The Centre for Health Protection said on Tuesday it was investigating the latest cluster involving pupils from SKH St Michael’s Primary School in North Point.
The five were part of a group of 70 students and teachers who visited Xi’an between March 11 and 14.
Health authorities said one of the five developed symptoms of acute gastroenteritis such as vomiting on the last day of the tour. Four others sitting nearby the pupil who first fell ill began showing similar symptoms two days later.
Two sought medical attention but none required hospitalisation, officials said, adding their symptoms matched that of a norovirus infection.
“The centre’s preliminary investigation results show that the cases are likely to be infected through person-to-person transmission or an environment contaminated by vomit,” it said.
A number of secondary school pupils have come down with gastroenteritis after visiting the mainland on exchange tours, with the government forming a task force to look into the recent outbreak.
The Education Bureau has reminded schools that have upcoming exchange tours to learn about the latest information on disease prevention and control, a spokesperson said.
According to the bureau’s website on national education activities, it has urged schools to “avoid providing higher-risk food such as raw and cold food, siu mei and shellfish seafood when organising student exchange activities [on] the mainland”.
It has also asked contactors of exchange programmes across the border who help to organise trips to stop supplying such dishes.
