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Background: Influenza viruses gradually accumulate point mutations, reducing the effectiveness of prior immune protection., Methods: Children aged 9-14 years received 2010-2011 trivalent inactivated influenza vaccine (TIV). Vaccination history, hemagglutination-inhibition (HI) titers, and cell-mediated immune responses were assessed to investigate the cross-reactivity with past and future influenza virus strains., Results: 2010-2011 TIV induced significant T-cell responses and HI titers of ≥160, with a fold-rise of ≥4 and titers of ≥100 maintained for >7 months in the majority of children. Pre-existing memory B cells in these children differentiated quickly to antibody-secreting cells to the new vaccine antigens. Children vaccinated in the previous year maintained high HI titers well into 2010, demonstrating elevated HI titers against A/Perth/16/2009, the future (in 2010-2011) H3N2 component. Prior vaccination enhanced CD8 + T-cell responses to A/Perth/16/2009. Children vaccinated with the prior 2009-2010 seasonal vaccine also demonstrated higher preexisting levels of interferon γ-secreting CD4 + CD69 + T cells to 2009 pandemic influenza A(H1N1). Children previously vaccinated with 2009-2010 seasonal influenza vaccine also showed greater expansion of tumor necrosis factor α-secreting CD8 + CD69 + T cells to 2009 pandemic influenza A(H1N1) upon vaccination in the 2010-2011 season than those who were not previously vaccinated., Conclusions: Seasonal influenza viruses continuously drift, which allows them to circumvent protective immunity, but conserved epitopes provide immunological cross-reactivity in children through either vaccination directly or through prime/boost in the prior influenza season., (Published by Oxford University Press for the Infectious Diseases Society of America 2016. This work is written by (a) US Government employee(s) and is in the public domain in the US.)

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., (© The Author 2014. Published by Oxford University Press on behalf of the Infectious Diseases Society of America.)