Infectious diseases expert Ivan Hung on Sunday called on Hongkongers, especially the elderly and those who are immunocompromised, to take every possible measure to guard against the coronavirus.

He said while Covid has transitioned into an endemic, those who are vulnerable can still develop very severe diseases if they're infected.

The University of Hong Kong medical professor said high-risk groups should get their booster jabs as soon as possible, adding that it is perfectly fine to mix vaccines.

"The crossover of inactivated and mRNA vaccines is extremely safe and very effective. And some studies actually showed that if you have been receiving BioNTech and cross over to receive the Moderna vaccine, the antibody, the immunogenicity is even better," Hung said at a press conference.

Speaking at the same event, an expert in paediatrics, Mike Kwan, said now is the time to get jabbed, as there's currently a Covid upsurge around the world with the more-transmissible JN.1 variant being the dominant strain.

"This is an important timing for receiving a vaccine, in fact, today," he said.

"Because two weeks later we're having our Lunar New Year festive period. It takes two weeks' time for us to develop antibodies to protect ourselves after vaccination."

Kwan stressed that children infected with Covid could develop serious complications like acute pneumonia and acute necrotising encephalitis.