Using ILI surveillance to estimate state-specific case detection rates and forecast SARS-CoV-2 spread in the United States

Justin D Silverman; Alex D Washburne

We identified excess ILI cases by first subtracting cases due to influenza and then subtracting the seasonal signal of non-influenza ILI (Figure 1 ). Many states, including Washington, New York, Oregon, Pennsylvania, Maryland, Colorado, New Jersey, and Louisiana, have had a recent surge in number of non-influenza ILI cases far in excess of seasonal norms. For example, in the second week 70 of March, 2020, Oregon saw 50% higher non-influenza ILI than it had ever seen since the inception of the ILINet surveillance system within the US. We find that with 95% probability approximately 4% of all outpatient visits in Oregon during this time were for ILI that could not be explained by either influenza or the normal seasonal variation of respiratory pathogens. We find that as the seasonal surge of endemic non-influenza respiratory pathogens declines, this excess ILI correlates 75 more strongly with state-level patterns of newly confirmed COVID cases suggesting that this surge is a reflection of ILI due to SARS-CoV-2 (Pearson ρ > 0.35 and p < 0.01 for the last three weeks; Figure S1A ).