The "safest bet" for some families will be not to have family gatherings this Christmas in order to stop the spread of the coronavirus, WHO technical lead for COVID-19 Maria Van Kerkhove said on Monday.

"In some situations, the difficult decision not to have a family gathering is the safest bet," she told a virtual briefing in Geneva.

Daily new coronavirus infections in Russia hit a new high on Monday, with authorities reporting a record 25,173 new cases.

The latest figure brings the country's total to more than 2.1 million.

The government coronavirus task force also reported 361 deaths on Monday, raising the total since the start of the pandemic to 36,500.

Russia, which currently has the world's fifth largest number of confirmed cases, has been swept by a rapid coronavirus resurgence since September.

Meanwhile, Chinese authorities are testing millions of people, imposing lockdowns and shutting down schools after multiple locally transmitted coronavirus cases were discovered in three cities across the country last week.

As temperatures drop, widescale measures are being enacted in Tianjin, Shanghai and Manzhouli, even though the number of new cases remains low compared to the other countries that are recording new waves of infections.

Experts and government officials have warned that the chance of the virus spreading will be greater in cold weather.

On Monday, the National Health Commission reported two new locally transmitted cases in Shanghai over the previous 24 hours, bringing the total to seven since Friday.

China has recorded 86,442 cases overall and 4634 deaths since the virus was first detected in the central Chinese city of Wuhan late last year.

The two latest cases confirmed in Shanghai were close contacts of another airport worker who was diagnosed with COVID-19 earlier in November.

On Sunday night, the city's Pudong International airport decided to test its workers, collecting 17,719 samples through the early hours of Monday morning.

Videos on social media purportedly from workers showed what appeared to be chaotic scenes at the airport as they were given last-minute orders to get tested.

In the videos, people are seen standing in large groups pushing back and forth against officials in hazmat suits.

Shanghai has been more selective with mass testing, targeting people associated with a particular place, such as the airport or the hospital where someone who has tested positive had worked, rather than an entire district.

In Tianjin, health workers have collected more than 2.2 million samples for testing from residents in the Binhai new district after five locally transmitted cases were discovered there last week.

In Manzhouli, a city of more than 200,000 people, local health authorities are testing all residents after two cases were reported on Saturday.

They also shut down all schools and public venues and banned public gatherings such as banquets.

China has resorted to its heavy, top-down approach each time new cases of local transmission are found - shutting down schools and hospitals, locking down residential communities and entire neighbourhoods, and testing millions.

WHO chief scientist Soumya Swaminathan on Monday welcomed an announcement from Astrazeneca, which became the latest major drug maker on Monday to say its vaccine could be about 90 per cent effective.

The vaccine results are "encouraging and we look forward to seeing the data as we do with other promising results of recent weeks," she said.

"We welcome the efforts of Oxford/AZ to make the vaccine affordable and easy to store, which will be good for countries and people everywhere."

AstraZeneca's vaccine is seen as cheaper to make, easier to distribute and faster to scale-up than rivals.

Globally, more than 58.72 million people have been reported to be infected by the coronavirus since the start of the outbreak and 1,391,367 have died.

Australian Associated Press