Background: Questions have surfaced about the epidemiology of the 2009 H1N1 pandemic influenza virus, differences between spring and fall waves and differences between the pandemic and previous influenza seasons.
Objective: To characterize hospitalizations during waves of the pandemic and to compare the pandemic with recent seasonal epidemics.
Methods: Instructions regarding testing for influenza virus infection were disseminated. Demographic and basic clinical data were collected on patients with laboratory confirmed influenza A. Similar, although more limited, data were collected as part of influenza surveillance during previous seasons. Group comparisons were calculated using a Z-test and hospitalization rates were compared using Poisson regression with a linear spline term.
Results: Pandemic activity at JHH occurred in 2 distinct waves: wave 1 comprised 177 cases peaking in mid-June; wave 2 comprised 399 cases (as of Nov 7) peaking in early Oct. The previous 6 seasons peaked between mid-Dec and mid-March and comprised 117 to 532 cases. Pandemic cases ranged in age from 0 to 78 yrs. During wave 1, 67% of cases were children (<18 yrs) compared to 55% in wave 2 (p=.01). A larger percent of cases were hospitalized in wave 2 compared to wave 1 (46% and 28% respectively, p<.01) but the ICU admission rate was similar between waves (7% and 10%, p=.24). The proportion of admitted pediatric and adult cases was similar during wave 1 (24% and 37%, p=.06); however, the proportion of adults admitted was greater during wave 2 (41% and 53%, p=.02). ICU admission rates were similar among pediatric and adult cases during both waves. Overall, the hospitalization rate was lower during wave 1 than the previous 6 seasons (p<.01), but the rate of hospitalizations during wave 2 was similar to those seasons (p=.18). These trends held true for pediatric cases and adult cases analyzed separately (wave 1 p<.01 and p=.02, wave 2 p=.49 and p=.18). Four (0.7%) pandemic cases died (ages 21 – 51 years); one was otherwise healthy and 3 had underlying medical conditions.
Conclusions: Pandemic virus circulation occurred at an unusual time of year and involved more cases than a typical seasonal epidemic. Illness severity, as indicated by hospitalization rate, was less than (wave 1) or similar to (wave 2) seasonal influenza epidemics. During wave 1, frequent testing in outpatient settings was encouraged to define the epidemiology of H1N1 in the community. This testing pattern likely biased the finding of reduced hospitalization rate during wave 1. The similar percent of cases admitted to the ICU during both waves suggests that virus virulence has not changed. The similar percent of pediatric and adults cases in an ICU could represent similarity in disease severity between groups but it is possible that any true difference is masked by differing thresholds for ICU admission.