Role of Heterogeneous Transmission in the Decline of COVID-19 Cases During Winter of 2020/2021 in Massachusetts

Importance Heterogeneity in transmission of COVID-19 is a significant multiscale phenomenon. However, the role of this heterogeneity in shaping the overall dynamics of disease transmission is not well understood. Objective To investigate the role of heterogeneous transmission among different towns in Massachusetts in shaping the dynamics of COVID-19 transmission, especially the recent decline during winter of 2020/2021. Design, Setting, Participants Analysis of COVID-19 data collected and archived by the Massachusetts Department of Public Health. Exposures The entire population of the state of Massachusetts is exposed to the virus responsible for COVID-19, to varying degrees. This study quantifies this variation. Main outcome measures Weekly observations, by town, on confirmed COVID-19 cases in Massachusetts, during the period (April 15 th , 2020 to February 9 th 2021). Results The relative decline in COVID-19 cases, during January 12 th , 2021 to February 9 th , 2021, in the group of towns with higher total accumulated cases in the period before January 12 th , 2021 is significantly larger than the corresponding relative decline in the group of towns with lower accumulated cases during the same period. Conclusions and Relevance Heterogeneous nature of transmission is playing a significant role in shaping the rapid recent decline (January 12 th to February 9 th , 2021) in reported cases in Massachusetts, and probably around the country. These findings are relevant to how we estimate the threshold defining “herd” immunity, suggesting that we should account for effects due to heterogeneity. Key Points Question Does heterogeneity in disease transmission play a role in shaping the overall dynamics of COVID-19 in Massachusetts, including the recent decline in cases during the 2020/2021 winter. Findings Based on analysis of data on cases in Massachusetts, the consistent and widespread decline of COVID-19 spread during winter of 2020/2021 (January 12 th , 2021 to February 9 th , 2021) appears to be shaped to a significant degree by the heterogeneous nature of transmission at the scale of different towns. Towns with a history of high (low) transmission rates during 2020 are experiencing a faster (slower) relative decline. Meaning We suggest that heterogeneity in transmission of COVID-19 may impact the dynamics of disease transmission including the emergence of “herd” immunity, in line with some recent theoretical studies. This finding deserves some attention from other research groups investigating “herd” immunity, and from federal and state public health authorities concerned with the future evolution of the pandemic.