Comparison of Viral Load in The Nasopharyngeal Swabs of Patients with Severe Acute Respiratory Syndrome Coronavirus 2 in Different Epidemic Seasons in Gunma Prefecture, Japan

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has repeatedly undergone mutations since its emergence, based on which, it has been assumed that there have been changes in its characteristics, including virulence and antigenicity. In this study, the viral loads in nasopharyngeal samples of patients with SARS-CoV-2 in Gunma Prefecture, Japan, from April 2, 2020, to April 1, 2023, were determined. The number of virus in samples in the Omicron-variant-prevalent period was higher than that in strains detected in samples before Week 50 of 2020; the B.1.1.284-prevalent period, the Alpha-variant-prevalent period, and the Delta-variant-prevalent period. Moreover, among the Omicron variants, the sub-lineage BA.5-prevalent period had more viral copies in the samples than in the BA.1-prevalent and BA.2-prevalent periods. Hence, the new variant may have released more viruses into the nasopharynx during the process of repeated mutations, resulting in widespread infection. The number of viruses detected in the nasopharyngeal samples showed an increasing trend with the evolution of the virus. Therefore, considering that the number of viruses in samples is also a vital factor contributing to the spread of the infection, it is important to determine the viral load in samples.