Two of China’s biggest cities ordered residents to undergo mass Covid-19 tests this week, highlighting the risk that fresh outbreaks could trigger new and economically costly lockdowns. Authorities in Tianjin, a northeastern metropolis of more than 13 million people, ordered citywide tests on residents to screen for coronavirus infections starting Monday evening, after reporting two new cases earlier in the day. These were the first locally transmitted infections detected in more than a week in the home to the biggest port in northern China and the main maritime gateway to Beijing.