Association between Haemagglutination inhibiting antibodies and protection against clade 6B viruses in 2013 and 2015.

BACKGROUND: The epidemiology of the pandemic A(H1N1) virus has been changing as population immunity continues to co-evolve with the virus. The impact of genetic changes in the virus on human's susceptibility is an outstanding important question in vaccine design. In a community-based study, we aim to (1) determine the genetic characteristics of 2009-2015 pandemic H1N1 viruses, (2) assess antibody response following natural infections and (3) assess the correlation of A/California/07/09 antibody titers to protection in the 2013 and 2015 epidemics.

METHODS: In a household transmission study, serum specimens from 253 individuals in Managua, Nicaragua were analyzed. Combined nose and throat swabs were collected to detect RT-PCR confirmed influenza infection and virus sequencing. Hemagglutination inhibition assays were performed and the protective titer for circulating H1N1pdm was determined.

RESULTS: Clade 6B pandemic H1N1 viruses predominated in Nicaragua during the 2013 and 2015 seasons. Our household transmission study detected a household secondary attack rate of 17% in 2013 and 33% in 2015. Infected individuals, including vaccinees, showed an apparent antibody response to A/California/07/09. Baseline titers of A/California/07/09 antibodies were found to associate with protection in both seasons. A titer of ≥1:40 correlated to a 44% protection in children, a 29% protection in adults 15-49years old and a 51% protection in adults 50-85years old.

CONCLUSION: In 2013 and 2015, antibody titers to A/California/07/09 associated with an infection risk reduction amongst exposed household contacts. This is consistent with a detectable vaccine effectiveness reported in a number of studies. Genetic changes in clade 6B viruses might have led to a reduced immunity in some whereas others might have been less affected. The use of human serologic data is important in virus characterization and if performed in a timely manner, could assist in vaccine strain selection.