Estimating Vaccination Effects on COVID-19 outbreak course in Japan

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

Background: Japan started vaccinations for COVID-19 in late February, 2021, mainly using BNT162b2 mRNA COVID-19 Vaccine. Object: We evaluated COVID-19 vaccination effects on infectiousness in Japan. Method: The effective reproduction number R(t) was regressed on vaccine coverage along with data of temperature, humidity, mobility, and countermeasures. We presumed two definitions for vaccine coverage: at least a single dose with a 12-day delay and a second dose. The study period was February, 2020 through May 16, 2021. Results: Estimation results indicate that vaccine coverage significantly and drastically reduced R(t) for both definitions. Results imply that increased first shot coverage by 1% point decreased R(t) by 0.5 with a 12 day delay. Moreover, increased second shot coverage by 1% point, decreased R(t) by 1 immediately. Discussion and Conclusion: Vaccination effects might be stronger because the periods when vaccination was started and rapid spread coincide with a decreasing phase of R(t).