Background: A variant strain of SARS-CoV-2, VOC202012/01, emerged in the UK in September , 2020 . Its infectiousness was estimated as higher than that of the original strain . Object: We estimated the infectiousness of the variant strain of SARS-CoV-2 in comparison to that of the original strain under conditions prevailing in Japan .
Methods: We estimated infectiousness through a simple susceptible-infected-recovered (SIR) model by strain . The study period was March 1-28 , 2020 . The information used for the study was available as of April 3 , 2020 .
Results: The estimated reproduction number of the SARS-CoV-2 variant strain was 1.799; its 95% CI was [1.642 , 1.938]. The onset date of the first case in Japan was estimated as December 4 ([ November 16, December 14] ), 2020 . However, infectiousness of the original strain was estimated as 1.123 ([ 1.093 , 1.166] ). Discussion and Conclusion: We demonstrated that infectiousness increased by 0.684 or 60%, increasing from the original to the variant . That finding might be comparable to that of a study conducted in the UK . However, the difference must be monitored continuously and carefully.