A top Hong Kong public health adviser has warned a potential fifth wave of Covid-19 infections could emerge in the next two weeks, hours after authorities identified the city’s first untraceable case with a mutant strain and evacuated hundreds from a residential block overnight. Chinese University respiratory medicine expert Professor David Hui Shu-cheong also said the government might need to look into tightening social-distancing measures once more if local coronavirus infections rose rapidly, particularly if unlinked cases made up more than 20 per cent of the total. Infectious disease expert Dr Joseph Tsang Kay-yan, meanwhile, suggested that authorities consider mass virus testing for the city’s foreign domestic workers, given that the 39-year-old woman carrying the mutant strain had met with other helpers during her incubation period.