Dr Mike Kwan, president of the Asian Society for Paediatric Infectious Diseases, said this especially applied to vulnerable groups.
Speaking after a radio programme, he said new viruses often appear during the summer holidays.
“Indeed it is a worry because other places in the world also have flu outbreaks,” Kwan said.
“After we travel to those areas and we bring back the virus to Hong Kong, this might cause another infection if the protection among the population in Hong Kong is not good.
Although students are not in school during the summer, Kwan pointed out that many still take part in group activities, making it easy for the flu to spread.
In response to the case of a seven-year-old boy who died from complications arising from a bout of influenza A, despite having been vaccinated, Kwan said some individuals can have severe immune reactions that lead to complications and a quick deterioration in their condition.
He said he hoped the case would not affect public confidence in vaccines, pointing out that the boy had been vaccinated some time ago and that his antibody levels might have since decreased.