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

Excess ILI appears to have peaked during the week starting on March 15th, leading the observed ILI dynamics to diverge from the overall epidemic dynamics implied by the growth rate of COVID deaths in the US. If the ILI dynamics were proportional to the epidemic curve then the two 155 could be related with a constant subclinical rate. However, the changing ratio between the ILI surge and the epidemic curves parameterized by the growth rate of US deaths suggests additional mechanisms may be behind the ILI slowdown. First, a slowdown in ILI outpatient arrivals could be due to decrease in care-seeking where patients with mild ILI are less likely to present to the hospital as evident in emergency departments across New York City. Adjusting our ILI prevalence 160 estimates based on the effect observed in New York City aligns ILI estimates more closely with predicted dynamics, yet the discrepancy remains. The remaining deviation could reflect more extreme changes in patient behaviors than those seen in New York City or successful interventions leading to lower transmission rates.