Temporal Associations Between Nonpharmaceutical Interventions and COVID-19 Transmission in Delaware

Understanding how nonpharmaceutical interventions temporally align with changes in COVID-19 transmission remains important for public health monitoring and evaluation. However, substate-level evidence remains limited. Delaware provides a useful setting for such analyses due to its small population and largely uniform statewide policy implementation during 2020.

We analyzed COVID-19 transmission dynamics in Delaware and its 3 counties (Kent, New Castle, and Sussex) between March 2020 and March 2023, with policy-focused analyses restricted to major statewide interventions implemented in 2020. Incident infections were reconstructed using a deconvolution-based approach, and the time-varying reproduction number (Rt) was estimated using the EpiEstim R package.

Rt estimates varied across policy periods and revealed substantial spatial heterogeneity. The most consistent increases in Rt were associated with the transition to hybrid school reopening, while decreases in Rt were associated with limits on social gatherings, the public mask mandate, and the stay-at-home advisory and overnight curfew. During Phase 1 reopening, Rt changes were small statewide, while Sussex County exhibited a larger increase than other counties.

COVID-19 transmission dynamics in Delaware during 2020 varied across policy periods and counties, illustrating substate heterogeneity under uniform statewide interventions and the use of Rt-based estimates for monitoring epidemic dynamics.