PENNSYLVANIA (WHTM) — The World Health Organization added the mu COVID-19 variant to its list of “Variants of Interest” at the end of August. According to, mu has been detected at low levels in 49 states in the U.S., including in Pennsylvania, but experts say that the delta variant is by far the primary concern currently.

Experts are keeping an eye on the mu variant, as its mutations suggest that it may be able to evade certain antibodies that protect people from serious coronavirus infections, National Institute of Allergy and Infectious Diseases Director Dr. Anthony Fauci said in a Sept. 2 press briefing. However, more research will be needed, Fauci said, because there is very little clinical data about the new variant.

Although other variants like alpha, beta, gamma, and lambda have also been found in the U.S., the delta variant dominates. Delta accounts for around 99% of all COVID-19 cases circulating in the United States now, said WellSpan Health infectious disease specialist and physician Dr. Eugene Curley.

In fact, Curley said, the original strain of the coronavirus is no longer circulating in the U.S. It has been surpassed by variants like delta.

The delta variant is more contagious than the original strain of COVID-19, and some studies indicate that the coronavirus vaccines may be less effective against the delta variant than against others. But, Curley said, the vaccines do still offer important protection from the virus.

“Ninety-plus percent of the hospitalizations and cases are still in unvaccinated people, so that lets you know that, hey, the vaccine is still working,” Curley said. WellSpan Health recently shared data indicating that 92% of all COVID-19 patients in its hospitals are unvaccinated.

Breakthrough infections in vaccinated individuals also tend to be less severe than infections in unvaccinated individuals, Curley noted.

As new variants develop, though, vaccines may not offer the same level of protection. “The doomsday scenario is what we would call an ‘escape variant,’ so that would be a variant that can penetrate any protection that any of the vaccines provide for us,” Curley said.

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Experts also worry about more contagious variants, like delta, which can infect more people more quickly than other strains, and they are also on the lookout for variants that can cause more severe disease.

Virus variants develop as viruses replicate inside living hosts, so getting more people vaccinated and protected against the coronavirus is key to preventing additional variants from developing, Curley explained.

Researchers will continue to monitor new variants in the U.S., Curley said, and data on new variant traits and vaccine efficacy against those variants is still being collected for the less prominent variants that are currently circulating in the country.