where 13 million people have been confined to their homes in one of the country's strictest lockdowns since the pandemic began. these images are from inside xi'an. sun chunlan told officials there to speed up testing and do more to isolate the close contacts of those found to be positive. more than 1200 cases have been found in the city in the past few weeks. dr xi chen from yale university is an expert on health policy and explained why china is continuing to pursue a strategic of �*zero covid'. china has been maintaining the zero tolerance policy for a long time, almost two years, so basically it's ramping up mass testing, frequent contact tracing, quarantining and strict border control. and although the case numbers are low, china has been worried about the west opening up, it will crowd out and make the system very stressful and there is a larger concern