Influenza and Influenza-like Illness (ILI) Outbreaks in School Settings in Maryland During the 2009 H1N1 Influenza Pandemic

Background: Influenza/ILI outbreaks are reportable in Maryland; in response to the 2009 H1N1 pandemic, a uniform surveillance definition for school outbreaks was introduced. Methods: Maryland developed revised definitions for influenza/ILI school outbreaks: five ILI visits to school health in one day AND a doubling of school baseline absenteeism rate within a 48 hour period OR or an ILI cluster (25% of a specific group). Conclusion of an outbreak was defined as <3 school health ILI visits each day for 3 consecutive school days AND decreased absenteeism. Variables collected for analysis included: grade level and type of school, date of outbreak onset and conclusion, and number of 2009 H1N1 lab confirmation. Results: Beginning in MMWR week 37, 199 influenza/ILI school outbreaks were reported to state public health, 95(48%) in elementary, 30(15%) in middle, and 16(8%) high schools. Of these 21.1% were lab confirmed as 2009 H1N1. The median duration of elementary school outbreaks was 14 days (range 3-80), middle school 14 days (range 6-29), and high school 10 days (range 5-48). Although outbreak reports for all school types peaked in MMWR week 42, earliest reports were from elementary schools, which preceded middle schools by two weeks, which preceded high schools by two weeks. Conclusion: These findings suggest that younger ages (elementary/middle vs. high school age) may be more impacted during influenza pandemics. Elementary and middle schools reported both a greater proportion and longer duration of outbreaks. Focusing flu surveillance and mitigation in elementary and middle schools may be useful strategies during future influenza pandemics.