COVID-19 may have continued to spread silently in Wuhan, China, during the spring of 2020, even after official government tallies had suggested the coronavirus had been stamped out, a new study suggests.

SARS-CoV-2, the virus that causes COVID-19, was first discovered in Wuhan in December 2019, and the city soon became the epicenter of what would become the COVID-19 pandemic. Cases peaked in Wuhan in February 2020 but soon declined rapidly, with just a few cases reported in late March. By early April, the city’s lockdown had ended, and later that month, Wuhan was declared coronavirus-free.