A month ago Hong Kong was sitting on a 17-day stretch without new local Covid-19 infections and looking least like a place that needed help to fight the pandemic. Now, thanks to risky exemptions from quarantine compounded by a relaxation of social-distancing rules, the city has been reeling under the third and most serious wave of infections. This has prompted Chief Executive Carrie Lam Cheng Yuet-ngor to seek help with testing and temporary hospital accommodation from the mainland. In a week of extraordinary events, such as a delay to elections and candidate disqualifications, this one still stands out. Amid cross-border political tensions, Lam’s move came as a surprise. Even by invitation, the involvement of scores of mainland medical staff on the ground sets a precedent under “one country, two systems”. With hindsight, it will warrant reflection. Now that it has been done, every effort must be made to achieve the goal of bringing the third wave under control. Close collaboration between the city’s health authorities and mainland advisers is paramount if the city is to take advantage of relevant experience from over the border. Plans for at least two temporary hospitals and expanded makeshift facilities at the AsiaWorld-Expo exhibition centre will boost capacity by 2,400 beds. Three more Covid-19 deaths as Hong Kong confirms 80 new cases Mainland experts will also play a key role in an expansion of daily coronavirus testing. Reports of a plan to test everyone prompted reservations from some local experts. University of Hong Kong microbiologist Dr Ho Pak-leung said it was more important to focus on tracing and testing close contacts of coronavirus patients. Respiratory medicine expert Professor David Hui Shu-cheong from Chinese University agreed, pointing out tests on everyone would require a citywide lockdown and enormous manpower. Hongkongers may be weary of the impact of Covid-19 on normal life. But that said, as they live and work under the most stringent social-distancing rules yet, they would hold officials to account for half measures and exemptions that failed to bring the contagion under control. Hopefully, cooperation between an advance team of mainland experts, local authorities and three private laboratories, which will supplement official testing, will produce an effective testing strategy for Hong Kong, whatever it takes.