HONG KONG (BLOOMBERG) - China's worst Covid-19 outbreak since Wuhan was preceded by an influx of imported cases from Hong Kong, showing the risk the city's virus crisis poses to the mainland.

Just across the border from Hong Kong, tech hub Shenzhen is emerging from a week-long lockdown called to tackle an outbreak of the Omicron variant.

Shanghai, meanwhile, is seeing its highest new case levels of the pandemic, with half the city locked down on Monday (March 28) for testing after a handful of infections ballooned into more than 2,000 in a matter of weeks.

An analysis by Bloomberg News found the uptick in cases in both cities - key entry points into the mainland - came around the same time or shortly after a surge in infections was recorded in quarantined travellers coming from Hong Kong, where the number of recorded new cases still hovers above 8,000.

While China isolates everyone coming into the country for at least two weeks in line with its "zero-Covid-19" policy, health officials in both Shenzhen and Shanghai have pointed to lax oversight in quarantine facilities for arrivals as a potential cause for the virus's spread.

Having gone long stretches Covid-19-free thanks to its zero-tolerance approach, China has typically been quick to point out the foreign sources of virus cases that seed outbreaks in the community, tracing flareups back to a traveller from Pakistan, for example, or people who came in from the US.

On this current wave, the National Health Commission has only said that China has come under increasing pressure from Covid-19 incursions as outbreaks in neighbouring regions surge. Data from Shenzhen's health authorities, however, show those travelling from Hong Kong accounted for the overwhelming majority of imported cases detected there since the beginning of the year, with over 93 per cent of the 901 confirmed infections from abroad as of March 26 originating from the financial hub.

In Shanghai, more than half of the imported cases reported by the city's health commission this month to March 26 originated from Hong Kong. In this period, locally acquired infections skyrocketed from two to 2,676.

The two cities have been top destinations for those fleeing Hong Kong's outbreak, with the number of people leaving the territory hitting records from late February, as the outbreak there quickly spiralled out of control. Mainlanders living in Hong Kong, in particular, have sought refuge at home, where the national caseload is still below the city's.

Quarantine hotels are effective at catching Covid-19 cases, but as the virus has evolved to become more infectious leaks have become more common. Places that utilised them as a key part of their pandemic response, including Australia and New Zealand, as well as Hong Kong, have all seen outbreaks spawned by escaped cases. It's one of the reasons most countries that sought to eliminate the virus early on in the pandemic have now - unlike China - pivoted to living alongside it, like the US and much of Europe.

Risk Recognised

Contact tracing data released by Shenzhen's health authorities show that the earliest domestic cases in February stemmed from staff working at quarantine hotels, while Shanghai's government has also acknowledged the role of "lapses in management" of imported cases in quarantine driving local outbreaks.

China may be mute on whether Hong Kong was the source of its current outbreak, but its actions indicate officials recognise the risk. After airlines added more flights in February to cater to the demand from fleeing mainlanders, authorities capped the number of passengers per plane to 50 per cent from Feb 25 on the Hong Kong-to-Shanghai route.

Flights to Shanghai - which is emerging as a key epicentre of the mainland's current wave - have been steadily pared. Until recently, more flights from Hong Kong went to Shanghai than anywhere else, but now Shanghai has fallen below Taipei, Singapore and Manila as the top destination, according to data from Cirium.

Similarly, at Hong Kong's land crossing with Shenzhen, cross-border truckers plying the flow of fresh food were also found to be infected, forcing drivers and close contacts to go into compulsory quarantine. The city has instead sought to maintain the supply of food through deliveries by train and ship from China.

Despite these actions and the lack of finger-pointing by mainland officials, the perception arrivals from Hong Kong have fuelled the mainland's outbreak has taken hold on Chinese social media. There was outcry in mid-March over pictures of Hong Kongers crowding beaches despite record numbers of cases. A post on Weibo - China's equivalent of Twitter - featuring photos of the beach-goers, was liked over 126,000 times and received more than 5,000 comments in a day before being removed by the platform. Users criticised Hong Kong residents as selfish and called for the withdrawal of support such as mainland medical staff, and for Hong Kong visitors to be banned from entering Shenzhen.

While Hong Kong has in the past pushed hard for reopening the border with the mainland, and Chief Executive Carrie Lam last week expressing optimism about the resumption of talks to do so, the threat still posed by the city's still-high caseload may give Chinese officials pause for thought.