“The duration [of the flu season] would be prolonged when there is a transition of the dominating virus strain,” the centre said. “It is believed that the current influenza season will persist for a period of time and more outbreaks and severe cases might be recorded in the coming weeks.”

While the city entered the current flu season in early January this year, the extension meant that it would last longer than the average flu period of eight to 12 weeks.

Dr Wong Hoi-kei, an epidemiologist at the centre, told a radio show on Friday morning that Hong Kong had recorded 20 severe cases of flu in children, including two deaths, since the start of the influenza season.

Only five – a quarter of the total – had been vaccinated.

“These figures reflected the importance of vaccinations and the effectiveness of the flu jab in preventing serious complications,” he said.

Wong added the child flu vaccination rate remained at about 60 per cent and appealed to parents to get their children jabbed as soon as possible as the shots take two weeks to take effect.

He explained that, even though the second fatality had been vaccinated, her death could have been a result of genetic or hereditary factors.

“Any vaccination, not just the flu vaccination, can’t be 100 per cent effective in preventing illness,” Wong said. “But it can largely reduce the risk of severe complications.”

Dr Lee Kwok-piu, the chairman of the Hospital Authority’s coordinating committee for paediatrics, warned that influenza could develop into pneumonia and that serious complications such as brain infection, heart inflammation, and even death, could result.

“There has been a clear upwards trend in the number of children seeking medical treatment for influenza in the past week or two, with most of them being upper respiratory tract infections,” he told the same radio show.

“The hospitals’ paediatric wards are close to capacity, but we believe we have enough manpower to cope,” he added.

Dr Mike Kwan Yat-wah, an honorary associate professor at the University of Hong Kong’s department of paediatrics and adolescent medicine, said a relatively low vaccination rate could give rise to alternating flu strains.

He said about 50 per cent of the city’s population had been inoculated with the latest seasonal flu vaccine.

“If we need to build an immunity barrier to prevent infections of alternating viruses, an overall vaccination rate would need to reach 70 to 80 per cent to do so,” Kwan said.

He added that as the city experienced a peak of H1 infections starting from March last year, the immunity against the virus subtype would decline roughly six months later, suggesting that people could also be more susceptible to the strain.

“When the population’s immunity is low, the flu season could be longer … we could see a majority of people getting infected before the transmission stops,” he said.

Kwan added there was no significant difference in terms of transmissibility or virulence between H1 and H3.

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Professor Ivan Hung Fan-ngai, an infectious disease expert from the same university, pointed out it was “quite common” to have two different flu virus strains in one season, especially when community immunity was low after Covid-19.

He said the low immunity was caused by a lack of thorough flu infections in the community during Covid-19 pandemic, together with the low flu vaccination rate.

Health authorities said Hong Kong underwent a 16-week winter flu season in 2015-16 after the dominance of flu A virus was overtaken by flu B later.

The six-year-old girl died this week after having contracted the H1 subtype virus and developing encephalopathy, also known as altered brain function.

The girl had a history of good health and had received a flu vaccine for this season, authorities said.

The news came just five days after the announcement of the death of an unvaccinated eight-year-old girl, who had also caught the H1 strain.

Girl, 6, in critical condition after contracting flu in Hong Kong

Kwan explained that a person could still develop complications following a flu jab under certain conditions, such as undiagnosed congenital immunodeficiency or metabolic disorders, a severe flu infection or another bacterial infection.

Health authorities said there had already been a “significant increase” in the number of flu-like outbreaks at schools and institutions in the week ending April 20 compared with the first week of the month, rising from 10 to 29.

“The [centre] appealed again to members of the public to heighten their vigilance, and people belonging to high-risk priority groups should receive seasonal influenza vaccination as soon as possible for prevention of severe disease and death.”