Association between Serum Antibody Titers against Influenza Viruses and the Risk of Influenza in Patients with Chronic Respiratory Diseases - a Prospective, Longitudinal Study

Abstract

Introduction: Patients with respiratory co-morbidities are the primary target group for influenza vaccination as chronic respiratory diseases pose a serious risk for unvaccinated individuals during influenza season. According to Centers for Disease Control and Prevention (CDC) guidelines, serum antibody titer of ≥ 40 is associated with “at least a 50% decrease in risk of acquiring influenza infection”. Methods: A prospective, longitudinal study was conducted in 345 patients with chronic respiratory diseases (April 2014-August 2015). Serum samples were tested for antibody levels against influenza A and B viruses using hemagglutination inhibition assay as per WHO protocol. These patients were followed up for one year and nasopharyngeal swab was collected from any patient who reported Influenza-like illness symptoms and subjected to real-time PCR for influenza viruses. Results: The seroprotective rates against influenza A (H1N1) pdm09, influenza A(H3N2), influenza B (Victoria and Yamagata) were 61%, 94%, 61%, 68% respectively. During the follow-up period of one year, 5 cases of influenza A/H3N2 and 7 cases of influenza B infection were identified while there were no cases of influenza A(H1N1)pdm09 infection. One case of influenza A/H3N2 infection was observed in an individual who had protective titers against the same virus while all 7 cases of influenza B infection occurred in people with non-protective titers against both the strains of influenza B viruses. Conclusion: Based on the findings of this study, it is proposed that in patients with chronic respiratory diseases influenza vaccination may be considered after more concrete information is obtained on correlates of protection against influenza subtypes.