Late surges in COVID-19 cases and varying transmission potential partially due to public health policy changes in 5 Western states, March 10, 2020-January 10, 2021

Objective This study investigates how the SARS-CoV-2 transmission potential varied in North Dakota, South Dakota, Montana, Wyoming, and Idaho from March 2020 through January 2021. Methods Time-varying reproduction numbers, R t , of a 7-day-sliding-window and of non-overlapping-windows between policy changes were estimated utilizing the instantaneous reproduction number method. Linear regression was performed to evaluate if per-capita cumulative case-count varied across counties with different population size. Results The median 7-day-sliding-window R t estimates across the studied region varied between 1 and 1.25 during September through November 2020. Between November 13 and 18, R t was reduced by 14.71% (95% credible interval, CrI, [14.41%, 14.99%]) in North Dakota following a mask mandate; Idaho saw a 1.93% (95% CrI [1.87%, 1.99%]) reduction and Montana saw a 9.63% (95% CrI [9.26%, 9.98%]) reduction following the tightening of restrictions. High-population counties had higher per-capita cumulative case-count in North Dakota at four time points (June 30, August 31, October 31, and December 31, 2020). In Idaho, North Dakota, and South Dakota, there was a positive correlation between population size and per-capita weekly incident case-count, adjusted for calendar time and social vulnerability index variables. Conclusions R t decreased after mask mandate during the region’s case-count spike suggested reduction in SARS-CoV-2 transmission.