Modeling layered non-pharmaceutical interventions against SARS-CoV-2 in the United States with Corvid

Dennis L Chao; Assaf P Oron; Devabhaktuni Srikrishna; Michael Famulare

When many NPIs are implemented simultaneously, R effective can be brought below 1.0 resulting in declining prevalence. One such example was shown in the top panels of Figure 3 , in which liberal leave, work-from-home, school closure, shelter-in-place, and home isolation of 50% of ascertained cases started on day 30 and schools re-opened on day 180. Because elimination did not occur, the epidemic rebounded when schools re-opened. To demonstrate stochastic effects in the model, we ran the same scenario with different random number seeds ( Figure 5 ). Using different random number seeds produced different numbers of cases when all interventions were in place from days 30 to 180, and these differences were amplified after schools re-opened. To understand what persistence looked like during those 150 days before schools re-opened, we analyzed transmission in one stochastic simulation run. When the interventions started on day 30, 261 people were infectious, 55 of whom were symptomatic on that day. The number of people who were infectious continued to rise for about 3 weeks (the infectious period in the model), peaking at 598 before declining. When schools re-opened on day 180, there were 315 infectious people who could re-start the epidemic when the schools opened. When school, workplace, community-targeted NPIs and home isolation were in effect, R effective was 0.96. Before schools and many workplaces closed, there were chains of transmission within schools and workplaces ( Figure 6 , blue and green nodes). When all NPIs were in effect, a larger proportion of transmission occurred within families ( Figure 6 , red nodes) and between families by contacts in the community ( Figure 6 , brown nodes). This scenario did not include interventions that reduced transmission within households, so they may sustain the outbreak by amplifying cases that infect new households.