As 25m people in Shanghai enter their fourth week in stringent isolation, authorities are playing an increasingly desperate game of cat and mouse.

But it’s not just omicron that they are attempting to stamp out. Chinese censors are battling to block an outpouring of online anger, including the “Voice of April” - a six-minute montage that documents the harsh realities of a lockdown that has turned the country’s financial capital into a ghost town.

Social media users are rallying to outwit censors to share the film, which collates snippets of government press briefings with residents’ descriptions of their time under lockdown, from struggles accessing medical care to criticism of inadequate food supplies. “Get well soon, Shanghai,” says the black and white film in closing.

Some 700 miles away in Beijing, a mass coronavirus testing order in 11 of the capital’s 16 districts - despite just 70 cases emerging since Friday - has prompted residents to panic-buy groceries, fearing they too may soon face weeks trapped at home.

The thought of China’s largest and wealthiest cities stuck in indefinite lockdowns has triggered turmoil in the financial markets, with the blue chip CSI 300 index tumbling 4.9pc on Monday - its steepest one-day loss since the initial coronavirus lockdown in Wuhan. The fall underlines a fear among investors that the latest draconian restrictions are set to eat into profits and economic growth.

Yet president Xi Jinping appears more wedded than ever to his ‘zero-Covid’ strategy.

“There is a health logic and a political logic that justifies the continued strategy,” says Dr Yanzhong Huang, a senior fellow for global health at the Council on Foreign Relations. “They’re not even considering co-existence with the virus as an option right now.”

Dr Huang pointed to comments made last week by Dr Liang Wannian, head of the expert panel leading the country’s Covid-19 response. During a briefing in Beijing, Dr Liang said it would be a “huge disaster” if China relaxed restrictions, describing the zero-Covid policy as “insurance for the 1.4 billion people” living in the country.

“The key is to effectively recognise and manage the source of transmission, cut transmission chains, and protect vulnerable groups so that the outbreak does not rebound on a large scale,” he added.

Part of the reason Beijing is so reliant on this approach is because the state has failed to vaccinate the most vulnerable, raising concerns that hospitals would be overwhelmed and death rates high if China lifts restrictions.

According to figures by Airfinity, a health analytics group, 88pc of the country’s population is fully vaccinated. This drops to 81pc among those over 60 years old and just 51pc for those over 80.

Booster take-up is even worse, at just 59pc among the over 60s and 20pc among the over 80s.

“China has realised what Singapore learnt late last year and Hong Kong faced more recently: that there is a remarkable culture linked hesitancy within Chinese seniors,” says Professor Dale Fisher at the National University of Singapore.

Part of this is down to precedent, argues Dr Huang. Mass-vaccination campaigns among older groups are not very common in China, where - unlike the flu-jab rollout in Britain - “there’s no effort to promote the flu vaccine among the elderly”.

A slew of scandals involving tainted food and drugs haven’t helped. In 2018, for instance, there was uproar after it emerged that a major pharmaceutical company, Changchun Changsheng, had violated standards in making at least 250,000 doses of vaccine for diphtheria, tetanus and whooping cough. It was later fined $1.32bn (£1bn).

“[This] undermined the trust in Chinese domestic vaccines,” says Dr Huang. Yet during the pandemic, China has been disparaging of foreign-made vaccines – especially the mRNA shots developed by Pfizer and Moderna – in an effort to champion its own.

Beijing also prioritised essential workers and healthcare workers in the early rollout of vaccines, and has done little to ease the concerns of the elderly.

“The government hasn’t done a very good job at sending a clear message about the safety and effectiveness of the vaccine,” says Dr Huang. “My impression is that [vaccinating the elderly] has not been seriously promoted at the local level, even today.”

Part of this is a consequence of resources having been dragged away from the vaccination campaign to instead help enforce the zero-Covid policy, adds Dr Huang, by mobilising medical staff to conduct PCR tests and impose quarantines.

Experts say low booster rates are a particular concern, given China’s sole reliance on homegrown shots. A recent study in Hong Kong found that two doses of Sinovac vaccine were 72pc effective against severe or fatal Covid-19 for over 60s - compared to 90pc for the Pfizer-BioNTech vaccine. After three doses, Sinovac’s effectiveness jumped to 98pc.

Based on the case-fatality rate in Hong Kong, Bloomberg Intelligence estimated a month-long Chinese outbreak could result in between 31,000 and 250,000 deaths.

"China's vaccination policies, particularly for those over 50 years of age is a real problem," says Professor Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota. "They still have more people 60 years and above not fully vaccinated than all the people who live in Japan.

"Their two vaccines do provide protection from serious illness, hospitalisation and death after three doses, but inferior to the results of a similar number of mRNA or adenovirus vector vaccines. They will not stop omicron with their current policies."

Internal politics is also driving the steadfast pursuit of zero-Covid.

Professor Steve Tsang at foreign policy think tank Chatham House, believes the situation risks dragging on until late 2022 when the Chinese Communist Party is expected to hold its 20th annual congress.

An admission that the zero-Covid policy was wrong would amount to a big loss of face for Xi - especially when diplomats and state media trumpet China’s seemingly low death rates on a daily basis. Official figures suggest fewer than 5,000 people have died of Covid, compared to almost one million in the US.

“Zero-Covid is Xi's policy,” says Prof Tsang. “Xi does not make mistakes and cannot be seen to make mistakes. So as far as China is concerned, that is the best policy and demonstrates how they are superior to the West.

“It cannot be changed unless Xi says otherwise. We know already that some senior Chinese medical and health services are uncomfortable with it and that there are political leaders who think the policy should not go on any longer,” he adds.

“But they cannot change it because changing it means challenging the big boss - and there is nothing more dangerous you can do in Chinese politics than say Xi Jinping is wrong.”