Infectious diseases expert Ivan Hung on Saturday said the influenza season has been disrupted because of a drop in population immunity after the Covid pandemic.

The University of Hong Kong professor said Hong Kong is currently at the end of the summer flu outbreak, and the winter surge will appear from February to April next year.

"What we have been seeing is that, there's a displacement of the flu season from previously... The winter peak usually happened in December, January, and February, now it's shifted to February, March and April," Hung told reporters after attending a radio programme.

"Basically both the flu and Covid will have a surge in cases every six months, that is of course due to a drop in (the) community antibody level."

Hung called on vulnerable groups such as the elderly and people who have chronic illnesses to get jabbed against the flu.

He added that senior citizens should take at least one booster shot against Covid every year.

"For individuals who are in good health and under the age of 65, they don't need to take the booster vaccine, provided that they have already had two to three doses of vaccines," he said.