JOHANNESBURG—A small clinical trial in South Africa found that AstraZeneca PLC’s Covid-19 vaccine doesn’t appear to protect recipients against mild and moderate illness from a fast-spreading new strain of the coronavirus first detected in the country, according to limited results released Sunday.
The trial, which enrolled around 2,000 volunteers with a median age of 31, was too small and its participants too young to draw broad conclusions on the vaccine’s overall efficacy in protecting against the disease caused by the coronavirus, especially when it comes to hospitalizations or death. However, its findings contribute to concerns that a mutating virus is rendering existing Covid-19 vaccines less effective and that shots will need to be updated to protect against new virus strains.
Johnson & Johnson and Novavax Inc., whose vaccines have yet to be authorized in any country, have also found that their shots were less effective in recent human clinical trials in South Africa, compared to trials in the U.S. or the U.K. But their vaccines were still found to be 50% or more effective at preventing mild or moderate cases of Covid-19 and even more potent at shielding recipients from severe illness and hospitalization from the new strain.
Sunday’s news release on the AstraZeneca trial in South Africa didn’t provide an efficacy rate for the vaccine. The Johnson & Johnson trial in South Africa included about 6,500 people while the Novavax one had 4,400.
The University of the Witwatersrand in Johannesburg, which conducted the AstraZeneca trial, said it couldn’t make an assessment of whether the vaccine prevented more severe cases of Covid-19, because the relatively young trial participants were at low risk of falling seriously ill.