By Julie Steenhuysen

(Reuters) – State health officials in South Carolina on Thursday said they found two cases of COVID-19 linked to a more transmissible coronavirus variant first detected in South Africa, the first cases of the mutated virus discovered in the United Sates.

Viruses are constantly changing, but the presence of the South African variant in the United States is especially concerning because several laboratory studies have shown that it reduces vaccine effectiveness.

So far, vaccines made by Moderna Inc and Pfizer Inc/BioNTech still appear protective against this variant of the coronavirus, but the companies this week said they are considering making new versions of their shots just in case. Moderna also plans to test giving a third shot of its vaccine to bolster antibody levels.

The U.S. Centers for Disease Control and Prevention said in a statement that it is aware of the cases and noted that there is no evidence the variant causes more severe disease.

The arrival of the South African variant follows predictions by the CDC this month that a more-transmissible U.K. variant could become the dominant strain in the United States by March.

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

(Reporting by Peter Szekely and Julie Steenhuysen in Chicago; additional reporting by Manojna Maddipatla in Bangalore; Editing by Chizu Nomiyama and David Gregorio)