China on Monday in its most extensive COVID-19 lockdown since the pandemic started has effectively shuttered Shanghai to conduct mass virus testing and control a growing outbreak in the country’s largest city and financial capital.

Government officials have managed smaller, previous outbreaks with limited lockdowns of housing compounds and workplaces in which the virus was spreading. But the sharp increase in recent days has resulted in them moving to more austere measures.

Roughly 3,500 new cases were reported Sunday in the city of 26 million people. However, almost all of those who tested positive were asymptomatic, according to the Associated Press.