HONG KONG — Researchers say a comparison of coronavirus samples taken from people recently infected in Hong Kong suggests the virus might have ceased to mutate, which would mean if has effectively adapted to humans and become more transmissible.

The team at Polytechnic University also discovered strains in more than 20 recently confirmed local cases were highly identical to those of some imported cases, proving they were the source.

The study shed light on how the pandemic was spreading across the city and authorities would examine the conclusions, Secretary for Food and Health Professor Sophia Chan Siu-chee said.

"The genetic sequencing done by the experts is effective empirical evidence that provides us with a clearer picture," Ms Chan said on a radio programme. "The Centre for Health Protection was unable to find the source earlier, but now there is evidence and we will follow up in a serious manner."

The research was carried out by the university's department of health technology and informatics, supported by the Department of Health.

The experts compared virus strain samples taken from different clusters since late June. Ones taken from infected people linked to the Bun Kee Congee and Noodle Shop in Tsz Wan Sha, where the earliest cases of the third wave emerged at the beginning of July, were identical to those taken from the Fulum restaurant in Tuen Mun on July 21.

"The coronavirus had continued to mutate during the first and second waves when we carried out similar research," said Associate Professor Gilman Siu Kit-hang. "The discovery is very different this time, proving either the virus has adapted to the human body, thus it has stopped mutation, or these cases all contracted the virus at the same venue in a short period of time."

Professor David Hui Shu-cheong of Chinese University agreed the virus was more infectious than during the first two waves as it had adapted to the human body more efficiently.

Mr Siu's team compared 26 local samples drawn from different clusters against ones taken from three imported cases. It found 19 virus strains drawn from the Kong Tai Care for the Aged Centre Limited in Tsz Wan Shan, taxi drivers, and the Shui Chuen O public housing estate were highly identical with strains taken from imported cases from the Philippines and Pakistan.

A few strains drawn from a patient and his family members living in Leighton Hill in Causeway Bay were also very similar with one collected from a pilot returning from Kazakhstan.

"As many of the imported cases stayed at the Crowne Plaza hotel in Causeway Bay, where many aircrew stay, the patients likely contracted the virus from these imported cases," Mr Siu said.

One remaining strain taken from a case in Sai Kung could not be linked to any other cases, thus his team believed there were at least three transmission chains in the community.

The Hong Kong government said on July 19 there was no evidence that the latest wave was attributable quarantine exemptions, saying that assumption was a misunderstanding.

On the radio programme, Chan added the government has plugged the loophole of exempting seafarers and aircrew from quarantine rules. Under the policy that takes effect on Wednesday, ships without any cargo trade via Hong Kong will no longer be allowed to change their crew in the city, while seafarers and aircrew members must obtain negative Covid-19 results before they enter the city. SOUTH CHINA MORNING POST