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Estimating Vaccination Effects and Variant Strains on COVID-19 outbreak course in Japan, as of August, 2021

By Junko Kurita, Tamie Sugawara, Yasushi Ohkusa

Posted 22 Jun 2021
medRxiv DOI: 10.1101/2021.06.20.21259209

Background: Japan started vaccinations for COVID-19 in late February, 2021, mainly using BNT162b2 mRNA COVID-19 vaccine. At that time, N501Y strain was emerging. Later, L452R strain appeared in Japan. Object: We evaluated effects of COVID-19 vaccination and variant strains on infectiousness in Japan. Method: The effective reproduction number R(t) was regressed on vaccine coverage and the shares of variant strains 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 July 18, as of August 10, 2021. Results: Estimation results indicate that vaccine coverage for both first and second doses wast significant, but positive. Moreover, infectiousness of the mutated strain N501Y decreased significantly, but that of L452R was not significant. Discussion and Conclusion: Estimation result for vaccine coverage implies that vccine did not reduced infectiousness at all. Also, mutated strain with higher infectiousness were not confirmed. Close and continuous monitoring is necessary.

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