Tohoku J. Exp. Med., 2022 May, 257(1)

Effectiveness of mRNA COVID-19 Vaccines in Japan during the Nationwide Pandemic of the Delta Variant

Tetsuya Akaishi,¹ Shigeki Kushimoto,² Yukio Katori,³ Noriko Sugawara,⁴ Kaoru Igarashi,⁵ Motoo Fujita,⁶ Shigeo Kure,⁴ Shin Takayama,¹ Michiaki Abe,¹ Akiko Kikuchi,¹ Kota Ishizawa,¹ Yoshiko Abe,¹ Hiroyuki Imai,⁷ Yohei Inaba,⁸ Yoko Iwamatsu-Kobayashi,⁹ Takashi Nishioka,¹⁰ Ko Onodera¹ and Tadashi Ishii¹

¹Department of Education and Support for Regional Medicine, Tohoku University Hospital, Sendai, Miyagi, Japan
²Division of Emergency and Critical Care Medicine, Tohoku University Graduate School of Medicine, Sendai, Miyagi, Japan
³Department of Otolaryngology-Head and Neck Surgery, Tohoku University Graduate School of Medicine, Sendai, Miyagi, Japan
⁴Department of Pediatrics, Tohoku University Graduate School of Medicine, Sendai, Miyagi, Japan
⁵Division of Craniofacial Anomalies, Tohoku University Graduate School of Dentistry, Sendai, Miyagi, Japan
⁶Department of Emergency and Critical Care Medicine, Tohoku University Hospital, Sendai, Miyagi, Japan
⁷Clinical Skills Laboratory, Tohoku University School of Medicine, Sendai, Miyagi, Japan
⁸Department of Radiation Disaster Medicine, International Research Institute of Disaster Science, Tohoku University, Sendai, Miyagi, Japan
⁹Department of Dental Infection Control, Tohoku University Hospital, Sendai, Miyagi, Japan
¹⁰Liaison Center for Innovative Dentistry, Tohoku University Graduate School of Dentistry, Sendai, Miyagi, Japan

The coronavirus disease 2019 (COVID-19) pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), remained a major global health concern in 2021. To suppress the spread of infection, mass vaccinations have been performed across countries worldwide. In Japan, vaccinations of the first and second doses for most of the nation were performed during the nationwide outbreak of the B.1.617.2 (Delta) variant with the L452R spike protein mutation, and the effectiveness of the vaccinations to suppress the spread of COVID-19 among the people in Japan remains uncertain. In this study, adults aged ≥18 years, who were in contact with patients with COVID-19 and underwent nasopharyngeal swab reverse transcription-polymerase chain reaction (RT-PCR) tests during August and September 2021 at a mass screening test center in Japan, were enrolled. In this period, more than 95% of the COVID-19 infections were reportedly caused by the Delta variant. As a result, a total of 784 adults with recent contact history, including 231 (29.5%) RT-PCR test-positive cases, were enrolled. The test positivity rate was lower in individuals who had been vaccinated twice than in unvaccinated individuals (12.5% vs. 39.0%, p < 0.0001), with the risk ratio of 0.32 (95% confidence interval 0.23-0.46). The vaccine effectiveness was the highest between 7-90 days after the second vaccine dose. In conclusion, two doses of mRNA COVID-19 vaccines effectively suppressed transmission in Japan during the nationwide pandemic of the Delta variant, estimated to have prevented 50-80% of the infection.

Keywords —— coronavirus disease 2019 (COVID-19); Delta variant; L452R spike protein mutation; mRNA vaccines; severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)

© 2022 Tohoku University Medical Press

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Tohoku J. Exp. Med 2022, 257, 1-6.

Correspondence: Tetsuya Akaishi, M.D., Ph.D., Department of Education and Support for Regional Medicine, Tohoku University Hospital, 1-1 Seiryo-machi, Aoba-ku, Sendai, Miyagi 980-8574, Japan.

e-mail: t-akaishi@med.tohoku.ac.jp