Investigating the relationship between interventions, contact patterns, and SARS-CoV-2 transmissibility
Filippo Trentini,
Adriana Manna,
Nicoletta Balbo,
Valentina Marziano,
Giorgio Guzzetta,
Samantha O’Dell,
Allisandra G. Kummer,
Maria Litvinova,
Stefano Merler,
Marco Ajelli,
Piero Poletti,
Alessia Melegaro
Affiliations
Filippo Trentini
Dondena Centre for Research on Social Dynamics and Public Policy, Bocconi University, Milan, Italy; Covid Crisis Lab, Bocconi University, Italy; Center for Health Emergencies, Bruno Kessler Foundation, Trento, Italy; Correspondence to: Dondena Centre for Research on Social Dynamics and Public Policy, Bocconi University, Via Roentgen 1, 20141 Milan, Italy.
Adriana Manna
Dondena Centre for Research on Social Dynamics and Public Policy, Bocconi University, Milan, Italy; Department of Network and Data Science, Central European University, Wien, Austria
Nicoletta Balbo
Dondena Centre for Research on Social Dynamics and Public Policy, Bocconi University, Milan, Italy; Department of Social and Political Sciences, Bocconi University, Milan, Italy
Valentina Marziano
Center for Health Emergencies, Bruno Kessler Foundation, Trento, Italy
Giorgio Guzzetta
Center for Health Emergencies, Bruno Kessler Foundation, Trento, Italy
Samantha O’Dell
Laboratory for Computational Epidemiology and Public Health, Department of Epidemiology and Biostatistics, Indiana University School of Public Health, Bloomington, IN, USA
Allisandra G. Kummer
Laboratory for Computational Epidemiology and Public Health, Department of Epidemiology and Biostatistics, Indiana University School of Public Health, Bloomington, IN, USA
Maria Litvinova
Laboratory for Computational Epidemiology and Public Health, Department of Epidemiology and Biostatistics, Indiana University School of Public Health, Bloomington, IN, USA
Stefano Merler
Center for Health Emergencies, Bruno Kessler Foundation, Trento, Italy
Marco Ajelli
Laboratory for Computational Epidemiology and Public Health, Department of Epidemiology and Biostatistics, Indiana University School of Public Health, Bloomington, IN, USA
Piero Poletti
Center for Health Emergencies, Bruno Kessler Foundation, Trento, Italy
Alessia Melegaro
Dondena Centre for Research on Social Dynamics and Public Policy, Bocconi University, Milan, Italy; Covid Crisis Lab, Bocconi University, Italy; Department of Social and Political Sciences, Bocconi University, Milan, Italy; Corresponding author at: Department of Social and Political Sciences, Bocconi University, Milan, Italy.
Background: After a rapid upsurge of COVID-19 cases in Italy during the fall of 2020, the government introduced a three-tiered restriction system aimed at increasing physical distancing. The Ministry of Health, after periodic epidemiological risk assessments, assigned a tier to each of the 21 Italian regions and autonomous provinces. It is still unclear to what extent these different sets of measures altered the number of daily interactions and the social mixing patterns. Methods and findings: We conducted a survey between July 2020 and March 2021 to monitor changes in social contact patterns among individuals in the metropolitan city of Milan, Italy, which was hardly hit by the second wave of the COVID-19 pandemic. The number of daily contacts during periods characterized by different levels of restrictions was analyzed through negative binomial regression models and age-specific contact matrices were estimated under the different tiers of restrictions. By relying on the empirically estimated mixing patterns, we quantified relative changes in SARS-CoV-2 transmission potential associated with the different tiers.As tighter restrictions were implemented during the fall of 2020, a progressive reduction in the mean number of daily contacts recorded by study participants was observed: from 15.9 % under mild restrictions (yellow tier), to 41.8 % under strong restrictions (red tier). Higher restrictions levels were also found to increase the relative contribution of contacts occurring within the household. The SARS-CoV-2 reproduction number was estimated to decrease by 17.1 % (95 %CI: 1.5–30.1), 25.1 % (95 %CI: 13.0–36.0) and 44.7 % (95 %CI: 33.9–53.0) under the yellow, orange, and red tiers, respectively. Conclusions: Our results give an important quantification of the expected contribution of different restriction levels in shaping social contacts and decreasing the transmission potential of SARS-CoV-2. These estimates can find an operational use in anticipating the effect that the implementation of these tiered restriction can have on SARS-CoV-2 reproduction number under an evolving epidemiological situation.
