Influenza vaccine effectiveness in the United States during 2012-2013: variable protection by age and virus type

Abstract

Background: During the 2012-2013 influenza season, there was cocirculation of influenza A(H3N2) and 2 influenza B lineage viruses in the United States.

Methods: Patients with acute cough illness for ≤7 days were prospectively enrolled and had swab samples obtained at outpatient clinics in 5 states. Influenza vaccination dates were confirmed by medical records. The vaccine effectiveness (VE) was estimated as [100% × (1 - adjusted odds ratio)] for vaccination in cases versus test-negative controls.

Results: Influenza was detected in 2307 of 6452 patients (36%); 1292 (56%) had influenza A(H3N2), 582 (25%) had influenza B/Yamagata, and 303 (13%) had influenza B/Victoria. VE was 49% (95% confidence interval [CI], 43%-55%) overall, 39% (95% CI, 29%-47%) against influenza A(H3N2), 66% (95% CI, 58%-73%) against influenza B/Yamagata (vaccine lineage), and 51% (95% CI, 36%-63%) against influenza B/Victoria. VE against influenza A(H3N2) was highest among persons aged 50-64 years (52%; 95% CI, 33%-65%) and persons aged 6 months-8 years (51%; 95% CI, 32%-64%) and lowest among persons aged ≥65 years (11%; 95% CI, -41% to 43%). In younger age groups, there was evidence of residual protection from receipt of the 2011-2012 vaccine 1 year earlier.

Conclusions: The 2012-2013 vaccines were moderately effective in most age groups. Cross-lineage protection and residual effects from prior vaccination were observed and warrant further investigation.

Keywords: influenza; medically attended influenza; vaccine effectiveness.

Figure 1.

Trivalent inactivated influenza vaccine effectiveness (VE) against influenza A(H3N2) (A) and influenza B/Yamagata (B), by year of age among persons aged 1–75 years. VE estimates were derived by including an interaction term between age and current-season vaccination status in the logistic model, in which age was represented as a restricted cubic spline function with 5 knots based on percentiles. The confidence intervals represent the uncertainty around the point estimate at each age. VE data are adjusted for network site, subject age, presence of high-risk health conditions, and calendar time (2-week intervals).

Figure 2.

Age-specific trivalent inactivated influenza vaccine effectiveness (VE) of current and prior vaccination against influenza A(H3N2) among persons age ≥9 years, in which persons not vaccinated in either the current or prior season served as the reference group. VE data are adjusted for network site, subject age, presence of high-risk health conditions, and calendar time (2-week intervals). *P = .01 for comparison between the 2 vaccine groups. Abbreviation: CI, confidence interval.

Figure 3.

Age-specific trivalent inactivated influenza vaccine effectiveness (VE) of current and prior vaccination against influenza B/Yamagata among persons age ≥9 years, in which persons not vaccinated in either the current or prior season served as the reference group. VE data are adjusted for network site, subject age, presence of high-risk health conditions, and calendar time (2-week intervals). Abbreviation: CI, confidence interval.