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BACKGROUND: Gastrointestinal (GI) illness chief complaints from emergency department and urgent care center visits were used to develop an early warning tool for norovirus (NV) outbreak activity. The tool will provide an indicator of increasing NV outbreak activity in the community and allow for earlier public health action.
METHODS: GI illness visits were classified by ESSENCE based on chief complaint and compared to NV outbreaks across four years. An analysis period was defined as week 21 through week 20 of the subsequent year. Percent of GI illness visits and number of NV outbreaks were aggregated by week. Time-series, correlation, and logistic regression analyses were performed. Low NV outbreak activity weeks were those with 4 or fewer outbreaks, and high NV outbreak activity weeks were those with 5 or more outbreaks. Based on low NV outbreak activity weeks, baseline and threshold values for the weekly percent of GI illness visits were calculated for each analysis period. Baseline calculation was the average weekly percent of GI illness visits from week 21 to week 31 and threshold value was baseline plus two standard deviations. Weekly percent of GI illness visits was compared to the threshold value to serve as an indicator of increasing NV outbreak activity.
RESULTS: The study period was from May 18, 2008 to May 19, 2012. Weekly percent of GI illness visits were highly correlated with outbreaks of NV in the community (r =0.809, p <.0001). Median and mean number of NV outbreaks per week were 2 and 4, respectively (range 0-23). Median and mean percent of GI illness visits per week were 10.2% and 10.5%, respectively (range 8.9%-12.8%). Weeks with high NV outbreak activity were more likely to occur when the weekly percent of GI illness visits had surpassed the threshold value (OR =110.7, 95% CI: 31.9-384.8). On average, weekly percent of GI illness visits surpassed the threshold value 1.25 weeks prior to the start of high NV outbreak activity weeks (range 0-3).
CONCLUSIONS: The analysis identified that GI illness visits crossed a calculated threshold value on average 1.25 weeks before the initiation of high NV outbreak activity. The early warning tool developed through this analysis was implemented in Virginia during 2012, 2013 and 2014 in order to detect the transition of NV outbreak activity from low to high. The early warning tool successfully identified increasing NV outbreak activity and allowed public health practitioners to communicate timely risk mitigation messages to community partners.