The head of the World Health Organization has said China's zero-tolerance Covid-19 policy is not sustainable given what is known of the disease, in rare public comments by the UN agency on a government's handling of the virus.

"We don't think that it is sustainable considering the behaviour of the virus," WHO Director-General Tedros Adhanom Ghebreyesus told a media briefing.

Speaking after Tedros, WHO emergencies director Mike Ryan said the impact of a "zero-Covid" policy on human rights also needs to be taken into consideration alongside the effect on a country's economy from any Covid policy.

Mike Ryan (File image)

He also noted that China has registered 15,000 deaths since the virus first emerged in the city of Wuhan in late 2019 - a relatively low number compared with 999,475 in the United States and more than 500,000 in India.

With that in mind, it is understandable, Mr Ryan said, that one of the world's most populous countries would want to take tough measures to curb coronavirus contagion.

Still, China's zero-Covid policy has drawn criticism ranging from scientists to its own citizens, leading to a cycle of lockdowns of many millions of people, anguish and anger.

The continued outbreaks also underscore how difficult it is to stop the spread of the highly transmissible Omicron variant.

A health worker tests people at a swab collection site in Beijing today

Under zero-Covid, authorities lock down large population areas to stamp out viral spread in response to any coronavirus outbreak, even if just a small number of people test positive.

Shanghai's measures have been particularly strict, with residents allowed out of compounds only for exceptional reasons, such as a medical emergency. Many are not even allowed out of their front doors to mingle with neighbours.

Its quarantine policy has also been criticised for separating children from parents and putting asymptomatic cases among those with symptoms.