India's export ban on Covid-19 shots risks dragging the battle against the pandemic "back to square one" unless wealthy nations step in to plug a gaping hole in the Covax global vaccine-sharing scheme, health specialists said on Thursday.

Covax, which is critical for poorer countries, relies on AstraZeneca shots made by the Serum Institute of India, the world's largest maker of vaccines. It was already around 100 million doses short of where it had planned to be when India halted exports a month ago amid a surge in infections there.

Rich countries with plentiful Covid-19 vaccine stocks...