Estimating SARS-CoV-2 variant strain infectiousness in Japan as of March 28, 2021

Kurita, J.; Sugawara, T.; Ohkusa, Y. O.

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.