Delta variant likely to become dominant in US within weeks; may see July COVID-19 surges among unvaccinated

The delta coronavirus variant likely will become predominant in the U.S. within weeks, according to an analysis published June 21 by preprint server medRxiv.

The variant, first identified in India, accounts for at least 14 percent of all new U.S. infections, according to the analysis of more than 242,000 virus cases over the last six months. The study is not yet peer-reviewed.  

Genomics company Helix, which is contracted with the CDC to track variants, began the study after noting a drop in the prevalence of the alpha variant, first identified in the U.K., NPR reported. The researchers discovered the decline of the alpha variant in the U.S. was due to a rapid increase in two other variants: the gamma variant, first detected in Brazil, and the delta variant.  

"It looks like both of them are going to slowly push out alpha," said William Lee, PhD, researcher and vice president of science at Helix.

Vaccines appear to provide protection against all variants thus far. However, the delta variant is believed to be the most contagious strain yet and, among unvaccinated individuals, may trigger severe illness in more people than other variants.

"There still are big portions of the country where the rates of vaccination are quite low," Jeremy Luban, MD, a virologist at Worcester-based University of Massachusetts Medical School, told NPR. "The Helix paper shows that this delta variant is increasing in frequency — the speed at which it's increasing in frequency is greatest in those areas where vaccination rates are lowest."

The delta variant could prompt moderate COVID-19 surges through U.S. regions with a higher proportion of unvaccinated people, according to recent projections from the COVID-19 Scenario Modeling Hub. The projections indicate that cases may start to rise again sometime in July, especially if vaccination rates continue to stall.

During a June 21 news briefing, World Health Organization officials said the delta variant is the "fastest and fittest" strain yet, and it will "pick off" the most vulnerable people, especially in places with low vaccination rates.

The delta variant appears "more lethal because it's more efficient in the way it transmits between humans," said Mike Ryan, MD, executive director of WHO's health emergencies program.

The variant also is on track to become dominant worldwide, WHO said June 18. The agency declared delta a "variant of concern" last month, with the CDC giving it the same designation June 15.


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