The Omicron variant is spreading faster than any other strain of the coronavirus and it is probably present in most countries by now, the World Health Organization said on Tuesday.

The Omicron variant, which has a large number of mutations, was first detected in South Africa in November. Preliminary evidence suggests that it is more transmissible than the Delta variant and might reduce the efficacy of vaccines.

Tedros Adhanom Ghebreyesus, the global health agency’s director-general, said at a press briefing on Tuesday that 77 countries have reported Omicron cases so far. He expressed concern about Omicron being dismissed as a mild variant.

“Surely, we have learned by now that we underestimate this virus at our peril,” Tedros said.

The World Health Organization’s chief said that even if the Omicron variant causes less severe disease, unprepared health systems could be overwhelmed by “the sheer number of cases”.

Tedros cautioned countries that vaccines alone will not protect them from the new variant. He emphasised that people must continue to wear masks and practice physical distancing.

“It’s not vaccines instead of masks,” he added. “It’s not vaccines instead of distancing. It’s not vaccines instead of ventilation or hand hygiene. Do it all. Do it consistently. Do it well.”

The World Health Organization chief noted that in view of the emergence of Omicron, some countries have rolled out booster vaccine doses for their citizens. He added that moving forward, boosters could play an important role, especially for those who are at the highest risk of severe disease or death from Covid-19.

Tedros clarified that the World Health Organization was not against booster doses. However, he raised concerns that countries would again hoard vaccines for their booster rollout programmes, and this would increase inequity.

The director-general pointed out that 41 countries have still not vaccinated 10% of their populations, while 98 others have not reached 40% target.

Tedros said that with the end of inequity, the pandemic would end. “If we allow inequity to continue, we allow the pandemic to continue,” he warned.