A genetic study of viral samples from infected passengers on a flight into Hong Kong later found to have a record number of Covid-19 cases will be undertaken to determine whether in-flight transmission or quarantine facilities were to blame, with implications on tighter protocols required to contain such spikes.
The Post learned the University of Hong Kong (HKU) would be conducting genome sequencing on the 51 passengers of Vistara flight UK6395 from New Delhi on April 4, while a review of official data showed 22 of them, or around 43 per cent, were only confirmed to have had the infection during their hotel quarantine on the 15th day of their arrival.
Thirty-nine of those passengers were also asymptomatic when their infections were confirmed, figures that on Wednesday sparked calls for more frequent testing of foreign arrivals and placing those from high-risk countries in government facilities.
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At least one expert questioned whether hotel transmission was partly the reason behind the large number of infected passengers from just one flight, while others queried the reliability of pre-departure Covid-19 tests and the possibility of in-flight transmission.
Hong Kong’s focus is firmly on imported cases, with the city marking its first day without a local coronavirus infection in more than two weeks.
The one imported case on Wednesday involved a 31-year-old woman who recently arrived from the Philippines and carried the N501Y mutant strain. The new infection took the total number of confirmed cases to 11,704, with 209 related deaths. Fewer than five preliminary-positive cases were reported.
In a separate development, one of two designated flights bringing residents back to Hong Kong from Britain departed on Wednesday, carrying hundreds of passengers.
The arrivals, expected to land in the early hours of Thursday, would be quarantined for 21 days at the Rambler Garden Hotel in Tsing Yi. The second flight would depart on April 28.
Flights from Britain were banned in December after the emergence of a mutant strain in the country.
The large number of infections linked to the Vistara flight had prompted HKU infectious disease expert Dr Ho Pak-leung to question whether some cases were transmitted during hotel quarantine rather than being imported, given the mean incubation period for Covid-19 was around five days.
It was previously reported that 53 infections were imported but the Department of Health on Wednesday clarified that two were yet to be determined.
Concerns about infection control measures at Hong Kong’s 30 designated quarantine hotels were also heightened after the first case of a locally transmitted coronavirus variant was detected in a 29-year-old man who arrived from Dubai and might have been infected while in isolation.
Viral samples from the 51 infected passengers would be sent to HKU’s school of public health for whole genome sequencing, the health department told the Post.
The procedure would give officials and researchers a better understanding on the source and route of transmission, including on any links to quarantine hotels. The department said epidemiological investigations and relevant contact tracing on the cases were continuing.
Dr Gilman Siu Kit-hang, an associate professor at Polytechnic University’s health, technology and informatics department, believed that a primary reason for the “exceptionally high” caseload was potential “false negative” tests as India was besieged by infections.
In the week before April 4, Covid-19 cases in India were starting to surge, with around 53,000 to 93,000 new infections daily.
“It is entirely possible their laboratories became unable to cope with the high demand for testing,” Siu said, adding that the country had recently experienced 260,000 cases in a day.
It was widely believed in the scientific community, including experts in Hong Kong, that in-flight transmissions were unlikely because of tight infection control measures such as sparse seating arrangements, as well as adequate ventilation and air changes.
But Siu said a spread could still have happened on flight UK6395 if a highly infectious patient had not worn a mask properly, or if germs had contaminated common facilities such as toilets when a passenger did not flush the water closet with the lid closed.
Professor Leo Poon Lit-man from HKU’s school of public health also believed transmission on the plane was a possibility.
“In-flight transmission has happened before,” Poon said. “If those passengers were found to carry a similar strain of the virus, it could suggest there was transmission during the time they were at airport check-in counters, boarding gates or aircraft cabin.”
He said follow-up checks should be done to see if the passengers had worn masks and followed other infection control measures throughout the journey.
Although travellers from India were required to present a negative Covid-19 test result before boarding a flight to Hong Kong, Poon questioned the accuracy.
“If someone could transmit the virus, their viral load should not be low and should be able to be detected by the test,” he said.
The Ramada Hong Kong Grand View in North Point and Ramada Hong Kong Harbour View in Sai Ying Pun housed the highest number of infected UK6395 passengers. The former had 18 passengers, and the latter eight.
Poon said evidence of in-hotel transmission could only be established if genome sequences of Covid-19 cases in a facility were similar, while taking into account other epidemiological data such as whether the people had met before.
Siu believed weekly testing, rather than the current regime of screening on the first, 12th and 19th day of the person’s 21-day isolation, could stop a potential hotel spread similar to transmissions at the Ramada Hong Kong Grand in Tsim Sha Tsui, where the 29-year-old man stayed.
HKU’s Ho also suggested introducing extra tests on the fifth and eighth day of arrival to detect infections in a more timely manner.
Government pandemic adviser Professor David Hui Shu-cheong also suggested more frequent testing.
“For those travelling from high-risk countries, we should keep the quarantine period at 21 days so we can observe whether they have infections. But extending the quarantine to 28 days might deter people from coming to Hong Kong at all,” he said.
While several S-shaped door hooks used in quarantine hotels to deliver meals were previously suspected of being the means of transmission behind the infection of the 29-year-old Dubai engineer, Hui said the ones where he stayed tested negative for the coronavirus, leaving the source of infection still a mystery.
Cross-infections were made more likely when many travellers from countries deemed by the government to be “extremely high-risk” such as India and the Philippines, quarantined in the same hotel, Siu said.
He said Hong Kong should restore the practice of putting returnees from extremely high-risk countries, which also included Brazil, Ireland, South Africa and Britain, into government-run quarantine centres to lower the chance of a spread.
Workers were worried about contracting mutant strains at quarantine hotels, unions said.
Alex Tsui Hau-lai, chairman of the Hong Kong Hotel Employees Union, said he had not heard about hotels setting up air purifiers in corridors yet, but some properties had put small tables outside rooms for meal deliveries.
“If confirmed cases are found, hotels can partially cordon off the levels in question, so that staff can stay away from the infected zone,” he said, adding the area should only resume services after thorough cleaning and 14 days.
Some migrant worker volunteer groups also said they would stop dropping off donated goods to newly hired helpers undergoing quarantine.
Meanwhile, an expert panel on vaccine effects assessed two cases involving pregnant women, both aged 32. One suffered a miscarriage while the other case involved fetal death after the women received Covid-19 jabs.
“After reviewing the relevant clinical information and pathology results, the expert committee concluded that there was no evidence indicating association with vaccination,” a statement said.
The committee noted the hospital would further investigate the cause of the fetal death.
The panel also assessed four reports of post-vaccine deaths received by the Hospital Authority between April 5 and 18. They involved four men, aged 54 to 92.
All had been vaccinated more than 14 days before they died and there was no clinical evidence based on preliminary autopsy findings that their outcomes were caused by the vaccines, the panel found.
It noted the two Covid-19 vaccines authorised for use in Hong Kong were safe, effective and of good quality and the benefits of taking them outweighed the risks.
Additional reporting by Gigi Choy and Ng Kang-chung
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