"The clock is still ticking on vaccine equity," WHO director-general Tedros Adhanom Ghebreyesus told a news briefing on Thursday, as he revealed the pressure on COVAX to meet its immunization targets.

A "serious challenge" remains for vaccine equity and availability, he said, despite some 35 billion doses having been delivered to more than 78 countries through the WHO-backed COVAX initiative.

The program aims to start the vaccine rollout to all health workers and vulnerable people by April 10 – the 100th day of the year.

But Ghebreyesus said that he had been forced to urge WHO partner countries last week to share 10 million more doses "immediately" in a bid to meet the COVAX target.

The extra doses are an "urgent stopgap measure" to start vaccine rollouts in 20 countries, but no government has yet come forward to make up the shortfall, Ghebreyesus said.

"I'm still hopeful that some forward-looking and enlightened leaders will step up," he added.

Many of the countries that have not received any Covid-19 vaccines are poorer nations, such as Yemen, which only received its first batch of 360,000 doses on Wednesday.

Some richer countries and regions have already ordered considerably higher numbers of vaccine doses than strictly required for inoculating their populations, including the UK, US, EU, Japan, Australia and Canada.