Effective detection of the 2009 H1N1 influenza pandemic in U.S. Veterans Affairs medical centers using a national electronic biosurveillance system

The 2008-09 influenza season was the time in which the Department of Veterans Affairs (VA) utilized an electronic biosurveillance system for tracking and monitoring of influenza trends. The system, known as ESSENCE or Electronic Surveillance System for the Early Notification of Community-based Epidemics, was monitored for the influenza season as well as for a rise in influenza cases at the start of the H1N1 2009 influenza pandemic. We also describe trends noted in influenza-like illness (ILI) outpatient encounter data in VA medical centers during the 2008-09 influenza season, before and after the recognition of pandemic H1N1 2009 influenza virus. We determined prevalence of ILI coded visits using VA's ESSENCE for 2008-09 seasonal influenza (Sept. 28, 2008-April 25, 2009 corresponding to CDC 2008-2009 flu season weeks 40-16) and the early period of pandemic H1N1 2009 (April 26, 2009-July 31, 2009 corresponding to CDC 2008-2009 flu season weeks 17-30). Differences in diagnostic ICD-9-CM code frequencies were analyzed using Chi-square and odds ratios. There were 649,574 ILI encounters captured representing 633,893 patients. The prevalence of VA ILI visits mirrored the CDC's Outpatient ILI Surveillance Network (ILINet) data with peaks in late December, early February, and late April/early May, mirroring the ILINet data; however, the peaks seen in the VA were smaller. Of 31 ILI codes, 6 decreased and 11 increased significantly during the early period of pandemic H1N1 2009. The ILI codes that significantly increased were more likely to be symptom codes. Although influenza with respiratory manifestation (487.1) was the most common code used among 150 confirmed pandemic H1N1 2009 cases, overall it significantly decreased since the start of the pandemic. VA ESSENCE effectively detected and tracked changing ILI trends during pandemic H1N1 2009 and represents an important temporal alerting system for monitoring health events in VA facilities.