Two Distinct Surveillance Methods to Track Hospitalized Influenza Patients in New York State During the 2009–2010 Influenza Season

Abstract

To better understand the severity of 2009 H1N1 influenza disease, enhanced surveillance of patients hospitalized with influenza was conducted during the 2009-2010 influenza season in New York State through existing Emerging Infections Program surveillance and a newly established sentinel hospital surveillance program. The 2 surveillance systems were compared to determine consistency across surveillance modalities and reveal the strengths and weaknesses of each to accomplish comprehensive influenza surveillance. Similar variables from the aggregate data collected from each system were compared and differences were analyzed in detail. New York State. Hospitalized adult and pediatric patients detected through 2 influenza surveillance programs. Significant differences in age distribution, timing of illness onset, illness complications, underlying medical conditions, critical care admissions, use of mechanical ventilation, and illness outcomes. Both surveillance systems saw the highest numbers of confirmed influenza infection among patients hospitalized in early fall 2009, with sharp declines thereafter. Sentinel hospital surveillance continued to detect hospitalizations for influenza-like illness that were not due to 2009 H1N1 influenza well into March 2010. Compared to influenza surveillance conducted through the Emerging Infections Program, the sentinel hospital influenza surveillance program tended to detect a sicker population of children and adults, including a higher rate of critical illness and mechanical ventilation, and among adults, higher rates of some underlying medical conditions. There were no differences in disease outcomes detected between the 2 systems. Although the 2 surveillance systems were complementary, inherent methodologic variations revealed important differences at season conclusion. The lessons learned should be used to determine the best way to allocate resources to meet the needs of future state and national influenza surveillance efforts.