tetano
Editor, Senior Moderator
J Infect Dis
. 2026 Jul 11:jiag352.
doi: 10.1093/infdis/jiag352. Online ahead of print.
Influenza Attack Rates and Vaccine Effectiveness by Prior-Season Infection and Vaccination: A 17-Season Analysis
Naoki Kawai[SUP] 1 [/SUP], Takuma Bando[SUP] 1 [/SUP], Takashi Kawashima[SUP] 1 [/SUP], Shinro Matsuura[SUP] 1 [/SUP], Tetsunari Maeda[SUP] 1 [/SUP], Woon Joo Lee[SUP] 1 [/SUP], Shin Nagao[SUP] 1 [/SUP], Hareaki Yamamoto[SUP] 1 [/SUP], Kazuo Mori[SUP] 1 [/SUP], Tokunao Amemiya[SUP] 1 [/SUP], Osame Tanaka[SUP] 1 [/SUP], Ken-Ichi Doniwa[SUP] 1 [/SUP], Ietaka Satoh[SUP] 1 [/SUP], Hideyuki Ikematsu[SUP] 1 [/SUP]
Affiliations
Background: Vaccine effectiveness (VE) is commonly interpreted as a measure of protection but may be influenced by differences in baseline risk across populations. The effects of prior-season influenza infection and vaccination on both attack rates (ARs) and VE have not been fully characterized.
Methods: We analyzed 148,108 person-seasons from a 17-season prospective, community-based cohort study in Japan (2002-2003 through 2018-2019). Influenza infection was confirmed using rapid antigen testing. ARs among current-season unvaccinated individuals (a proxy for baseline risk) and vaccinated individuals were compared across groups defined by prior-season infection and vaccination. Adjusted odds ratios (aORs) and relative aORs (rORs) for current-season vaccination were estimated using multivariable logistic regression, with laboratory-confirmed influenza A infection (yes/no) as the outcome.
Results: Among current-season unvaccinated individuals, ARs were higher in those with prior-season vaccination than in those without (14.0% vs 8.6% in children; p<0.001). ARs were similar regardless of prior-season infection but higher with prior-season vaccination. Current-season vaccination reduced odds of influenza A infection (aOR, 0.52; 95% CI, 0.47-0.57). This reduction was more pronounced among individuals with prior-season infection (aOR, 0.35; 95% CI, 0.25-0.49) and less pronounced among those with prior-season vaccination (aOR, 0.59; 95% CI, 0.50-0.70). Protection remained across all groups (aOR <1).
Conclusions: Prior-season infection was associated with enhanced VE of current-season vaccination and similar ARs among vaccinated individuals, whereas prior-season vaccination was associated with attenuated but effective VE. These findings highlight the importance of interpreting AR alongside VE, as ARs reflect differences in baseline risk.
Keywords: attack rate; cohort study; influenza A; prior-season infection; prior-season vaccination; vaccine effectiveness.
. 2026 Jul 11:jiag352.
doi: 10.1093/infdis/jiag352. Online ahead of print.
Influenza Attack Rates and Vaccine Effectiveness by Prior-Season Infection and Vaccination: A 17-Season Analysis
Naoki Kawai[SUP] 1 [/SUP], Takuma Bando[SUP] 1 [/SUP], Takashi Kawashima[SUP] 1 [/SUP], Shinro Matsuura[SUP] 1 [/SUP], Tetsunari Maeda[SUP] 1 [/SUP], Woon Joo Lee[SUP] 1 [/SUP], Shin Nagao[SUP] 1 [/SUP], Hareaki Yamamoto[SUP] 1 [/SUP], Kazuo Mori[SUP] 1 [/SUP], Tokunao Amemiya[SUP] 1 [/SUP], Osame Tanaka[SUP] 1 [/SUP], Ken-Ichi Doniwa[SUP] 1 [/SUP], Ietaka Satoh[SUP] 1 [/SUP], Hideyuki Ikematsu[SUP] 1 [/SUP]
Affiliations
- PMID: 42435455
- DOI: 10.1093/infdis/jiag352
Background: Vaccine effectiveness (VE) is commonly interpreted as a measure of protection but may be influenced by differences in baseline risk across populations. The effects of prior-season influenza infection and vaccination on both attack rates (ARs) and VE have not been fully characterized.
Methods: We analyzed 148,108 person-seasons from a 17-season prospective, community-based cohort study in Japan (2002-2003 through 2018-2019). Influenza infection was confirmed using rapid antigen testing. ARs among current-season unvaccinated individuals (a proxy for baseline risk) and vaccinated individuals were compared across groups defined by prior-season infection and vaccination. Adjusted odds ratios (aORs) and relative aORs (rORs) for current-season vaccination were estimated using multivariable logistic regression, with laboratory-confirmed influenza A infection (yes/no) as the outcome.
Results: Among current-season unvaccinated individuals, ARs were higher in those with prior-season vaccination than in those without (14.0% vs 8.6% in children; p<0.001). ARs were similar regardless of prior-season infection but higher with prior-season vaccination. Current-season vaccination reduced odds of influenza A infection (aOR, 0.52; 95% CI, 0.47-0.57). This reduction was more pronounced among individuals with prior-season infection (aOR, 0.35; 95% CI, 0.25-0.49) and less pronounced among those with prior-season vaccination (aOR, 0.59; 95% CI, 0.50-0.70). Protection remained across all groups (aOR <1).
Conclusions: Prior-season infection was associated with enhanced VE of current-season vaccination and similar ARs among vaccinated individuals, whereas prior-season vaccination was associated with attenuated but effective VE. These findings highlight the importance of interpreting AR alongside VE, as ARs reflect differences in baseline risk.
Keywords: attack rate; cohort study; influenza A; prior-season infection; prior-season vaccination; vaccine effectiveness.