The chairman of the Hong Kong Society for Infectious Diseases on Thursday said it would "not be surprising" if some students might have already caught norovirus – which can cause gastroenteritis – before departing for an exchange trip on the mainland.

More than 50 secondary students and teachers from three local schools who had eaten meals at the same restaurants on an exchange trip to Shaoguan, Guangdong, were reported to have contracted gastroenteritis on Tuesday.

Speaking on an RTHK radio show, Dr Wilson Lam revealed that norovirus activity has significantly increased globally compared to last year.

He pointed out that the virus can be spread if there is vomit present.

"For example, if there's a confined space in school buses or coaches, if a student vomited, it can absolutely cause students nearby to be infected," he said.

"Or by having contact with areas where the virus has landed can also result in indirect contact transmission."

He also revealed that hand sanitisers are ineffective against norovirus, and advised the public to wash their hands with soap instead.

Hong Kong Subsidised Secondary Schools Council chairman Lee Yi-ying revealed to the same programme that travel agencies will not allow schools to postpone tours if they suspect there may be infectious diseases at the destination.

But schools can talk to agencies about visiting different attractions or restaurants, she said.

Lee added that if a student feels unwell, teachers will take them to the doctor after they obtain their parents' consent.

Lawmaker Tang Fei told the same show that there are sufficient guidelines for schools to follow during study tours.

For instance, at least one participating staff should have a first aid licence, he said.

Schools should activate the crisis management mechanism if they encounter similar cases, he added.