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

Here, we quantify background levels of non-influenza ILI over the past 10 years and identify a recent surge of non-influenza ILI starting the first week of March, 2020. This surge of excess ILI correlates with known patterns of SARS-CoV-2 spread across states within the US, suggesting the surge is unlikely to be due to other endemic respiratory pathogens, yet is orders of magnitude larger than the number of confirmed COVID cases reported. Together this suggests that the true prevalence of SARS-CoV-2 within the US is much larger than currently appreciated and that the syndromic case detection rate is approximately 1%, corresponding to at least 28 million new ILI cases due to SARS-CoV-2. Our analysis provides empirical corroboration of previous hypotheses 55 of substantial undocumented cases yet places the estimated undocumented case rate an order of magnitude higher than prior reports [6] . The SARS-CoV-2 prevalence estimates obtained from the ILI surge are consistent an epidemic doubling time of less than 3.5 days. A 3.5 day doubling time is substantially faster than many prior reports [7, 8] yet is consistent with the 3-day doubling time of observed deaths due to COVID within the US. Our findings support a conceptual model 60 for COVID spread in the US in which more rapid spread than previously reported is coupled with a larger undiagnosed population to give rise to currently observed trends. Finally, we find that the ILI surge peaks the week starting March 15, and we discuss the potential explanations for this phenomenon.