• Hiroshi Yokomichi, Mie Mochizuki, Sayaka Horiuchi, Megumi Kushima, Ryoji Shinohara, Reiji Kojima, Tadao Ooka, Yuka Akiyama, Kunio Miyake, Sanae Otawa, Zentaro Yamagata, and Japan Environment and Children's Study Group.
• Department of Epidemiology and Environmental Medicine, University of Yamanashi, 1110 Shimokato, Chuo, Yamanashi, Japan. Electronic address: hyokomichi@yamanashi.ac.jp.
• Prev Med. 2023 Aug 1; 173: 107599107599.

AbstractWe measured the association between history of influenza vaccination by age 2 years and influenza virus (IFV) infection at ages 3 and 4 years by relative risk reduction. We also examined the association between history of IFV infection by age 2 years and recurrent IFV infection at age 3 years. This study included 73,666 children from a large Japanese birth cohort. Among children vaccinated never, once or twice when aged under 2 years, 16.0%, 10.8% and 11.3%, respectively, had been infected with IFV by age 3 years, and 19.2%, 14.5% and 16.0%, respectively, by age 4 years. Compared with no history of influenza vaccination, vaccination at ages 1 and/or 2 years reduced the risk of IFV infection at age 3 by 30%-32% and at age 4 by 17%-24%. The relative risk of recurrent IFV infection at ages 3 and 4 years increased in proportion to the number of prior infections by age 2. One-season-prior influenza vaccination history reduced the IFV infection risk at age 3 years by 25%-42%. Influenza vaccination most effectively protected children at age 3 who lacked older sibling(s) and did not attend nursery school. One-season-prior IFV infection increased the relative risk of recurrent infection at age 3 years (1.72-3.33). In conclusion, influenza vaccination-induced protection may partly extend to the next season. Owing to the relative risk reduction by influenza vaccination and the increased relative risk of IFV infection from prior-season infection, annual influenza vaccination is recommended.Copyright © 2023 The Authors. Published by Elsevier Inc. All rights reserved.

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