Low H3N2 vaccine effectiveness (VE) in 2012-2013 in adults may have been caused by low vaccine immunogenicity in a subset of the population, according to a recent study. This is in contrast to analyses based on ferret studies, in which it was due to egg-adaptation of the vaccine strain. Researchers investigated the basis of low VE in 2012-2013 by evaluating whether vaccinated and unvaccinated individuals were infected by different viral strains and assessing the serologic responses to IVR-165, A/Victoria/361/2011, and 3C.2 and 3C.3 strains in an adult cohort before and after vaccination. They found:

• No significant genetic differences were found between the strains that infected vaccinated and unvaccinated individuals.
• Vaccination increased titers to A/Victoria/361/2011 and 3C.2 and 3C/3 representative strains as much as to IVR-165.
• Only ∼one-third of the cohort achieved a ≥4-fold increase in titer.