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New coronavirus variants could lead to a rapid rise in Covid-19 cases, and can “dangerously accelerate the trajectory of the pandemic,” US health officials warned Wednesday.

“I know these variants are concerning, especially as we’re seeing signs of progress,” US Centers for Disease Control and Prevention Director Dr. Rochelle Walensky said during a White House briefing on Wednesday. “I’m talking about them today because I am concerned, too.

“The continued spread of variants that are more transmissible could jeopardize the progress we have made in the last month if we let our guard down.”

In a report released Wednesday, researchers from the CDC and the Minnesota health department detailed cases of the B.1.1.7 variant – first identified in the United Kingdom – among eight Minnesota residents. Earlier modeling data has suggested this variant, which may be more transmissible, could become the predominant variant in the United States in March.

The new study, published in CDC’s Morbidity and Mortality Weekly Report, described people ages 15 to 41 whose samples were collected from mid-December through mid-January; five reported Covid-19-like symptoms and three were asymptomatic.

Among the eight people, three had a history of international travel in the two weeks before they became ill, including two who traveled to West Africa and one who traveled to the Dominican Republic, and three others had traveled to California. One person was exposed to the virus at home, and another in the community. None had a history of travel to the UK.

Identification of the variant in Minnesota “highlights the importance of mitigation measures such as mask use, physical distancing, avoiding crowds and poorly ventilated indoor spaces, isolation of persons with diagnosed COVID-19, quarantine of close contacts of persons with COVID-19, and adherence to CDC travel guidance,” the report says.

During the White House briefing, Walensky said it’s “more important than ever for us to do everything we can to decrease the spread.”

“Fewer cases means fewer opportunities for the variants to spread and fewer opportunities for new variants to emerge,” she said.

On Tuesday, the CDC reported at least 1,299 cases of coronavirus strains first spotted in the UK, South Africa and Brazil have been reported in the United States. The vast majority of these cases, 1,277, are the B.1.1.7 variant originally detected in the UK. This variant has been found in 41 states and Washington, DC. Roughly a third are in Florida.

Nineteen of those 1,299 are the B.1.351 variant first identified in South Africa. The P.1 strain first linked to Brazil has been discovered among two cases in Minnesota and one in Oklahoma.

These numbers don’t represent the total number of such cases circulating in the United States, but rather, only those found by analyzing positive samples.

Rapid increase in cases is ‘not inevitable’

In a separate report released by the CDC on Wednesday, researchers in Zambia and elsewhere described how the detection of the B.1.351 variant in South Africa coincided with a rapid rise of cases in Zambia – and the variant might have become the dominant strain there.

The B.1.351 variant also might be circulating elsewhere in southern Africa, where many countries reported rapid increases in Covid-19 case numbers in December and January, the report said.

“Spread of the B.1.351 variant is of public health concern because of the potential for increased transmissibility and, thus, increases in cases, hospitalizations, and deaths,” researchers wrote.

In a related commentary also published Wednesday, Walensky, Dr. Henry Walke from the CDC and Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, wrote that genomic sequencing identified no cases of the B.1.351 variant in Zambia in March to early December 2020, but the variant was identified in 96% of samples sequenced from a one-week period in mid-December. That corresponded with a 16-fold increase in Covid-19 incidence in Zambia from early December to early January.

“The possibility of a similar experience in the US is a real threat,” Walensky and her co-authors wrote in the medical journal JAMA. “However, such an outcome is not inevitable; the US and other countries have the capability to prevent this outcome from occurring with a strong and immediate public health response.”

They note the US Department of Health and Human Services’ SARS-CoV-2 Interagency Group was developed to increase coordination among agencies, including to rapidly identify and monitor variants of concern. Agencies are evaluating testing, studying reinfection and assessing therapeutics. They continue to study how vaccines are impacted by the variants, and research “breakthrough” infections that occur after vaccination.

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Although US Covid-19 cases are falling, they said reducing transmission is key, as is accelerated vaccination.

There’s also a push to increase genome sequencing surveillance in the United States – state health departments are handling 750 samples per week, and CDC is now contracting with commercial labs to “significantly increase” surveillance to more than 6,000 samples per week.

“A concerted and well-coordinated public health effort, together with rapid and widespread uptake of effective vaccines, is essential to remain ahead of the inevitable evolution of variants that could dangerously accelerate the trajectory of the pandemic,” Walensky and her co-authors wrote.