Workers in PPE test residents, including elderly and children, in a locked-down Shanghai residential district. Shanghai, a city of 25 million and China's economic engine room, has become the heart of the country's biggest outbreak since the peak of the first virus wave in Wuhan over two years ago, rattling the country's adherence to a strict zero-Covid policy. Residents locked down since early April have complained of food shortages and over-zealous officials forcing them into state quarantine, as authorities rush to construct tens of thousands of beds to house Covid-19 patients with daily infections topping 20,000. IMAGES