Influenza virus infections and immunity in a cohort of school-age children over a 5-year period

Abstract

BACKGROUND: School-aged children experience high rates of influenza virus infections and associated illnesses each year, and are a major source of transmission in the community. However, information on the cumulative incidence of infection in specific epidemics is scarce, and there are limited studies with sufficient follow-up to identify the strength and duration of protection against reinfection. METHOD: Between August 2009 and February 2010 in Hong Kong, 796 households including 1,182 children were enrolled in a cluster-randomized trial and followed up for approximately for one year. One child in each household was randomized to receive 2009-10 seasonal TIV or saline placebo. We extended follow up of participants through to September 2014, losing approximately 15% of participants each year. Follow-up included serum collection at least once per year, and home visits to collect respiratory specimens from any ill individuals. The primary outcome measure was influenza virus infection in participants indicated by a four-fold or greater increase in antibody titers between paired serum specimens, or by RT-PCR confirmation of influenza on a respiratory specimen. RESULTS: Over the five years of follow-up, we included 4,141 person-years of follow-up of children 6-17 years of age. We identified 12 distinct influenza epidemics across the 5 years. Between 23% and 39% of children experienced laboratory-confirmed influenza virus infections each year, with over 75% of children experiencing at least one infection across the five year period. We found statistically significant evidence of protection against H1N1pdm09 virus infection in year 2 and again in year 4 associated with prior H1N1pdm09 infection in year 1, consistent with homosubtypic immunity lasting multiple years. We also identified a statistically significant protective effect of H3N2 infection in 2010 on H3N2 infection in 2012. There was no evidence that heterosubtypic immunity spanned one or more years. CONCLUSION: Influenza virus infections are common in school-age children in Hong Kong. These results increase our understanding of influenza epidemiology and immunity across multiple years following natural infections.