Evaluation of Passive and Active Surveillance of Hospitalized Influenza Patients in Michigan during the Circulation of the 2009 Novel Influenza Strain, October–November, 2009

Background:  Novel Influenza A H1N1 2009 was first identified in the United States in April 2009 and rapidly spread across the nation. The Michigan Department of Community Health (MDCH) partnered with the Centers for Disease Control and Prevention (CDC) to conduct active surveillance for hospitalized cases of influenza. This unique influenza season provided an opportunity to evaluate passive versus active influenza surveillance.

Objective: The objectives of this study were to estimate the level of Michigan influenza hospitalization under-reporting by passive surveillance, evaluate the demographics of influenza cases reported by passive relative to active surveillance, and to offer recommendations for improvement of passive reporting during future influenza seasons.

Methods: Three counties were enrolled in active surveillance. Residents were followed throughout the influenza season beginning 9/15/09. Residents admitted to a hospital who had a positive test result for any antigenic strain of influenza within two weeks prior to, or three days post-, admission were considered cases. Infection preventionists (IPs) reported cases to MDCH where epidemiologists reviewed medical records for risk factor data using a CDC-designed data collection tool. Data were entered into a database, managed, and submitted weekly to the CDC. Michigan residents of the remaining 80 counties hospitalized for influenza were reported to MDCH via the electronic Michigan Disease Surveillance System (MDSS). Data on confirmed and probable cases from both systems was exported for 10/6/2009 through 11/3/2009.  Age-specific rates were calculated using US census data and used to estimate standardized statewide hospitalizations.  

Results: Active surveillance produced 109 laboratory-confirmed, hospitalized influenza cases for three counties while passive surveillance totaled 422 cases for the remainder of the state. The projected number of hospitalized influenza cases was 2595, equating passive surveillance sensitivity to 16.3%. The overall relative risk of being reported hospitalized was 5.7 (95% CI 5.1, 6.4) times more likely among active surveillance than passive surveillance county residents. Age-specific reporting significantly differed for pediatric and adult cases, with relative risks of reported hospitalizations were 3.2 (2.7 to 3.7) and 9.6 (8.0 to 11.4) times greater for adults and pediatric cases respectively in active surveillance counties than non. 

Conclusions: Hospitalized influenza cases are more likely to be reported in counties using active rather than passive surveillance. Future recommendations include clear reporting communications for IPs and local health departments and increased access and training for health care providers on MDSS to increase the level of reporting using passive surveillance systems.