The novel coronavirus disease 2019 (COVID-19) was first reported to the World Health Organization by the WHO country office in China in December 2019 and has since become a worldwide pandemic. In Japan, COVID-19 is a designated infectious disease under the Infectious Disease Control Act, meaning that hospitalization or isolation measures are required for infected persons. It has been noted that in some cases, patients show positive PCR test results even after discharge from the hospital upon meeting the discharge criteria for COVID-19. However, so far, there is a lack of substantial evidence on the pathogenesis of reinfection or relapse in patients with COVID-19. We report 4 cases of COVID-19 who showed repeat-positive PCR test results for SARS-COV-2 after discharge from the hospital, and assessed their infectivity using clinical information collected via a public epidemiological survey. All 4 cases showed a repeat-positive PCR test results within 40 days of the initial onset of symptoms. All the PCR test results showed high Ct values of 33 or higher in the repeat-positive test. In addition, neutralizing antibodies were detected in all cases within 3 days from the date of the repeat-positive test. Furthermore, an epidemiological survey was conducted in 18 persons who were in close contact with the 4 cases, and 11 of them tested negative by the PCR test, and no case of secondary infection was found. Based on these findings, the risk of secondary infection from the 4 cases was considered as low. No specimens collected at the time of the first infection or virus culture test results were available for further evaluation. Issues remain to be resolved in respect of the systems needed for cooperation with healthcare providers and the laboratory testing required for the evaluation of re-infection. In order to further elucidate the pathogenesis of COVID-19, and to provide appropriate medical care, it is essential to evaluate the infectivity of patients with a repeat-positive PCR test and to accumulate further knowledge about the disease.
