The highly infectious omicron coronavirus variant causes less severe disease than the delta strain but it remains a "dangerous virus", particularly for those who are unvaccinated, the head of the World Health Organization (WHO) said on Wednesday.

Speaking at a news briefing, director general Tedros Adhanom Ghebreyesus said more than 90 countries were yet to meet the target of vaccinating 40% of their populations and more than 85% of people in Africa were yet to receive a single dose, reports Reuters.

"We mustn't allow this virus a free ride or wave the white flag, especially when so many people around the globe remain unvaccinated," he said.