A deficit in COVID-19 vaccine doses facing the COVAX facility in June and July may compromise the efficiency of the roll-out, according to the World Health Organization (WHO).

WHO Senior Adviser Bruce Aylward told journalists in the Swiss city of Geneva that the current number of doses was "about 200 million doses behind where we want to be".

"We are setting up for failure if we don't get early doses. We are not on track yet: we don't have enough doses from enough countries early enough to get the world on track to get out of this," Aylward said on Friday.

An estimated 150 million vaccine doses have been pledged by rich countries but Aylward cautioned the issue would not be dealt with conclusively because of two big problems.

"Number one, very little is committed to the June-July period, which means we're going to still have this gap," Aylward noted.

"The other problem is just the volume. If we are going to get on track to get at least 30-40 percent of the world population vaccinated this year we got to get another 250 million people vaccinated between now and the end of September," he added.

Amid concerns that low-income countries, especially those in Africa, would be left behind in the COVID-19 vaccine distribution, a global pooled procurement mechanism, the COVAX facility, co-led by the WHO, was formed.

The facility has already delivered more than 80 million doses to 129 territories.

However, the facility has experienced delays and limited supply which have forced other countries to seek more doses elsewhere, including via bilateral deals which may be unfavourable.

(With input from agencies)