Age-related differences in influenza type/subtype among patients hospitalized with influenza, FluSurv-NET—2017–2018

Background: The 2017-18 influenza season had the highest rates of influenza hospitalizations since the 2009 H1N1 pandemic. We used data from the Influenza Hospitalization Surveillance Network (FluSurv-NET) to identify unique characteristics of the 2017–18 season.

Methods: We included all patients residing within a FluSurv-NET catchment area, and hospitalized with laboratory-confirmed influenza during 2017-18. We used multiple imputation, including age, surveillance site, and month of hospital admission as predictors, to impute influenza A subtype for 40–64% of cases across seasons with an unknown subtype. We calculated influenza hospitalization rates by type/subtype per 100,000 population. We compared 2017-18 rates to rates during 4 prior seasons: 2016-17, 2015-16, 2014-15, and 2013-14.  

Results: The overall unadjusted hospitalization rates per 100,000 population varied from 31.5 during 2015–16 to 105.1 during 2017-18. After imputing A subtype, the 2017-18 season had the highest rates observed for H3N2 (62.8) and B (28.5) than in any previous season, and the 3^rd highest rate of H1N1 (13.5) (Figure 1A). During 2017-18, rates in adult ≥65 years peaked 3 weeks before they peaked in children 0-4 years. In contrast, during the 4 prior seasons, rates in adults ≥65 years peaked during the same week or 1 week after they peaked in children 0-4 years. During 2017-18, the distribution of influenza type/subtypes varied significantly by age group (p <0.0001); for example, the proportion of cases with H1N1 ranged from 19 to 29% in those <65 years to only 7% in those ≥65 years. During 2017-18, H1N1 (the non-predominant A virus) contributed >25% of A cases across all age groups (except ≥65 years) versus all prior seasons where the non-predominant A virus contributed <10% of A cases across all age groups (except ≥65 years) (Figure 1B-F).

Conclusions: Several unique characteristics may have contributed to the high hospitalization rates observed during 2017-18. Rates in older adults, who were predominantly infected with H3N2, peaked several weeks prior to children in contrast to prior seasons.  Higher overall rates of H3N2 and B were observed in 2017-18 compared to these prior seasons and substantial H1N1 co-circulation also occurred with marked variability by age group.