Several unprecedented characteristics might have contributed to the extremely high influenza hospitalization rates observed during the 2017-18 season, a recent study.

National Centers for Disease Control and Prevention researchers and colleagues pointed out that rates in older adults, who were predominantly infected with H3N2, peaked several weeks prior to children, which was different from prior seasons.

Hospital bed. Source: Getty

In addition, they noted that higher overall rates of H3N2 and B were observed in 2017-18 compared to previous flu seasons, and that substantial H1N1 co-circulation also occurred with surprising variability by age group.

Noting that the 2017-18 influenza season had the highest rates of influenza hospitalizations since the 2009 H1N1 pandemic, researchers said they used data from the Influenza Hospitalization Surveillance Network (FluSurv-NET) to identify unique characteristics of the 2017–18 season.

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

Results indicated that the overall unadjusted hospitalization rates per 100,000 population varied from 31.5 during 2015–16 to 105.1 during 2017-18. The data confirmed how severe last year’s influenza season was, according to the researchers, who said that, after imputing A subtype, the 2017-18 season was found to have the highest rates observed for H3N2 (62.8) and B (28.5) than in any previous season, and the 3rd highest rate of H1N1 (13.5).

How different age groups were affected by the flu remain puzzling, however. The study determined that, during 2017-18, rates in adult 65 and older peaked 3 weeks before they peaked in children 0-4 years old. Yet, during the 4 prior seasons, rates in older adults peaked during the same week or 1 week after they peaked in children 0-4 years.

In addition, during the 2017-18 flu season, the distribution of influenza type/subtypes varied significantly by age group (p <0.0001). The researchers offered this example: The proportion of cases with H1N1 ranged from 19 to 29% in those younger than 65 vs. only 7% in those 65 and older.

At the same time, H1N1 (the non-predominant A virus) contributed more than 25% of influenza A cases across all age groups except 65 and older, compared to all prior seasons where the non-predominant A virus contributed fewer than 10% of A cases across all age groups except 65 and older.

“Several unique characteristics may have contributed to the high hospitalization rates observed during 2017-18,” the CDC authors concluded. “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.“