Infectivity of omicron BA.5 comparison with original strain and other mutated strain of SARS-Cov-2 in Japan

Abstract

Background: Pervious studies indicated that BA.5 sublineage of Omicron variant strain of SARS-CoV-2 has more infective than BA.2.

Object: The object of this study was to estimate infectivity of BA.5 controlling other factors that might affect BA.5’s infectively including vaccine effectiveness and waning, the mutated strain other than , the Olympic Games, countermeasures, and other factors that might affect BA.5’s infectively.

Method: The effective reproduction number R(t) was regressed on shares of BA.5 as well as  vaccine coverage, vaccine coverage with some delay, temperature, humidity, mobility, share of the other mutated strains, counter measures including Go To Travel Campaign and an Olympic Games and countermeasures. The study period was February, 2020 through July 22, 2022, as of August 12, 2022.

Results : We selected the specification with 120 days lag of waning. In this specification, mobility, some state of emergency, vaccine coverage and those with lag, and proportion of delta and omicron BA.2 were significant with the expected sign. Conversely, proportion of omicron BA.1 were significant but with the unexpected sign. The estimated coefficient of BA.5 was negative but insignificant. Goto Travel Campaign was significantly negative and thus it reduced infectively. The Olympic games was negative but in significant and thus it did not raise infectively.

Discussion: The obtained estimated results showed that BA.5 did not have higher infectivity than the original strain and lower than delta or omicron BA.2 variant strains. Though it might be inconsistent the previous research, this study might be more reliable than the previous studies. Because BA.5. outbreak had not cease yet and we cannot predict its dynamics, the estimated BA.5 infectivity will change over time.