JOHANNESBURG—Scientists will meet on Thursday to advise South Africa’s government on its next steps after a study suggested the dominant local coronavirus variant may reduce antibody protection from Pfizer’s COVID-19 vaccine by two-thirds. The laboratory study, published in the New England Journal of Medicine, dealt a new blow to the country hardest-hit by the pandemic on the African continent. The government and its advisers must weigh whether to wait for vaccines that might be more effective against the more infectious 501Y.V2 variant, or try to vaccinate people quickly to avert further infections and deaths. South Africa had been counting on the Pfizer shot, developed with German partner BioNTech, to step up its vaccination program after administering the first Johnson & Johnson (J&J) doses on Wednesday. Earlier this month, it placed AstraZeneca vaccinations on hold because of interim data showing its jab offered minimal protection against mild to moderate illness from …