China’s first wave of coronavirus cases may have been four times bigger than the country reported, according to a Hong Kong study.

China had reported more than 55,000 cases as of Feb. 20 — but the true number would have been far greater if the country had used a later adopted, broader definition of what constitutes a COVID-19 case, The Guardian reported.

The original criteria for identifying Covid-19 was very narrow, the paper reports, citing a new study published in the Lancet.

“The case definition was initially narrow and was gradually broadened to allow detection of more cases as knowledge increased, particularly milder cases and those without epidemiological links to Wuhan, China, or other known cases,” the study’s authors wrote.

The new data revises the epidemic’s growth rate and doubling times, the authors noted.