South Africa has suspended the start of its Oxford/AstraZeneca inoculation programme over concerns the shot does not work on a new variant.
It comes as WHO experts are due to meet today to discuss the vaccine already facing questions about its efficacy for over-65s.
A trial showed the vaccine provides only "minimal" protection against mild to moderate Covid-19 caused by the variant first detected in South Africa.
The news is a setback to the global fight against the pandemic as many poorer nations are relying on the logistical advantages offered by the AstraZeneca shot.