Identification of H5N1-specific T-cell responses in a high-risk cohort in vietnam indicates the existence of potential asymptomatic infections.

(See the editorial commentary by Epstein, on pages 4–6.) Influenza H5N1 remains endemic in domestic poultry in large parts of Asia, and although the total number of human infections is relatively small, sporadic human cases with a high risk of death are still being reported [1, 2]. Since 2003, >500 human cases of highly pathogenic influenza A H5N1 have been reported, with 119 cases occurring in Vietnam [3]. At present, H5N1 influenza cannot be transmitted readily between humans, but the possibility remains of a recombination between H5N1 and other influenza viruses, resulting in a virulent and easily transmissible virus [4]. The reported frequency and severity of H5N1 infection in humans is almost certainly biased by the underdetection of mild or asymptomatic cases: leading to an underestimate of the number of cases and an overestimate of the case fatality rate. The extent of this bias is indicated by seroprevalence surveys that have reported anti-H5 antibody prevalence in exposed groups of between 0% and 12% [5–9]. The presence of virus-neutralizing antibody is important for protection against influenza, and antibodies that recognize specific hemagglutinin (HA) subtypes can give an indication of recent infection history [10, 11]. However, measurement of H5N1-specific neutralizing antibodies has been problematic because the traditional hemagglutination inhibition (HAI) assay has low sensitivity for the detection of H5N1 antibodies [12]. Alternative assays that have been used for the detection of H5N1 infection include horse erythrocyte HAI, microneutralization, and pseudoparticle assays [13]. These assays are all subject to false-positive reactions due to the presence of cross-subtype neutralizing antibodies [14]. Measuring interferon (IFN) γ secretion by peripheral blood mononuclear cells (PBMCs) after stimulation with pools of HA peptides from different influenza strains demonstrates the specificity and reactivity of T cells generated by strain-specific vaccination [15–17] or memory cells generated after natural infection [18–21]. In this study, we set out to investigate the rate of infection with H5N1 virus in a community in rural Vietnam that had previously experienced H5N1 cases in both poultry and humans by measuring the prevalence of specific T-cell responses against the HA and neuraminidase (NA) of H5N1 influenza virus. We also compared T-cell responses against the proteome of seasonal H3N2 and the HA and N1 of H5N1 and H1N1 influenza in the community cohort with those in a group of persons who had recovered from H5N1 infection and in healthy controls with no exposure to H5N1.