Participating laboratories in the U.S. report the total number of norovirus tests performed that week and the number of those testing positive to the CDC weekly. Because reporting delays may be expected for some laboratories, the data shown for the most recent weeks may be less complete than others. Each point in the trend graphs below shows the average percentage of tests that were positive from three adjacent weeks: the specified week and the weeks before and after. This is also known as the 3-week centered moving average.
In the United States, norovirus outbreaks occur most often during the late fall, winter, and early spring. There may be variation in the timing of cases between regions and between communities within the same region.
