The scale of the COVID-19 outbreak in Wuhan early this year may have been nearly 10 times the recorded tally, a study conducted by China’s public health authorities indicates, leaving the city where the coronavirus first took hold still well short of the immunity required to protect against a potential resurgence.

About 4.4% of those tested were found to have specific antibodies that can fight off the pathogen that causes COVID-19, indicating they were infected some time in the past, according to a serological survey of more than 34,000 people conducted in April by the Chinese Center for Disease Control and Prevention. The data was released late Monday.

That ratio would suggest that with Wuhan home to about 11 million people, as many as 500,000 residents may have been infected, nearly 10 times more than the 50,000 confirmed cases reported by health authorities in mid-April, when the survey was conducted.