Widespread testing, case isolation and contact tracing may allow safe school reopening with continued moderate physical distancing: A modeling analysis of King County, WA data

Abstract

Background: In late March 2020, a "Stay Home, Stay Healthy" order was issued in Washington State in response to the COVID-19 pandemic. On May 1, a 4-phase reopening plan began. We investigated whether adjunctive prevention strategies would allow less restrictive physical distancing to avoid second epidemic waves and secure safe school reopening.

Methods: We developed a mathematical model, stratifying the population by age, infection status and treatment status to project SARS-CoV-2 transmission during and after the reopening period. The model was parameterized with demographic and contact data from King County, WA and calibrated to confirmed cases, deaths and epidemic peak timing. Adjunctive prevention interventions were simulated assuming different levels of pre-COVID physical interactions (pC_PI) restored.

Results: The best model fit estimated ~35% pC_PI under the lockdown which prevented ~17,000 deaths by May 15. Gradually restoring 75% pC_PI for all age groups between May 15-July 15 would have resulted in ~350 daily deaths by early September 2020. Maintaining <45% pC_PI was required with current testing practices to ensure low levels of daily infections and deaths. Increased testing, isolation of symptomatic infections, and contact tracing permitted 60% pC_PI without significant increases in daily deaths before November and allowed opening of schools with <15 daily deaths. Inpatient antiviral treatment was predicted to reduce deaths significantly without lowering cases or hospitalizations.

Conclusions: We predict that widespread testing, contact tracing and case isolation would allow relaxation of physical distancing, as well as opening of schools, without a surge in local cases and deaths.

Keywords: Age structured model; Contact tracing; Epidemiology; Mathematical modeling; Physical distancing.

Fig. 1

Simplified diagram of the modeling analysis.

Fig. 2

Model calibration and validation. Model fitting to 5 sources of King County data assuming gradual scale up of social distancing between March 8 and March 29: A)-B) Cumulative and daily cases and deaths. Red dots represent data up to April 30, thick lines represent the best model fit while other acceptable trajectories are shown in grey. Green bands show 80% range from acceptable trajectories. C)-D) Age distributions of cases and deaths as of April 15. Bars represent data, green dots and ranges represent the best fit and other acceptable trajectories included in the analysis. Reopening plan is implemented between May 15 and July 15 by restoring 60% of pC_PI in all age groups.

Fig. 3

Model best fit projections under different intervention scenarios. A) Time variation of the physical interactions for specific levels of pC_PI restored B-D) Daily deaths for specific levels of pC_PI restored; E) Cumulative deaths from the beginning of the outbreak to Sep 1. Reopening plan is implemented from May 15 to July 15 by restoring different levels of pC_PI as shown in the upper left corner of each panel.

Fig. 4

Model projections under different combinations of reopening scenarios and adjunctive interventions in terms of: A) Cumulative deaths by Nov 1. B) Maximum daily deaths by Nov 1; C) Time from the start of reopening to reach 15 deaths daily. Reopening plan is implemented between May 15 and July 15 by restoring 60% of pC_PI in all age groups (Baseline) or age groups 1–3 only (Protect seniors). Schools reopen on Sept.1.

Fig. 5

Model projections under different levels of diagnostic rates among: A) asymptomatic, pre-symptomatic and symptomatic cases due to mass testing programs; B)–C) symptomatic cases due to test & isolate programs. Reopening plan is implemented between May 15 and July 15 by restoring different levels of pC_PI in all age groups on the x-axis. Heatmap represent the proportion of parameter sets (UI) (from 0 to 1) for which daily deaths remain at or below 15 through November 1.

Fig. 6

Model projections with different asymptomatic assumptions: A)- B) cumulative deaths by May 15 and Sept.15; C)-D) cumulative incidence at May 15 and Sept 15. Reopening plan is implemented between May 15 and July 15 by restoring 60% of pC_PI in all age groups on the x-axis. Boxplots show median and interquartile range (IQR) with whiskers extending to the smallest/largest value no further than 1.5 ∗ IQR based on 100 accepted parameter sets per scenario.