Impact of repeated vaccination on vaccine effectiveness against influenza A(H3N2) and B during 8 seasons

Abstract

Background: Recent studies suggest that influenza vaccination in the previous season may influence the effectiveness of current-season vaccination, but this has not been assessed in a single population over multiple years.

Methods: Patients presenting with acute respiratory illness were prospectively enrolled during the 2004-2005 through 2012-2013 influenza seasons. Respiratory swabs were tested for influenza and vaccination dates obtained from a validated registry. Vaccination status was determined for the current, previous, and prior 5 seasons. Vaccine effectiveness (VE) was calculated for participants aged ≥9 years using logistic regression models with an interaction term for vaccination history.

Results: There were 7315 enrollments during 8 seasons; 1056 (14%) and 650 (9%) were positive for influenza A(H3N2) and B, respectively. Vaccination during current only, previous only, or both seasons yielded similar protection against H3N2 (adjusted VE range, 31%-36%) and B (52%-66%). In the analysis using 5 years of historical vaccination data, current season VE against H3N2 was significantly higher among vaccinated individuals with no prior vaccination history (65%; 95% confidence interval [CI], 36%-80%) compared with vaccinated individuals with a frequent vaccination history (24%; 95% CI, 3%-41%; P = .01). VE against B was 75% (95% CI, 50%-87%) and 48% (95% CI, 29%-62%), respectively (P = .05). Similar findings were observed when analysis was restricted to adults 18-49 years.

Conclusions: Current- and previous-season vaccination generated similar levels of protection, and vaccine-induced protection was greatest for individuals not vaccinated during the prior 5 years. Additional studies are needed to understand the long-term effects of annual vaccination.

Keywords: influenza; vaccine effectiveness.

Figure 1.

Unadjusted and adjusted vaccine effectiveness (VE) of various vaccination histories against influenza A(H3N2) virus infection–associated medically attended acute respiratory illness, age ≥9 years. ^aAdjusted for age, sex, high-risk conditions, season, interval (days) from onset to sample collection, and influenza diagnosis code in prior seasons. ^bAmong persons with ≥5 years of continuous residency. ^cP = .01 for comparison between the 2 groups. ^dP = .01 for comparison between the 2 groups. Abbreviations: aVE, adjusted vaccine effectiveness; CI, confidence interval; uVE, unadjusted vaccine effectiveness.

Figure 2.

Unadjusted and adjusted vaccine effectiveness (VE) of various vaccination histories against influenza B virus infection–associated medically attended acute respiratory illness, age ≥9 years. ^aAdjusted for age, sex, high-risk conditions, season, interval (days) from onset to sample collection, and influenza diagnosis code in prior seasons. ^bAmong persons with ≥5 years of continuous residency. ^cP = .05 for comparison between the 2 groups. Abbreviations: aVE, adjusted vaccine effectiveness; CI, confidence interval; uVE, unadjusted vaccine effectiveness.