The World Health Organization warned wealthy countries on Thursday against hoarding COVID-19 vaccines for booster shots as they try to fight off the new Omicron variant, threatening supplies to poorer countries where inoculation rates are low.

Many Western nations have been rolling out boosters, targeting the elderly and people with underlying health issues, but worries about the fast-spreading Omicron have prompted some to expand their programmes.

The WHO recommended boosters instead for those with health issues, or those who have received an inactivated shot...