Close to half a million people in Tokyo are likely to have been infected with COVID-19 by March this year — nearly quadruple the number of official cases announced by the Metropolitan Government — according to a recent study.

The estimate was made by a team of researchers led by the Tokyo Metropolitan Institute of Medical Science that examined 23,234 randomly selected serum samples from patients at 14 Tokyo hospitals taken between Sept. 1, 2020, and March 31 this year, checking for SARS-CoV-2 antibodies.

Although none of the patients whose samples were examined had been diagnosed with COVID-19 or exhibited any symptoms at the time of their blood collection, the researchers found that the overall seroprevalence, or percentage of individuals in a population who have developed antibodies against the disease, was 1.83%, while the rate last March — the most recent month in which samples were tested — was 2.7%.