Estimating unobserved SARS-CoV-2 infections in the United States

Alex Perkins; Sean M. Cavany; Sean M Moore; Rachel J Oidtman; Anita Lerch; Marya Poterek

Estimates of the proportion of imported symptomatic infections that were detected, ρ travel , and the infectiousness of asymptomatic infections relative to symptomatic infections, , varied based on the values of the other parameters. In general, higher values for parameters expected to increase transmission (e.g., R) were associated with higher estimates of ρ travel (Table S1 ). Compared to a baseline median estimate of ρ travel = 0.39 (95% PPI: 0.15 -0.90) with R = 1.97, the estimate of ρ travel was 0.83 (95% PPI: 0.47 -0.99) with R = 2.7 and 0.08 (95% PPI: 0.04 -0.19) with R = 1.5. For a shorter serial interval with a mean of 4.7 days, the estimate was ρ travel = 0.52 (95% PPI: 0.19 -0.96), and with a longer mean serial interval of 7.5 days, the estimate was 0.06 (95% PPI: 0.03 -0.14). The estimated value of ρ travel was also lower if the CFR was low (ρ travel = 0.20, 95% PPI: 0.08 -0.53), compared to the scenario with a higher CFR (ρ travel = 0.54, 95% PPI: 0.21 -0.96). Higher ρ travel estimates correspond to fewer undetected imported infections; therefore, fewer undetected importations are required to account for the observed number of local deaths through March 12 if the CFR is high, R is high, or the serial interval is short. In addition, when we based the timing of importations on international incidence (excluding China after travel restrictions were implemented on February 3) the estimate of ρ travel was 1.00 (95% PPI: 0.98 -1.00) due to the increased probability of early importations -and more time for local infections to increase -under this scenario. There was greater uncertainty in our estimates under most sensitivity scenarios, and in most scenarios the estimates of ρ travel and were positively correlated (Fig. S5) .