The Centers of Disease Control and Prevention (CDC) estimated in a study released Thursday that the Alpha variant made up about 66 percent of COVID-19 infections in the U.S. in mid-April.
The study highlights how quickly a new variant can emerge, spread and become the dominant strain in the U.S.
The CDC predicted the Alpha variant, known by the scientific name B.1.1.7, became the dominant strain during the two-week period ending with March 10.
But during the last weeks of December, the CDC estimated the strain first discovered in the U.K. made up 0.2 percent of U.S. COVID-19 cases, showing the Alpha variant’s speedy spread over four months.
The research also measured the prevalence of the Gamma variant, also known as P.1, which made up an estimated 5 percent of cases in the U.S. between April 11 and 24. This represents a rise from when the CDC predicted the strain first discovered in Brazil made up 0.1 percent of cases in the two-week period ending with Jan. 30.
The agency projects that in the two-week period April 25 to May 8, the Alpha variant made up 72.4 percent of cases, and the Gamma strain made up 6.2 percent of cases.
Experts have warned that variants of COVID-19 present the potential to be more transmissible, cause more serious illness and evade therapeutics and vaccines.
In this study, the researchers called for the country to conduct “robust and timely genomic surveillance” in order to track the variants and respond appropriately, including through targeting distribution of vaccines and monoclonal antibody treatments.
In the two-week period ending in April 24, the study predicted the Alpha strain made up the highest percentage of cases, 74.1 percent, in Department of Health and Human Services (HHS) region six, which includes Arkansas, Louisiana, New Mexico, Oklahoma and Texas.
HHS region five, with Illinois, Indiana, Michigan, Minnesota, Ohio and Wisconsin, saw the highest proportion of Gamma strain cases in that time period.
The CDC started national surveillance of COVID-19 variants with genomic sequencing in November, with the number of sequences ramping up over the past few months. In the two-week period ending in Jan. 2, 3,275 sequences were conducted, or 0.1 percent of positive COVID-19 tests. In April, that boosted to 25,000 sequences, making up 3.1 percent of positive tests.
But last month, The New York Times reported that the agency moved to only track breakthrough cases — cases that occur in a fully vaccinated person — if that patient is hospitalized or dies.
The decision sparked criticism from experts who were concerned the limited tracking could allow an undetected variant capable of thwarting the vaccine to spread quicker, although the CDC argued it was focusing on the most important cases.