The Sinovac and Sinopharm vaccines were 52% effective against Delta infection and 60% for symptomatic disease.

But researchers warn against depending on infection as a strategy, given the higher risks to unvaccinated persons of hospitalisation, long-term impacts, and death, compared to vaccinated people.

The authors found that the risk of hospital admission was roughly 80% lower for those with Omicron than Delta, and that for those in hospital the risk of severe disease was roughly 30% lower.

Lab tests are under way to determine vaccine efficacy against Omicron with results expected within weeks.

Whether Omicron will evade vaccine protection and whether it will cause more serious illness will be far less concerning if the new variant remains relatively contained.

WHO chief warns against falling into a false sense of security.

China's most widespread Delta outbreak has affected 21 provinces, regions and municipalities.

The simulation covered different aspects of a crisis, including mass inoculation, ordering curfews and imposing a flight ban.

This brings the total number of infections categorised as variants of concern and interest to 3,830.

Over the last three months, the daily average number of cases has fallen by 36% although the virus is still infecting 50 million people every 90 days.

None completely rules out what some call a 'doomsday scenario', but they express increasing confidence that many countries will have put the worst of the pandemic behind them in the coming year.

The researchers underline that this does not weaken the argument for vaccination as the best way of reducing serious illness from Covid-19, adding that booster shots are required.