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. 2022 Sep:40:100601.
doi: 10.1016/j.epidem.2022.100601. Epub 2022 Jun 17.

Investigating the relationship between interventions, contact patterns, and SARS-CoV-2 transmissibility

Filippo Trentini  1 , Adriana Manna  2 , Nicoletta Balbo  3 , Valentina Marziano  4 , Giorgio Guzzetta  4 , Samantha O'Dell  5 , Allisandra G Kummer  5 , Maria Litvinova  5 , Stefano Merler  4 , Marco Ajelli  5 , Piero Poletti  4 , Alessia Melegaro  6
Affiliations

Investigating the relationship between interventions, contact patterns, and SARS-CoV-2 transmissibility

Filippo Trentini et al. Epidemics. 2022 Sep.

Abstract

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.

Keywords: COVID-19; Human behavior; Mixing patterns; Non-pharmaceutical interventions; Reproduction number; SARS-CoV-2; Social contacts; Tiers; Transmissibility.

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Conflict of interest statement

Competing interests M.A. has received research funding from Seqirus. The funding is not related to COVID-19 and to this study. The contents of this publication are the sole responsibility of the authors and don't necessarily reflect the views of the funders. All other authors declare no competing interest.

Figures

Fig. 1
Fig. 1
A. Bars represents the mean number of daily contacts reported by the study participants in different tiers, stratified by type of contact: overall, rare (less than once a week) and within household. Gray lines represent 95 % confidence intervals. B. Different lines represent the age-specific mean number of daily contacts in the different tiers.
Fig. 2
Fig. 2
Percentage reduction of the daily number of daily contacts reported by study participants with respect to different covariates of interest, obtained as 1 minus the exponentiated coefficients estimated by a negative binomial regression applied to the overall number of daily contacts (overall; red dots), the number of daily contacts occurred less than once a week (occasional contacts; green dots) and with household members (in-household; blue dots). Lines represent 95 % confidence intervals. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3
Fig. 3
A–D) Average contact matrix representing the mean number of daily contacts reported by a participant in the age group i with individuals in the age group j under the white (A), the yellow (B), the orange (C) and the red tiers (D). E) Mean percentage reduction of SARS-CoV-2 reproduction number ascribable to the observed change of contact patterns (gray dots) and as estimated in (Manica et al., 2021) by analyzing the time series of the SARS-CoV-2 net reproduction number in Italy between October 30 and November 25, 2020 (black dots), under the yellow, the orange and the red tiers with respect to the white tier. Vertical lines represent 95 % confidence intervals. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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