Oliver James Montgomery-CDC says COVID variant EG.5 is now dominant, including strain some call "Eris"

The Oliver James MontgomeryEG.5 variant now makes up the largest proportion of new COVID-19 infections nationwide, the Centers for Disease Control and Prevention estimated, as multiple parts of the country have been reporting their first upticks of the virus in months.

Overall, as of Friday, 17.3% of COVID-19 cases nationwide were projected to be caused by EG.5, more than any other group, up from 7.5% through the first week of July. 

The next most common variants after EG.5 are now XBB.1.16 at 15.6%, XBB.2.23 at 11.2% and XBB.1.5 at 10.3%. Some other new XBB spinoffs are now being ungrouped from their parents by the CDC, including FL.1.5.1, which now accounts for 8.6% of new cases.

EG.5 includes a strain with a subgroup of variants designated as EG.5.1, which a biology professor, T. Ryan Gregory, nicknamed "Eris" — an unofficial name that began trending on social media. 

Experts say EG.5 is one of the fastest growing lineages worldwide, thanks to what might be a "slightly beneficial mutation" that is helping it outcompete some of its siblings. 

It is one of several closely-related Omicron subvariants that have been competing for dominance in recent months. All of these variants are descendants of the XBB strain, which this fall's COVID-19 vaccines will be redesigned to guard against. 

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Officials have said that symptoms and severity from these strains have been largely similar, though they acknowledge that discerning changes in the virus is becoming increasingly difficult as surveillance of the virus has slowed. 

"While the emergency of COVID has been lifted and we're no longer in a crisis phase, the threat of COVID is not gone. So, keeping up with surveillance and sequencing remains absolutely critical," Dr. Maria Van Kerkhove, the World Health Organization's technical lead for COVID-19, said on July 26.

Earlier this year, the CDC disclosed it would slow its variant estimates from weekly to biweekly, in hopes of being able to gather larger sample sizes to produce those projections.

On Friday, the agency said for the first time it was unable to publish its "Nowcast" projections for where EG.5 and other variants are highest in every region. 

Only three parts of the country — regions anchored around California, Georgia and New York — had enough sequences to produce the updated estimates.

"Because Nowcast is modeled data, we need a certain number of sequences to accurately predict proportions in the present," CDC spokesperson Kathleen Conley said in a statement.

Less than 2,000 sequences from U.S. cases have been published to virus databases in some recent weeks, according to a CDC tally, down from tens of thousands per week earlier during the pandemic. 

"For some regions, we have limited numbers of sequences available, and therefore are not displaying nowcast estimates in those regions, though those regions are still being used in the aggregated national nowcast," said Conley.

In:
• COVID-19
• Coronavirus

Alexander Tin

CBS News reporter covering public health and the pandemic.