Infectiousness in omicron variant strain and bA.2 variant in Japan

Background Omicron variant strain dominated since the beginning of 2022. Its infectivity was supposes to be higher than Delta variant strain or strains in past. Object We estimated prevalence of omicron variant strain, particularly bA.2 variant and COVID-19 vaccine effectiveness of the third dose in Japan as well as controlling for waning of second dose of vaccine, other mutated strains, the Olympic Games, and countermeasures. Method The effective reproduction number R( t ) was regressed on shares of omicron variant strain and bA.2 and vaccine coverage of the third dose, as well as along with data of temperature, humidity, mobility, share of the other mutated strains, and an Olympic Games and countermeasures. The study period was February, 2020 through February 21, 2022, as of March 15, 2022. Results Estimation results indicated that waning of the second dose vaccine e with 150 days prior was the most appropriate specification. Moreover, bA.2 of omicron variant strain has higher infectively than other variant strain or traditional strain. Discussion Because of data limitation since emerging bA.2, the estimated its infectively will change over time.