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Influenza Attack Rates and Vaccine Effectiveness by Prior-Season Infection and Vaccination: A 17-Season Analysis.
Naoki Kawai1, Takuma Bando1, Takashi Kawashima1
1Influenza Study Group of the Japan Physicians Association, Tokyo Medical Association Building 4F, 2-5 Kanda-Surugadai, Chiyoda-ku, Tokyo 101-0062, Japan.
The Journal of Infectious Diseases
|July 11, 2026
Summary
Prior influenza infection enhances vaccine effectiveness (VE), while prior vaccination slightly reduces it. Understanding attack rates (ARs) alongside VE is crucial for accurate protection assessment.
Area of Science:
- Epidemiology
- Immunology
- Public Health
Background:
- Vaccine effectiveness (VE) interpretation can be confounded by baseline risk differences.
- The impact of prior influenza infection and vaccination on attack rates (ARs) and VE requires further characterization.
Purpose of the Study:
- To investigate how prior-season influenza infection and vaccination influence current-season attack rates (ARs) and vaccine effectiveness (VE).
Main Methods:
- A 17-season prospective cohort study in Japan (2002-2019) involving 148,108 person-seasons.
- Laboratory-confirmed influenza A infection was assessed.
- Multivariable logistic regression estimated adjusted odds ratios (aORs) for current-season vaccination, comparing ARs across groups with different prior infection/vaccination histories.
Main Results:
- Prior-season vaccination was associated with higher ARs in unvaccinated individuals compared to no prior vaccination.
- Current-season vaccination reduced influenza A odds (aOR=0.52).
- VE was more pronounced with prior infection (aOR=0.35) and less pronounced with prior vaccination (aOR=0.59), but remained protective across all groups.
Conclusions:
- Prior influenza infection enhances VE, while prior vaccination attenuates it, though VE remains effective.
- ARs vary based on prior vaccination status, highlighting their role in reflecting baseline risk.
- Interpreting AR alongside VE provides a more comprehensive understanding of vaccine protection.
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