A government pandemic advisor on Wednesday called on schools to work harder to boost the vaccination rate among pupils.

Lau Yu-lung, who is also Chair Professor of Paediatrics at the University of Hong Kong, made the comment after the Education Bureau said secondary schools that wish to resume full-day, face-to-face classes from November must ensure at least 90 percent of their students are triple-jabbed.

Speaking on a radio programme, the medical expert said slightly more than half of all secondary school pupils in Hong Kong have had their third Covid shot.

"As long as we work harder and schools promote vaccination, such as by directly asking students why they do not get vaccinated, whether there are any problems, or even inviting doctors to explain, and also having principals and teachers work harder in urging pupils to get jabbed, it's not difficult to reach an inoculation rate of 80 to 90 percent," Lau said.

But Dion Chen, chairman of the Hong Kong Direct Subsidy Scheme Schools Council, told RTHK's Hong Kong Today programme that it won't be easy for secondary schools to hit the 90 percent target in two months' time.

Chen, also a school principal, said younger students aged between 12 and 17 are required to wait several months between their second and third jabs.

"For the senior students at secondary schools, I believe it's not too difficult to reach that 90 percent. But for the whole school, it would take a little bit of time," Chen said.