[Transmission of SARS-CoV-2 through contact with a SARS-CoV-2-infected individual in the presymptomatic or asymptomatic state in Japan: A case series].

Objectives　There is little evidence supporting the transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) from presymptomatic or asymptomatic SARS-CoV-2-infected individuals in Japan, where the incidence of SARS-CoV-2 infection is lower than that in other developed countries. This study aimed to determine whether SARS-CoV-2 transmission can occur from presymptomatic or asymptomatic SARS-CoV-2-infected individuals.Methods　We surveyed all directors of Japanese public health centers for index cases and secondary patients who possibly contracted SARS-CoV-2 infection from a presymptomatic or asymptomatic SARS-CoV-2-infected individual who came under their care before June 20, 2020. The professional staff at the centers routinely perform contact tracing of infected persons based on the guidelines of the Infection Control Act. Four authors independently reviewed reports of 9 index cases of SARS-CoV-2-infected individuals with 17 secondary patients from 8 prefectures and examined the cases to determine whether transmission from a SARS-CoV-2-infected individual in the presymptomatic or asymptomatic state occurred.Results　We reported 7 index cases with 13 secondary patients. 1) An elderly woman acquired SARS-CoV-2 infection from her sustained asymptomatic granddaughter at home, 2) 4 guests and 1 accompanying child waiting at a hair salon acquired infection from the presymptomatic female hair stylist, 3) 2 inpatients acquired infection from a presymptomatic nurse while providing nursing care in close contact, 4) an elderly couple acquired SARS-CoV-2 infection from their presymptomatic relative who was in the 50s during household care at their home, 5) a man acquired SARS-CoV-2 infection from a presymptomatic adult neighbor in an enclosed space with poor ventilation, 6) a presymptomatic man had transmitted infection to another man at a coffee shop while having a discussion on business, and 7) a man in his 50s acquired SARS-CoV-2 infection from a presymptomatic man during 50 minutes of close contact at their office and in a car. These secondary patients had no other likely routes of infection. The interval between the date of symptom onset in the presymptomatic index case and the secondary patient ranged from 2 to 6 days. The incidence rates at the time these infections occurred in the corresponding prefectures ranged from 0.00 to 6.56 cases/1 million person-days.Conclusion　We report the first case of SARS-CoV-2 transmission from a sustained asymptomatic index case in Japan. All secondary patients came into close contact with presymptomatic index cases in areas with poor ventilation.