Epidemiological Characteristics of SARS-CoV-2-Positive Samples in Japan: A Retrospective Study

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causes coronavirus disease 2019 (COVID-19). In Japan, infections began in January 2020, and the number of infections has continued to increase. Specimens requested by medical institutions were measured for ORF1a/b, N-gene, and S-gene using TaqPath SARS-CoV-2 real-time polymerase chain reaction detection kits in our biomedical laboratory from January to August 2021. The distribution of threshold cycle (Ct) values of positive samples was analyzed separately every 2 months. The Ct values of saliva samples were normally distributed for all genes, except for the May/June S-gene. By contrast, nasopharyngeal swab samples showed a bimodal distribution from January to June, with a shift toward lower median values in July/August. The results suggest that viruses with different properties may be captured in saliva and nasopharyngeal samples.