Geographical Variation in SARS-CoV-2 Transmission Potential in Massachusetts

This ecological study aimed to investigate changes in the time-varying reproduction number (Rt) of SARS-CoV-2 across six regions of Massachusetts from 2020 to 2022 and to evaluate the impact of various nonpharmaceutical interventions (NPIs) implemented in 2020 by examining associated changes in the Rt.

COVID-19 incident case data from the Johns Hopkins University database were adjusted for reporting delays using deconvolution and for underreporting via a Poisson-distributed multiplier of 4. Negative and zero counts were corrected using imputation. Rt was estimated using R package EpiEstim (Version 2.2-4) with a 7-day sliding window from 2020 to 2022 and with non-overlapping time windows between policy changes in 2020.

From 2020 to 2022, Massachusetts experienced five COVID-19 surges, linked to the wild-type strain and emerging variants, with Rt exceeding 1 during each wave and stabilizing at or dropping below 1 during low-incidence phases. School closure and gathering restrictions, the first major intervention, were associated with a 14.7% statewide reduction in Rt (95% credible interval (CrI): -23.6%, -5.6%), with greater reductions in high-density areas such as Boston (-16.9%; 95% CrI: -26.9%, -7.5%). No statistically significant changes in Rt were found to be associated with other NPIs in 2020, including the mask mandate, reopening phases, travel restrictions and quarantine requirements, and curfews.

Our findings highlight the different NPIs' varying impacts on COVID-19 transmission dynamics across regions in Massachusetts in 2020 and underscore the importance of early interventions for future pandemic preparedness.