Omicron spreading at unprecedented rate, 'probably in most countries': WHO

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His comments came amid growing evidence that the new variant might also be better at skirting vaccine protections than previous ones, but could cause milder symptoms.

But Tedros cautioned against "dismissing Omicron as mild."

"Surely, we have learned by now that we underestimate this virus at our peril," he said.

"Even if Omicron does cause less severe disease, the sheer number of cases could once again overwhelm unprepared health systems."

The WHO chief also voiced concern that many countries are rushing to provide booster vaccine doses in response to the spread of Omicron, in light of the persistent glaring inequity in vaccine access between wealthy and poorer countries.

"WHO is concerned that such programmes will repeat the Covid-19 vaccine hoarding" seen previously this year, he said.

He said there was not yet enough data to show a third dose is needed to effectively protect healthy adults against the variant, although he said that "as we move forward, boosters could play an important role."

At the same time many vulnerable people in poorer countries have not yet received a single dose.

"Let me be very clear: WHO is not against boosters. We're against inequity. Our main concern is to save lives everywhere," Tedros said.

"It’s a question of prioritisation," he said.

"The order matters. Giving boosters to groups at low risk of severe disease or death simply endangers the lives of those at high risk who are still waiting for their primary doses because of supply constraints."