Home quarantine, false negative airport testing and lax supervision of hotel isolation could be behind the latest surge in community Covid-19 infections, after a study found a quarter of recently imported cases were not detected on arrival.

Dr Gilman Siu Kit-hang, the Polytechnic University academic who led the research, also warned that Hong Kong was on the cusp of a dreaded fourth wave of infections, as he presented fresh evidence that a new coronavirus strain in the city probably came from Nepal and India.

“If you ask me, I would say the fourth wave hasn’t started yet, but it’s likely we are on the brink of it, compounded by the winter flu season … and the absence of a vaccine,” said the associate professor at PolyU’s health, technology and informatics department.

Hong Kong overcame a third Covid-19 wave that started July, but has since been fighting several outbreaks of locally transmitted infections, with clusters emerging from a staycation holiday in Mui Wo and a bar in Tsim Sha Tsui.

All of the eight cases confirmed in Hong Kong on Monday were imported and included travellers from the South Asian nations of India, Pakistan and Nepal, pushing the city’s total to 5,466, with 108 related deaths.

The city also recorded seven more outbreaks of upper respiratory tract infections on Monday, most of them in primary schools, taking the total number in this flu season to 310, affecting 2,969 people.

Dr Chuang Shuk-kwan, head of the Centre for Health Protection’s communicable disease branch, last week revealed that a University of Hong Kong study had found that a new coronavirus strain – different from the one prevalent in the local third wave – had taken hold in the city.

The strain was also found to be a more than 99 per cent match, in terms of viral genome sequencing, to the one discovered among infected travellers from Nepal.

The separate study from PolyU, published in the academic journal Emerging Microbes & Infections, gave fresh backing to that theory.

Apart from confirming the similarities between the Hong Kong strain and the one in Nepal and India, which both belonged to the same “family tree”, Siu’s team also found that the new “GH” type circulating in the city had 22 mutations from the coronavirus’ “GR” form, which was prevalent locally between July and early October.

Siu said this meant that the latest strain was newly imported, as the evolutionary rate in Hong Kong, or the speed at which the virus could mutate, between July and October was only 2.53 per month, far lower than the 22 mutations found in the PolyU study.

“To see it from an optimistic point of view, this means we have won the battle and largely broken the chain of transmissions after months of collective effort against the virus,” Siu said.

But the findings also carried a worrying message, he added, as it showed the virus had yet again slipped through the border defences, leading to local outbreaks.

Citing official data from the health authorities, Siu said 25 per cent of the 268 imported cases recorded in the city in September and October, were only confirmed during the two-week quarantine period following their arrival to the city, rather than on the day of landing.

That meant there were 73 cases over those two months of infected people entering the city and starting their quarantine before their health status came to light, which has been attributed to false negative results from tests taken at the airport.

Siu said some patients who had just become infected, perhaps on the flight or during their transit in another city, could have a low viral load and fall under the radar of the PCR nucleic acid test, which cannot detect cases recording less than 250 copies of virus per millilitre.

A previous hospital trial had shown some patients on the first day of infection could carry a viral load as low as between 51 and 90 copies per millilitre, a measure of the quantity of virus in a given volume of fluid.

The false negative results, combined with allowing people to quarantine at home, could lead to the virus spreading to family members of travellers, Siu explained.

The government has tightened quarantine rules and, starting from last Friday, all arrivals apart from those from China had to complete their 14-day period of isolation in hotels.

Sui said another loophole existed in the form of allowing people to visit those quarantining in hotels. He confirmed its existence by taking it upon himself to bring takeaway food into a hotel and enter the lift, which he managed without being questioned by staff.

“Even if those under quarantine didn’t leave their hotel rooms, they could have infected their family members who might go and visit them and even share a meal with them.”

To plug the loopholes, Siu suggested the health department stationed officials at hotels to monitor compliance, and to arrange special coaches for travellers heading into quarantine, to minimise the chances of them spreading the virus on public transport.

Meanwhile, Hong Kong Tourism Board recorded 7,817 arrivals in October, a 99.8 per cent drop on that month last year. Over the first 10 months of the year, inbound arrivals slumped 92.2 per cent to 3.56 million people, from 50 million in the same period in 2019.

But with the sector gearing up for the launch of a travel bubble between Hong Kong and Singapore on Sunday, some experts have expressed concern that increasing the international flow of people could expose the city to higher health risks.