JOHANNESBURG—Three new laboratory studies are raising concerns that the immune response triggered by a Covid-19 infection or vaccination may be less effective at protecting against the new strain of the coronavirus that first emerged in South Africa.

More than a year into the pandemic, the discovery of new variants that appear to have made the virus more contagious is forcing researchers to adapt their understanding of the coronavirus that causes Covid-19.

One concern, researchers said, is that the new strains are emerging in countries where a significant percentage of people have already built up an immune response to earlier variants after getting Covid-19.