But the limits have costs. In the case of new scrutiny on imports, scientists widely believe that they will do little to keep people from getting infected. Amid a fresh Covid-19 outbreak in the port city of Dalian, Chinese officials this week ordered businesses there that use imported frozen foods to stop their operations.

In a neighboring Hebei Province, officials tested hundreds of packages after several workers at a children’s clothing factory were found to have Covid-19. In Guangxi, a province 1,200 miles to the south of Hebei, officials went even further, testing every person who had touched or even received a package from the factory.

Not a single person outside the factory reported testing positive.

In addition to inspecting imported frozen foods, China has required that packages coming from overseas be sanitized, and has encouraged people to use masks and gloves when receiving deliveries. International health bodies have said there is a minimal chance of transmission on surfaces like cardboard.