The Hospital Authority on Monday urged citizens to vaccinate against seasonal influenza, citing uncertainties for the upcoming winter peak.

According to the Centre for Health Protection, flu cases in Hong Kong typically surge twice every year - between January and April, and then from July to August.

But the winter peak this year lasted 28 weeks until mid-July, more than double the normal interval of 12 weeks.

The government is bringing back its seasonal influenza vaccination programmes from Thursday.

Eligible groups will be able to participate either free of charge, or enjoy a partial subsidy. They include anyone 50 or above, or between six months and 18, or those who are pregnant, reside in residential care homes or work in healthcare institutions.

Matthew Luk, a family medicine consultant at United Christian Hospital, explained the jabs this year have been specially formulated to safeguard citizens from the prevalent flu strains, namely the H1, H3 and influenza B viruses.

"From time to time, the influenza virus will change a little bit, the predominant strain in the community will change...so we encourage citizens to have the flu jab because for every year, the components of the vaccine, we change a little bit to make it more suitable for [protection against] the virus of that time," he said.

He warned that catching a flu alone may not be lethal, but it could lead to complications including acute bronchitis, pneumonia, encephalopathy or other life-threatening conditions.

"Although children and the elderly are more prone to critical conditions brought by respiratory infections, healthy adults may also suffer from such conditions. Taking the jab can effectively reduce the chances of suffering from these illnesses," Luk said.

The doctor reassured citizens they can inoculate themselves against the flu and Covid at the same time.