SARS-CoV-2 Omicron sublineage BA.2 replaces BA.1.1: genomic surveillance in Japan from September 2021 to March 2022

ABSTRACT

ObjectiveThe new emerging Omicron strain of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is currently spreading worldwide. We aimed to analyze the genomic evolution of the shifting Omicron virus subtypes. MethodsThe study included 1,297 individuals diagnosed as SARS-CoV-2 positive by PCR test or antigen quantification test from September 2021 to March 2022. Samples were analyzed by whole genome sequencing analysis (n=489) or TaqMan assay (n=808). ResultsAfter the outbreak of the SARS-CoV-2 Delta strain, the Omicron strain spread rapidly in Yamanashi, Japan. BA.1.1 was the predominant sublineage of the Omicron strain from January to mid-February 2022, but the number of cases of sublineage BA.2 began to increase after mid-February, and this sublineage was shown to have replaced BA.1.1 by the end of March 2022. We observed higher viral and antigen levels of sublineage BA.2 than of sublineage BA.1.1 in nasopharyngeal swab samples. However, no difference in viral load by patient age was apparent between sublineages BA.1.1 and BA.2. ConclusionsA transition from sublineage BA.1.1 to sublineage BA.2 was clearly observed over approximately one month. Omicron sublineage BA.2 was found to be more transmissible owing to its higher viral load regardless of patient age.