A coronavirus variant first identified in South Africa has been detected for the first time in the United States in two South Carolina patients, health officials said on Thursday.

Medical experts said arrival of the variant presents an alarming new challenge in efforts to contain a raging pandemic that has claimed at least 430,000 American lives in 11 months, as authorities struggle to launch the largest mass-vaccination campaign in U.S. history.

All viruses mutate frequently, and scientists have identified several variants of the novel coronavirus found to be more transmissible than the original strain.

But the presence of the variant detected in South Africa, which has shown no evidence of causing more severe disease, is nonetheless especially concerning because several laboratory studies have found that it reduces vaccine and antibody therapy efficacy.

Another concern, according to Vivek Murthy, nominated to be the next U.S. surgeon general, is that the variant may require larger doses of antibody therapeutics for effective treatment.

Confirmation of two patients with that variant in South Carolina comes days after the Minnesota Department of Health identified the first known U.S. case of another highly contagious variant that was detected in Brazil.

Yet a third form of the virus found in the UK that is more infectious, and associated with higher mortality, made its first U.S. appearance last month in Colorado and has since been detected in at least 28 states.

The U.S. Centers for Disease Control and Prevention (CDC) has predicted the UK-detected variant could become the dominant strain in the United States by March.

"The thing that's troublesome now that we really need to keep our eye on are these variants," Dr. Anthony Fauci, the leading U.S. infectious disease specialist, said in an interview on MSNBC. "The one that is of greater concern and that really could be problematic is the mutant that is now dominant in South Africa."

'Eye of the hurricane'

The CDC said in a statement that it was aware of the South Carolina cases and noted that there is no evidence the variant is any more deadly than the original virus.

The South Carolina Department of Health and Environmental Control said the two newly discovered cases of the variant, a strain known to scientists as B.1.351, appear to be unrelated to one another.

The agency said neither of the two adults who contracted it had a recent travel history that would account for their infections, suggesting the variant was transmitted locally.

The variant identified in South Africa has been detected in more than 30 countries.

The CDC has said the arrival of more transmissible coronavirus variants emphasizes the need for rapid rollout of vaccines to limit the ability of the virus to continue mutating.

Scientists have warned that another onslaught of infections could loom on the horizon as more transmissible forms of the virus spread before collective immunity can be achieved through widespread vaccination programs.

"Even though the total new cases has gone down from around 250,000 confirmed cases a day to 180,000 a day, that's the eye of the hurricane. The backside is now coming," Dr. Peter Hotez, dean of tropical medicine for Baylor College of Medicine, told MSNBC.

He said accelerating immunizations is more urgent than ever with the arrival of the variant identified in South Africa.

Fauci agreed, saying, "We could have some difficult times that we have to be prepared for."