The coronavirus vaccine developed by Moderna triggers an immune response that protected in laboratory tests against two variants of the virus first detected in Britain and South Africa, the company said Monday.

The finding was largely encouraging but contained a clear warning sign: Even though the vaccine generated disease-fighting antibodies that worked against the South African variant, that efficacy was diminished. Moderna said there was a reduction in response, prompting the company to design a new potential vaccine that could be added to the current two-dose regimen.