We examined influenza vaccine effectiveness (VE) in a prospective cohort of households with children over the past several influenza seasons; substantial VE was observed against influenza A (H1N1)pdm09 viruses that were well matched to vaccine viruses in the 2013-2014 season. In contrast, the 2014-2015 influenza season was characterized by widespread circulation of influenza A (H3N2) viruses that were antigenically drifted from vaccine viruses; we assessed VE against these viruses.
Enrolled households had ≥3 members, including ≥2 children <18 years. Specimens collected from subjects reporting acute respiratory illnesses were tested for influenza by RT-PCR. Influenza vaccination was self-reported with date and location of receipt and documented by medical records and registries. VE was estimated in Cox proportional hazards models that allowed vaccination status to vary by time and adjusted for age and high-risk health condition. VE was estimated overall and stratified by age group, infection source (community vs household-acquired influenza), and vaccination in the prior season (2013-2014).
A total of 1431 participants, including 862 (60%) children <18 years, from 340 households were enrolled. Overall, 195 (14%) subjects had high-risk health conditions, and 982 (69%) had evidence of receipt of at least one dose of 2014-2015 influenza vaccine. Influenza A (H3N2) virus was identified in 166 (12%) individuals; infection risks were similar across age groups (<9 years: 15%; 9-17 years: 11%; ≥18 years 10%). 112 (94%) of 119 viruses tested by pyrosequencing belonged to the drifted 3C.2a genetic group. VE against influenza A (H3N2) was -3% (95% CI: -55% to 32%) and similarly ineffective by age group and infection source (Figure 1). VE did not significantly vary by prior vaccination (Figure 2).
Absence of VE against influenza A (H3N2) viruses in 2014-2015 was expected given mismatch between the vaccine and circulating viruses, and is consistent with findings from the same season in the ambulatory care setting. However, VE estimated in the hospital setting in the same season was substantially higher, despite a similarly high proportion of drifted viruses, highlighting the continued need for annual evaluations in multiple settings to better understand variation in VE in different populations.
J. G. Petrie,
R. E. Malosh, None
E. T. Martin, None
A. M. Fry, None
A. S. Monto, Sanofi Pasteur: Consultant and Grant Investigator , Consulting fee and Grant recipient
GSK: Consultant , Consulting fee
Novavax: Consultant , Consulting fee