Infectivity of Omicron BA.5 Comparison with Original Strain and Other Mutated Strain of SARS-CoV-2 in Japan

Abstract

Background: Earlier studies have indicated the BA.5 sublineage of Omicron variant strain of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) as more infective than BA.2. Object: This study estimated BA.5 infectivity while controlling other factors possibly affecting BA.5 infectivity including vaccine effectiveness, waning effectiveness, other mutated strains, Olympic Games, and countermeasures. Method: The effective reproduction number R(t) was regressed on shares of BA.5 and vaccine coverage, vaccine coverage with some delay, temperature, humidity, mobility, shares of other mutated strains, countermeasures including the Go to Travel Campaign, and the Olympic Games and associated countermeasures. The study period was February 2020–July 22, 2022, using data available on August 12, 2022. Results: A 120 day lag was assumed to assess waning. Mobility, some states of emergency, vaccine coverage and those with lag, and the Delta and Omicron BA.2 proportions were found to be significant. The omicron BA.1 proportion was significant, but with an unexpected sign. The estimated coefficient of BA.5 was negative but not significant. The Go to Travel Campaign was significantly negative, indicating reduced infectivity. The Olympic Games were negative but not significant, indicating that they did not raise infectivity. Discussion: The obtained estimated results show that BA.5 did not have higher infectivity than the original strain. It was lower than either Delta or Omicron BA.2 variant strains. That finding might be inconsistent with results obtained from earlier studies. This study controlled several factors potentially affecting R(t), though the earlier studies did not. Therefore, results from this study might be more reliable than those of earlier studies.