Johnson & Johnson's single-dose Covid-19 vaccine has an overall efficacy of 66 per cent, the company has said after a huge multi-continent trial, but the shot does not protect as well against a variant first detected in South Africa.

Encouragingly, the vaccine was able to prevent 85pc of severe Covid-19 cases across all geographical regions however — a key indicator lauded by experts, according to an AFP report.

Its effectiveness against all forms of the disease — mild, moderate and severe — was as high as 72pc in the United States, but fell to 57pc in South Africa, where a more transmissible new variant is dominant. In Latin America it was 66 percent effective.

“We're proud to have reached this critical milestone and our commitment to address this global health crisis continues with urgency for everyone, everywhere,” the company's CEO Alex Gorsky said.