By Tessa Wong
When Air China Flight CA910 touched down at the Chinese city of Nanjing on 10 July, it was carrying more than just a planeload of passengers.
One of those people travelling from Moscow had the Delta variant of Covid-19. After they left the plane, staff from the Nanjing Lukou airport swooped in to pick up their rubbish.
According to Chinese officials, when those cleaners exited the aircraft they brought the virus along with them to the outside world – sparking what has now become China's widest outbreak since Wuhan.
In the last few weeks, at the height of the summer travel season, Delta has been detected in at least 16 Chinese provinces and municipalities. Many of the clusters have been linked to Nanjing.
Though there are a few hundred cases – relatively low for a country of 1.4 billion people – many are unnerved that the virus has appeared in major cities including Beijing, Shanghai and Wuhan.
In response, China has fallen back on familiar methods. Millions have been tested, sometimes more than once. Cities have gone into lockdown, while transport links in some areas have been cut.
It's what experts call a zero-tolerance or elimination strategy, seen not just in China but also other places such as Australia, New Zealand and Singapore.
But the astonishing speed of Delta's spread has also prompted questions about whether the approach is truly sustainable in China, in the face of a more transmissible Covid variant.
Already there had been signs of people letting down their guard, experts have pointed out.
Before Nanjing, there were several smaller outbreaks in Guangdong and along the borders with Russia and Myanmar.
Mask-wearing had become less commonplace than at the start of the pandemic, and mass gatherings had become the norm again. A theatre performance at the tourist destination of Zhangjiajie, in Hunan province, attended by around 2,000 people has been identified as a potential super-spreader event in the current outbreak.
State media have also pointed out "glaring loopholes" at Nanjing airport.
Officials believe the plane cleaners did not follow Covid protection protocols, and admitted that the flight was allowed to land even though it had been barred from flying multiple times for carrying Covid-positive passengers.
The quick swing from relaxed attitudes to hard lockdown illustrates a common issue in Chinese governance where there is often little room for nuance, according to virologist Jin Dongyan of Hong Kong University.
"We have the saying, 'kill it when you catch it, chaos when you let go'. The Chinese style is very extreme," he told the BBC.
Meanwhile, some are worried about whether Chinese vaccines are effective after authorities revealed that many of the early Nanjing cases were people who were fully vaccinated.
Health authorities have given public reassurances, even as they consider giving booster shots. Shao Yiming of the Chinese Center of Disease Control and Prevention said while there was no vaccine that could prevent Covid infection, "currently [vaccines] could still control all variants of the virus".
China has already administered more than 1.7 billion vaccine doses, though it has not said how many people are fully vaccinated.
But unlike other countries which have opened up after mass vaccinations, China does not seem willing to change course with the way it has reacted to the latest outbreak, say experts.
"There appears to be this lack of confidence in their vaccines that justifies this continuance of strategy," Professor Yanzhong Huang, senior fellow for global health at the Council on Foreign Relations, told the BBC.
A recent Global Times editorial rejected the idea of a UK-style re-opening, saying it was "almost politically inconceivable" as it would result in "unimaginable social costs and pain". It called instead for a "dynamic zero-Covid" approach with "controllable windows" to the outside world.
But in a Caixin commentary top medical expert Zhang Wenhong acknowledged that the latest outbreak "once again reminds us of the ever-present virus."
"Whether we like it or not, there will always be risks in the future," he said, adding that among other things China should "promote a return to normal life while protecting its citizens from fear of the virus".
It may not be easy to switch to what experts call a mitigation strategy, which focuses on reducing deaths rather than cases.
One of the biggest challenges for authorities, as Zhang appeared to hint at, is how to convince a risk averse Chinese public.
"There was this massively traumatic experience they had, seeing what happened in Wuhan where their health systems were completely overwhelmed. If they reopen, they are afraid that the Chinese health system would not be able to handle [another surge], especially in rural areas," said Prof Huang.
Prof Jin said the way state media depicted the virus had also fuelled this fear, with some outlets "presenting the Indian outbreak as if it was the end of days, and the situation in the UK and US was shown to be hellish."
There is also the question of losing face.
The successes of zero Covid allowed the Chinese government "to claim that this approach is superior to the Western approach, which is associated with general failure in containing the virus, and even claim the superiority of the Chinese political system," said Prof Huang.
"If they start abandoning it and turn to mitigation, they are basically endorsing a Western approach that they had trashed."
With relatively few deaths since Wuhan and an economy on the rebound, some in China may not feel the need to change.
But a long-term zero Covid strategy also has its risks.
Mass lockdowns affect poorer people much more than others, and also affect a population's mental health over the long run, pointed out Nancy Jecker, a professor of bioethics at the University of Washington School of Medicine.
"If China doesn't shift fast enough, the effects would be more severe at all levels of society," she said. She called for a more nuanced approach, such as more localised lockdowns and prioritising certain groups, like allowing schools to stay open while gyms and restaurants remain closed.
Prof Huang also warned of a long-term image problem for China as other countries move to reopen. Fellow zero Covid countries Australia and Singapore recently announced plans pegged to 80% vaccination rates.
Eventually the world would be split into two types of countries, said Prof Jecker: those who continue to pursue a zero Covid strategy, and those who have switched to mitigation.
"But ultimately we may not have a choice but to accept it – in a post-pandemic phase, deaths will recede but the virus may reappear annually like the cold," she said.
"If that's right, then China will have to live with it."
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By Tessa Wong