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Scientists in the United States are anxiously watching the Delta variant of Covid-19, as it spreads through an unevenly vaccinated American public and an economy that is rapidly reopening.

The Delta variant, first identified as B.1.617.2 in India, is believed to be more transmissible than both the original strain of Covid-19 and the Alpha strain, first identified in the United Kingdom.

“We’ve moved [Delta] to the top of our list of variants to study,” said Andrew Pekosz, a professor in Johns Hopkins University’s molecular microbiology and immunology department, and an expert in how viruses interact with the respiratory system.

“The data out of the UK showing how quickly the Delta variant became the dominant variant there is strong evidence that it is more transmissible than the Alpha variant, which we already thought was more transmissible than the original lineages,” said Pekosz.

The Delta variant is spreading at an uncertain time in the US. Covid-19 cases have fallen far below the winter peak, from an average of more than 250,000 new diagnoses a day in January to about 14,000 a day in June. Fewer cases have coincided with fewer hospitalizations and deaths.

This has led state after state to lift all social distancing guidelines, including in California, which gave the green light to large indoor gatherings such as sporting events. Now, social distancing and mask requirements are largely operating on the honor system.

But, even as pandemic guidelines recede, Delta has roughly doubled every two weeks in the US, a pattern once followed by Alpha, the variant first discovered in the UK, which eventually came to represent the vast majority of new US infections. The Delta variant has also delayed the UK’s planned reopening.

According to the CDC, at the end of May Alpha represented almost 70% of infections in the US. But in mid-March, it represented only 26% of cases. Similarly, Delta once represented only 2.5% of cases of Covid-19 by mid-May. But two weeks before that, it represented only 1.3% of cases. Again, two weeks before that in April, it represented just 0.6% of cases.

The doubling of cases has led some, such as the former Food and Drug Administration (FDA) commissioner Dr Scott Gottlieb, to predict that Delta may represent as much as 10% of US cases by mid-June.

The CDC officially elevated Delta to a “variant of concern” this week. A “variant of concern” designation puts Delta in the same category of increased surveillance as Alpha and Gamma (the variant first identified in Brazil).

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