

The Centers for Disease Control and Prevention (CDC) and other federal health agencies are tracking the spread of these strains, which the agency deemed concerning” because they appear to spread more readily than the original virus. The agency is providing verified information about what is distinct about these variants in terms of transmissibility, vaccine effectiveness, and treatment.

The CDC notes that increased COVID-19 transmission “might threaten strained health care resources, require extended and more rigorous implementation of public health strategies, and increase the percentage of population immunity required for pandemic control.”

Here's what the CDC knows about the variants of concern.

The UK strain (B.1.1.7)

The United Kingdom variant — B.1.1.7 — was discovered in September 2020 and carries the most mutations among the variants. The CDC says it is 50% more transmissible than the original COVID-19 strain.

In January, UK-based researchers reported that this variant might be more deadly than others. According to the CDC, there is a likely increase in severity, which the agency determined using hospitalization and case fatality rates.

B.1.1.7 was first detected in the US in December 2020.

The UK COVID-19 variant has been detected in every state.

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The South Africa strain (B.1.351)

The South Africa strain — B.1.351 or 501.V2 — was detected in October 2020. It shares some mutations with B.1.1.7 but emerged independently. There is no evidence that the variant is more or less severe than the original virus, but some evidence suggests certain therapeutic antibodies may not be effective against this form of the virus. Preliminary evidence also shows that the Moderna vaccine might be less effective against this strain, but the CDC cautions that more studies are needed.

The Brazil strain (P.1.)

The Brazil variant — P.1. — was first identified in travelers from Brazil tested at an airport in Japan in early January. This variant contains a set of additional mutations that may affect antibodies’ ability to recognize the strain.

California strains (B.1.427 and B.1.429)

The CDC identified these two strains in February 2021 and classified these as variants of concern in March. These strains have an estimated 20% increase in transmissibility and have some effect on the effectiveness on treatment.

Federal health officials say there is an ongoing need for surveillance, tracking, and vaccine deployment worldwide. A recent paper co-authored by Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, says studying virus changes may help develop a vaccine that protects against most or all variants.

Emerging SARS-CoV-2 Variants
Emergence of SARS-CoV-2 B.1.1.7 Lineage — United States, December 29, 2020–January 12, 2021
Variant Proportions in the U.S.

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