The study from South Africa’s University of the Witwatersrand and Oxford University showed the vaccine had significantly reduced efficacy against the South African variant, according to a Financial Times report published earlier in the day.

Among coronavirus variants now most concerning for scientists and public health experts are the so-called British, South African and Brazilian variants, which appear to spread more swiftly than others.

“In this small phase I/II trial, early data has shown limited efficacy against mild disease primarily due to the B.1.351 South African variant,” an AstraZeneca spokesperson said in response to the FT report.