China's National Health Commission (NHC) reported 20,472 Covid-19 cases on April 6, the highest daily tally given since the start of the pandemic as millions in locked-down Shanghai began a new round of testing.

The country’s “zero-Covid” strategy has come under immense strain as cases spike, with around 25 million residents of Shanghai – China’s largest city and economic engine room – ordered to stay at home as the authorities struggle to contain the outbreak.

Until March, China had kept daily cases low with snap localised lockdowns, mass testing, and strict restrictions on international travel.

But the caseload has hit thousands per day in recent weeks, with Shanghai driving the surge of the highly transmissible Omicron variant.

The city locked down in phases last week and complaints have swelled online of fresh food shortages caused by logistics disruptions and panic buying, which has left many residents waking early to try to beat the queues on grocery apps.

State broadcaster CCTV reported that the city was to launch a fresh round of tests on the entire population on April 6.

Shanghai’s “prevention and control situation is very severe”, NHC official Lei Zhenglong said on April 6, adding that the outbreak is still “in its peak phase”.

It was the latest dour warning suggesting a long run in lockdown may be ahead, while city health officials have converted a convention centre into a makeshift Covid hospital for 40,000 people.

Most of April 6’s new cases were asymptomatic. Authorities reported no new deaths, in a country which says only two people have died of the virus in nearly two years.

Yet China faces low vaccination rates, especially among the elderly, leaving officials with a high-wire balancing act of maintaining public health while keeping the economy moving.

In Shanghai quarantine facilities are bulging with people who test positive – even if they are asymptomatic – as city officials stick rigidly to virus protocols.

Those include separating Covid-positive babies and children from parents who test negative, a policy that has anguished worried families.

City officials on April 6 said parents of some child patients with “special needs” would now be allowed to remain with their Covid-positive children.