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.