Over 500 cases were identified retrospectively with COVID-19 symptom onset before the first officially reported case in Italy's Lombardy region, according to a study published in peer-reviewed academic journal Epidemics on Saturday.

Researchers from local health authorities and research institutions reviewed and analyzed official records of the first 16,665 lab-confirmed COVID-19 cases in Lombardy from January 30 to March 9, 2020.

"We identified 527 cases with symptom onset before the detection of the first case, on February 20 [2020], 89.2 percent of which were hospitalized and 27.5 percent who died," read the research paper titled "The early phase of the COVID-19 epidemic in Lombardy, Italy."

The results also suggested that SARS-CoV-2, the virus that causes COVID-19, was already circulating in "at least 222 out of 1,506 municipalities with sustained transmission across all the Lombardy provinces" before the first case.

At the time of the notification of the first case, "symptomatic cases were already present in all Lombardy provinces," said the research, which "may explain the large number of critical cases experienced by this region in a very short time frame."