Validation of the United States isolation termination criteria using virus culture results of the omicron variant in Japan

Abstract

Cycle threshold (Ct) values obtained from real-time reverse-transcription polymerase chain reaction (RT-PCR) tests are commonly used to determine COVID-19 isolation discharge from hospitals. However, not all patients with low Ct values are infectious, and studies evaluating infectivity through virus cultures in Japan remain limited. This study assessed patients with Ct values <30 at 10 days after symptom onset and compared the clinical characteristics of seven patients with positive virus culture results and 11 patients with negative virus culture results to validate the Centers for Disease Control and Prevention (CDC) isolation criteria in Japan.

Among non-immunosuppressed patients with mild-to-moderate COVID-19, 80 % were virus culture-negative 10 days after symptom onset, even when the Ct values were <30. In contrast, only 33 % of patients with severe-to-critical symptoms tested negative on virus culture. All immunosuppressed patients with severe-to-critical symptoms consistently tested positive in virus cultures. Notably, three patients with hematological disorders remained virus culture-positive 20 days after symptom onset.

These findings generally align with the CDC criteria. For non-immunosuppressed patients with mild-to-moderate COVID-19, isolation release after 10 days is supported without the need for additional RT-PCR testing. However, for non-immunosuppressed patients with severe-to-critical symptoms and immunocompromised patients, particularly those with hematological disorders, isolation may be safely discontinued when Ct values exceed 30 after 10 days. This study provides valuable insights into identifying individuals suitable for isolation release based on SARS-CoV-2 testing in Japan.