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How to Interpret Infectious Disease Baseline Activity

Here is an example of a baseline activity plot for Respiratory Syncytial Virus (RSV), a leading cause of hospitalization and admission to the intensive care unit for babies and toddlers during the winter. The location is Washoe County. The vertical axis on the left is the number of RSV cases per week. The horizontal axis across the bottom is the date.

Observed Case Counts
68% Confidence
95% Confidence

The lighter and darker areas of the graph are the limits of baseline activity, which comes from years of historical data. On the left hand side of the graph, you can see a broad area of color. This part of the graph illustrates where most of the disease activity occurs, which is roughly from December to April. The darker shaded area is where we expect to see about 2/3 (68%) of our cases reported. The lighter area is where we expect to see the vast majority (95%) of our cases reported. In the above example, we used data from 2000 to 2014 to generate the baseline activity and compared that to what was actually observed for 2015 (the red line). The fact that the number of cases we actually saw fell within the limits of baseline activity indicates we observed routine activity.

In the next example below, we see a more active season of RSV activity (2016). We note that in January and February there were more cases than we would normally expect, but still within expected range.

Sometimes we see current disease activity that falls outside of typical baseline activity. This usually means the disease activity is unusual compared to prior years. The graph below shows an unusually early season of influenza type A in Washoe County.

If you have any further questions about how to interpret infectious disease baseline activity, please use our contact form found at the bottom of the main website page.