A new variant of the coronavirus has been detected in the United States, posing yet another public health challenge in a country already losing more than 3,000 people to the coronavirus disease 2019 (Covid-19) every day.

The mutated version of the virus, first identified in South Africa, was found in two cases in South Carolina. Public health officials said it’s almost certain that there are more infections that have not been identified yet. They are also concerned that this version spreads more easily and that vaccines could be less effective against it.

The two cases were discovered in adults in different regions of the state and do not appear to be connected. Neither of the people infected has traveled recently, the South Carolina Department of Health and Environmental Control said Thursday.

“That’s frightening,” because it means there could be more undetected cases within the state, said Dr. Krutika Kuppalli, an infectious diseases physician at the Medical University of South Carolina in Charleston. “It’s probably more widespread.”

The arrival of the variant shows that “the fight against this deadly virus is far from over,” Dr. Brannon Traxler, South Carolina’s interim public health director, said in a statement. “While more Covid-19 vaccines are on the way, supplies are still limited. Every one of us must recommit to the fight by recognizing that we are all on the front lines now. We are all in this together.”

Viruses constantly mutate, and coronavirus variants are circulating around the globe, but scientists are primarily concerned with the emergence of three that researchers believe may spread more easily. Other variants first reported in the United Kingdom and Brazil were previously confirmed in the US.

As the variants bring a potential for greater infection risks in the US, pandemic-weary lawmakers in several states are pushing back against mask mandates, business closures and other protective restrictions ordered by governors.

States including Arizona, Michigan, Ohio, Maryland, Kentucky and Indiana are weighing proposals to limit their governors’ abilities to impose emergency restrictions. Wisconsin’s Republican-controlled Assembly had been expected to vote to repeal Democratic Gov. Tony Evers’ mask mandate, but lawmakers abruptly called off the vote Thursday in the face of broad criticism and out of concern it would jeopardize more than $49 million in federal aid. Pennsylvania lawmakers are considering a constitutional amendment to strip the governor of many of his emergency powers.

Governors argue that they need authority to act swiftly in a crisis, and limitations could slow critical emergency responses.