Complications and Outcomes of Pandemic 2009 Influenza A (H1N1) Virus Infection in Hospitalized Adults: How Do They Differ From Those in Seasonal Influenza?

Methods. A prospective, observational study was conducted in adults hospitalized with polymerase chain reaction-confirmed pH1N1 infection in 2 acute-care general hospitals from June 2009 to May 2010 (n = 382). Complications and outcomes were described and compared with those in a seasonal influenza cohort (2007-2008, same hospitals; n = 754). Results. Hospitalized patients with pH1N1 influenza were younger than those with seasonal influenza (mean age +/- standard deviation, 47 +/- 20 vs 70 +/- 19 years) and fewer had comorbid conditions (48% vs 64%). The rate of positive immunofluorescence assay results was low (54% vs 84%), and antiviral use was frequent (96% vs 52%). Most patients in both cohorts developed complicated illnesses (67.8% vs 77.1%), but patients with pH1N1 influenza had higher rates of extrapulmonary complications (23% vs 16%; P = .004) and intensive care unit admission and/or death (patient age < 35 years, 2.3% vs 0%; 35-65 years, 12.4% vs 3.2%; > 65 years, 13.5% vs 8.5%; adjusted odds ratio [OR] 2.13; 95% confidence interval [CI], 1.25-3.62; P = .005). Patients who received antiviral treatment within 96 h after onset had better survival (log-rank test, P < .001). However, without timely treatment, the mortality risk was higher with pH1N1 infection (9.0% vs 5.8% for seasonal influenza; adjusted OR, 6.85; 95% CI, 1.64-28.65; P = .008]. Bacterial superinfection worsened outcomes. Conclusions. Adults hospitalized for pH1N1 influenza had significant complications and mortality despite being younger than patients with seasonal influenza. Antiviral treatment within 96 h may improve survival.