The Centre for Health Protection said Hong Kong’s annual flu season lasted only about five weeks this year – from January 5 to February 8. 

“The influenza peaked around late January, before starting to decrease continuously. In early February, it returned to the baseline level,” a spokesman for the centre said.

That was shorter than the 14 weeks recorded last year, between December 30, 2018, and April 6, 2019.

Health authorities had found positive results in 18.76 per cent of respiratory specimens, down 11.34 points from 30.1 per cent at last year’s peak.

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The dominant virus in this influenza season was influenza A (H1), accounting for around 80 per cent of infections, followed by A (H3), at around 16 per cent, and very few influenza B positive detections.

Epidemiologists said the city had been able to avoid the double whammy of influenza and the coronavirus outbreak largely because of social distancing measures and heightened vigilance caused by the pandemic.

Infectious disease expert Dr Joseph Tsang Kay-yan said class suspensions in kindergartens, schools, and universities, in force since the Lunar New Year holiday ended on February 3, as well as work-from-home arrangements and cancellation of large-scale public events, had played a big role in curbing the spread of the flu.

“Seasonal influenza was earlier proved to spread quickly among schoolchildren,” he said.

Better personal and environmental hygiene practices, such as wearing masks and using hand sanitisers, also helped.

“The coronavirus outbreak has seen a lot of collateral damage, but this is a collateral benefit,” Tsang said.

Another factor according to him that helped bring a swift end to the winter flu was an expansion of the vaccination programme among people, with more than 1.3 million doses administered by the government this year – a year-on-year increase of 17 per cent.

Dr Leung Chi-chiu, chairman of the Hong Kong Medical Association’s advisory committee on communicable diseases, echoed Tsang’s comments and said good public health practices had helped break down “invisible routes of transmission” by subclinical patients in the community.

He also pointed to the sharp fall in influenza infections after the first coronavirus case was reported in the city on January 23.

Leung added travel restrictions had also helped lower imported infections. But he said the city still needed to guard against a potential resurgence of the seasonal influenza, particularly influenza B, since there was low herd immunity in the community as fewer people had been infected with that strain of the virus.

“We saw coronavirus infections rebound very quickly after some citizens seemed to have let down their guard and started going out and about again,” Leung said.

The flat influenza curve allowed the city to concentrate on battling the deadly coronavirus, Leung said.

“Otherwise our public hospitals would have been overwhelmed fighting two wars on two fronts at the same time,” he added.

Dr Ho Chung-ping, the association’s president, said while business at the city’s private clinics, including those from influenza cases, was down 50 per cent on average, many had flocked to public facilities to get tested when they presented symptoms such as a fever and cough.

“More rigorous testing this year has helped us contain the seasonal influenza too,” he said.