Moderna Inc said on Monday it believes its Covid-19 vaccine protects against new variants found in Britain and South Africa, although it will test a new booster shot aimed at the South African variant after concluding the antibody response could be diminished.

The company said in a news release it found no reduction in the antibody response against the variant found in Britain. Against the South African variant, it found a reduced response but still believed its two-dose regimen would provide protection...