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

ImportanceHeterogeneity 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. ObjectiveTo 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, ParticipantsAnalysis of COVID-19 data collected and archived by the Massachusetts Department of Public Health. ExposuresThe 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 measuresWeekly observations, by town, on confirmed COVID-19 cases in Massachusetts, during the period (April 15th, 2020 to February 9th 2021). ResultsThe relative decline in COVID-19 cases, during January 12th, 2021 to February 9th, 2021, in the group of towns with higher total accumulated cases in the period before January 12th, 2021 is significantly larger than the corresponding relative decline in the group of towns with lower accumulated cases during the same period. Conclusions and RelevanceHeterogeneous nature of transmission is playing a significant role in shaping the rapid recent decline (January 12th to February 9th, 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 PointsO_ST_ABSQuestionC_ST_ABSDoes 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. FindingsBased on analysis of data on cases in Massachusetts, the consistent and widespread decline of COVID-19 spread during winter of 2020/2021 (January 12th, 2021 to February 9th, 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. MeaningWe 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.