Covid-19 cases have stayed low in Hong Kong despite more mixing between mainland Chinese visitors and local residents, but flu infections have been rising since wearing masks became optional.

Health experts said people in the city and mainland China had built up hybrid immunity from natural infection and vaccination and this helped to keep the coronavirus in check since the border reopened early last month.

“Hong Kong has already established its hybrid immunity, and similarly for the mainland,” said Professor David Hui Shu-cheong, a Hong Kong government pandemic adviser. “When both places have hybrid immunity, there will not be much of a problem.”