Hong Kong is struggling to contain its fourth wave of Covid-19 infections, with more than 1,800 cases reported since late November 2020, most of which were locally transmitted. At the same time, a rising number of Hongkongers are heading to the Chinese mainland, where they are allowed to move freely after spending two weeks in quarantine. More than 21,000 people departed for the mainland via Shenzhen Bay Port and the Hong Kong-Zhuhai-Macau Bridge in the first week of December, according to…