Staying home is the best way to keep the Covid-19 virus at bay, so it would seem. But this is only true when residential buildings do not come with structural design issues or faulty sewage pipes. Even though the problems have long been recognised as contributing factors to the outbreaks of severe acute respiratory syndrome (Sars) in 2003 and the coronavirus epidemic since 2020, little has been done to solve them. With infections in their tens of thousands each day this month, Hong Kong authorities can no longer tell how many were caused by the so-called vertical spread. But there was evidence aplenty at the early stage of the fifth wave when the highly infectious Omicron variant prompted the evacuation of housing blocks in different districts. Recent decisions not to send patients with mild symptoms to hospitals or quarantine centres means both the sick and healthy are staying home these days, raising the risk of cross-household transmissions. The outbreaks within housing blocks is reminiscent to those during Sars, when a Kowloon Bay building with defective plumbing became a hotbed of the deadly respiratory disease. The episode set off alarm bells concerning health risks arising from the unauthorised alteration and poor maintenance of drainage pipes. The recurrence of vertical transmission for the past two years shows the matter still has not been given the attention it deserves. Hong Kong to chart own way out of pandemic, leader says as Singapore moves to ease rules A drainage system inspection programme by the Housing Authority in the wake of early Covid-19 outbreaks has further put the problem into perspective. It was found that vent pipes had been removed in 8.9 per cent of 460,000 homes inspected. The plan to spend HK$619 million (US$79 million) on improving 25,000 pipes in 1,315 blocks is long overdue, but as the saying goes, it is better late than never. The risk is not just confined to government subsidised housing. As the outbreaks and evacuations in the latest wave show, public and private housing are equally prone to infections, especially when pipes and other facilities are not properly maintained. But many old buildings have no management committees to lead renovation or there is no money to do so. The city’s dense population and high-rises may be here to stay, but there is no room for design flaws and poor maintenance.