• Formally known as AY.4.2, the "delta plus" subvariant includes two new mutations to the spike protein, A222V and Y145H, which allow the virus to enter the body.
• The AY.4.2 subvariant has been detected in at least five cases in the U.S.: in Washington, D.C., California, North Carolina, Washington state and Massachusetts.
• Francois Balloux, director of the Genetics Institute at University College London, said it could be 10%-15% more contagious than delta.

U.S. health officials are keeping a close eye on an emerging Covid-19 subvariant, dubbed "delta plus," that some scientists say may be more contagious than the already highly transmissible delta variant.

Formally known as AY.4.2, delta plus includes two new mutations to the spike protein, A222V and Y145H, which allow the virus to enter the body. Those mutations have been found in other Covid variants, so it's unclear how dramatically those changes affect the virus.

Francois Balloux, director of the Genetics Institute at University College London, said it could be 10%-15% more contagious than delta, which first appeared in India and spreads easier than Ebola, SARS, MERS and the 1918 Spanish flu, according to the Centers for Disease Control and Prevention.

Delta has an R-naught, or reproductive rate, of eight or nine, according to CDC Director Dr. Rochelle Walensky, meaning that every person who has Covid will spread it to up to nine other people. The "wild type" or original strain of Covid had an estimated R-naught of about three. Someone infected with the delta variant carries 1,000 times the viral load of the original Covid strain.

India's Ministry of Health reported in June that delta plus was more transmissible than the delta variant, adding that the subtype binds more strongly to lung cell receptors and could even reduce the effectiveness of monoclonal antibody treatments.

The mutation has been detected in the U.S., but there hasn't been a noticeable uptick in delta plus cases nationwide, Walensky said at a White House Covid briefing Wednesday.

"We particularly monitor for sublineages that could impact therapeutics, such as monoclonal antibodies and vaccines," Walensky said. "At this time, there is no evidence that the sublineage AY.4.2 impacts the effectiveness of our current vaccines or therapeutics."

The AY.4.2 subvariant has been detected in at least five cases in the U.S. since August: in Washington, D.C., California, North Carolina, Washington state and Massachusetts, according to Outbreak.info. The website collects data from GISAID, a global genomic database on Covid and influenza cases.

Top health authorities have cautioned for weeks that more powerful and potentially vaccine-resistant Covid variants could develop as long as widespread outbreaks continue to occur, fueled by billions of people worldwide who remain unvaccinated. White House chief medical advisor Dr. Anthony Fauci said in August that the U.S. could be "in trouble" if another mutation surpassed delta, asking the unvaccinated to get their shots in hopes of curbing a surge that crushed the nation's health-care systems this summer.

Delta plus could also eventually affect the age groups eligible to receive Covid booster doses, Dr. Peter Marks, the Food and Drug Administration's lead vaccine regulator, said Wednesday night. The FDA and CDC have authorized Covid boosters for a wide array of adults in the U.S. from all three manufacturers in the U.S.: Moderna, Johnson & Johnson and Pfizer.

"The exact age of that will be based on what we see of the emerging situation, which is quite dynamic right now because we continue to see reports of new variants coming up," Marks said. "And we're also seeing changes in the epidemiology of Covid-19 in our country right now with new hotspots coming up even as certain places die down."

Concerns over delta plus are running high in the U.K., where officials are battling a surge in cases and facing a renewed health crisis. Delta plus cases represented roughly 6% of all sequenced Covid cases as of the week beginning Sept. 27, according to the latest data from the country's Health Security Agency. The sublineage is "increasing in frequency" in the U.K., the agency noted, and doctors from the National Health Service Confederation in London are calling for a return to stricter Covid protocols heading into the winter.

But global health leaders are urging the public not to panic. Though the emergence of a Covid subtype isn't the same as an entirely new variant evolving, keeping track of delta's progression could allow the medical community to better understand the mutation, Dr. Sylvain Aldighieri, Covid-19 incident manager at the World Health Organization's regional branch for the Americas, said at a briefing Oct. 6.

"Looking to these additional changes, it may help researchers to track the variants on a fine scale," Aldighieri said. "But they do not imply any functional or biological difference."

— CNBC's Holly Ellyatt in London contributed to this report.