Respiratory medicine expert Leung Chi-chiu warned on Wednesday that a flu outbreak in Hong Kong is much more likely now people are no longer obliged to wear masks in public.

On an RTHK programme, Leung said younger children will be especially vulnerable, as they haven't had an opportunity to build immunity, and parents should get them vaccinated.

"In the past three years, the flu has almost disappeared in Hong Kong. Some, especially children under the age of six, have either never been exposed to the flu at all, or even if they have been exposed, the likelihood of catching it is small. In this case, their immunity to influenza is relatively low, and we have to be careful," he said.

"Even though schools do not require masks now, it's best for everyone to give themselves an adaptation period."

But government pandemic adviser Ivan Hung said it is "really necessary" for students to take their masks off.

"When schoolchildren are learning, especially when it comes to languages, they must see the movements of the mouth and lips."

Hung told an RTHK programme that schools are expected to see a slight rebound in flu or other upper respiratory tract infections, but this is good for students.

He said if pupils keep wearing masks, it will only delay outbreaks and a low level of viral spread in schools could help strengthen their immune systems.

Some children heading to school on Wednesday appeared to be cautious about going mask-free.

"I'm used to wearing a mask, and when you take it off, it feels strange," a secondary one student said.

A mother told RTHK, meanwhile, that she wanted her child to keep wearing a mask until the summer.

"[My kid] gets sick less often [with a mask on], and I think it is safer to wear it," she said.