Table 4. Results and estimated community asymptomatic and presymptomatic prevalence by week.
| IGI FAST data | Estimated community prevalence | |||||
|---|---|---|---|---|---|---|
| Collection Weeka | Positive | Negative | Inconclusive | Insufficient | Total | Asymptomatic/presymptomatic prevalenceb (Percent, 95% CIc) |
| 6/22/2020–6/28/2020 | 0 | 367 | 1 | 126 | 494 | 0.1% (0.04%, 0.35%) |
| 6/29/2020–7/5/2020 | 0 | 254 | 0 | 115 | 369 | 0.14% (0.06%, 0.45%) |
| 7/6/2020–7/12/2020 | 0 | 417 | 2 | 135 | 554 | 0.13% (0.05%, 0.4%) |
| 7/13/2020–7/19/2020 | 0 | 480 | 0 | 25 | 505 | 0.09% (0.04%, 0.3%) |
| 7/20/2020–7/26/2020 | 0 | 531 | 1 | 42 | 574 | 0.07% (0.03%, 0.22%) |
| 7/27/2020–8/2/2020 | 1 | 530 | 0 | 21 | 552 | 0.07% (0.03%, 0.2%) |
| 8/3/2020–8/9/2020 | 1 | 599 | 2 | 10 | 612 | 0.07% (0.03%, 0.22%) |
| 8/10/2020–8/16/2020 | 0 | 565 | 2 | 42 | 609 | 0.08% (0.03%, 0.24%) |
| 8/17/2020–8/23/2020 | 0 | 588 | 4 | 11 | 603 | 0.08% (0.03%, 0.26%) |
| 8/24/2020–8/30/2020 | 1 | 629 | 3 | 69 | 701 | 0.08% (0.04%, 0.26%) |
| 9/21/2020–9/27/2020 | 2 | 1650 | 1 | 36 | 1688 | 0.03% (0.01%, 0.1%) |
| 9/28/2020–10/4/2020 | 0 | 462 | 1 | 17 | 480 | 0.02% (0.01%, 0.08%) |
| 10/5/2020–10/11/2020 | 0 | 1497 | 1 | 65 | 1563 | 0.02% (0.01%, 0.08%) |
| 10/12/2020–10/18/2020 | 0 | 1068 | 2 | 28 | 1098 | 0.03% (0.01%, 0.13%) |
| 10/19/2020–10/25/2020 | 0 | 1077 | 0 | 4 | 1081 | 0.06% (0.02%, 0.21%) |
| 10/26/2020–11/1/2020 | 0 | 470 | 1 | 15 | 486 | 0.1% (0.04%, 0.35%) |
| Total | 5 | 11184 | 21 | 761 | 11971 | 0.08% (0.03%, 0.28%)d |
aWeeks 11–13 are excluded here due to the supply chain shortage that shut down testing.
bWeekly asymptomatic and presymptomatic prevalence was computed by summing the estimated daily new infections per 100,000 output from the ’covidestim’ package in R across the seven days of each week, dividing by a 700,000 person scaling factor to produce the weekly incidence rate, then multiplying by a 14-day duration of infectiousness to derive prevalence (see Methods). Finally, estimates were scaled by 40% to yield the asymptomatic and presymptomatic prevalence per week for the duration of the study period.
cChitwood et al. fixed the lower bound of their 95% confidence intervals at the reported case positive rate (lagged by delay time to presentation of symptoms). As a result, confidence intervals are not always evenly distributed (upper bounds exceed lower bounds).
dThe asymptomatic and presymptomatic prevalence for the entire study duration (6/23/2020–8/30/2020 and 9/21/2020–10/29/2020) was computed similarly as the weekly asymptomatic and presymptomatic prevalence, but summed the estimated daily new infections across the entire study duration, then followed the same steps.