Getting an initial course of Covid-19 vaccines to unprotected groups all over the world should remain a top priority before offering booster shots to vulnerable groups, World Health Organisation (WHO) Emergency Director Mike Ryan said yesterday.

Ryan said he expected the current vaccines would provide significant protection against hospitalisation and death, but that the data coming through was very preliminary and the WHO did not have enough of it yet to make a full determination.

"People are always asking should we be going for a primary vaccination or boosters, the reality is we should be doing both," Ryan told an online event.

"We should be focusing on getting those who are unvaccinated vaccinated as quickly as possible and then being able to give booster doses to those in vulnerable groups."

Ryan said Omicron would most likely replace Delta as the globally dominant variant over time and that governments needed to refocus on basic protective measures such as mask wearing and make sure their hospitals were ready, reports Reuters.

Even if Omicron turns out to be a milder disease in an individual patient, there was no doubt that the rapidly rising force of infection would generate more hospitalisations in the coming weeks just by the sheer weight of numbers, he added.

"Every single health system needs to get ready," Ryan said. "We are going to likely to see a very large Omicron wave. We need to protect the health system, we need to protect those we love. It's not rocket science, everyone knows how to do it."

The coronavirus has killed at least 5,311,914 people since the outbreak emerged in China in December 2019, according to a tally from official sources compiled by AFP yesterday.

China yesterday quarantined more than half a million people in Zhejiang province, an east coast economic hub, after it reported 44 infections -- almost all of the daily cases in China. The country reported its first Omicron variant case this week.

France said it could tighten border controls with the UK to protect against the Omicron variant, which is infecting a huge number of people in Britain.

The Omicron variant is showing a higher rate of reinfection that other variants and its growth rate is shortening in the United Kingdom, the Health Security Agency's chief medical adviser said yesterday.

"This is growing very fast with a growth rate of initially two to three days, and that growth rate seems to be shortening rather than lengthening," Dr Susan Hopkins said.

In US, California reinstated mask mandates in all indoor public spaces to try to curb a resurgence of cases, months after its major cities brought back the rule.

Brazil began requiring a vaccination certificate for foreign travellers arriving in the country despite the move being initially blocked by far-right President Jair Bolsonaro's government.