Aboard the Diamond Princess cruise ship, ¡°substantial transmission¡± of COVID-19 started prior to Feb. 5, when the health ministry quarantined the vessel and asked passengers to stay in their rooms, according to a state-run medical institution that analyzed data related to hundreds of patients.

A preliminary conclusion by the National Institute of Infectious Diseases (NIID) suggests that secondary transmission among passengers during the 14-day quarantine may not have been as rampant as initially feared by some experts.

However, transmission toward the end of the official quarantine period ¡°appears to have occurred mostly among crew or within passenger cabins,¡± the institute added in its report published Wednesday.