Social Distancing and SARS-CoV-2 Transmission Potential Early in the Epidemic in Finland

Abstract

Background: Information about social mixing patterns under heavy social distancing is needed to model the impact of nonpharmaceutical interventions on SARS-CoV-2 transmission.

Methods: We conducted a survey on daily person-to-person contacts during the early phase of the SARS-CoV-2 epidemic in Finland, one month after strong social distancing measures had been introduced nationwide. We defined a contact as exchange of at least a few words in proximity of another person. We also considered physical ("skin-to-skin") contacts separately. Based on 3,171 reported contacts by 1,320 participants of 1-79 years of age, we estimated age-stratified contact matrices essential in modeling virus transmission.

Results: Compared with contacts during prepandemic conditions, as learned from the Finnish part of the Polymod study, there was a 72% (95% credible interval, CI = 71, 74) reduction in the daily number of all contacts and a 69% (95% CI = 66, 73) reduction in the daily number of physical contacts in April 2020. The largest reduction, of almost 90%, occurred in physical contacts by individuals more than 70 years of age. The estimated reduction in the transmission potential of the virus attributable solely to reduced contact frequencies varied between 59% (whole population; physical contacts; 95% CI = 52, 68) and 77% (over 20-year olds; physical contacts; 95% CI = 70, 89).

Conclusions: We surmise that the large reduction in the daily numbers of social contacts in the early part of the SARS-CoV-2 epidemic in Finland was likely a major contributor to the steady decline of the epidemic in the country since early April.

FIGURE 1.

Distribution of the number of reported daily social contacts as reported by the 1,175 participants (18–79 years of age) and 145 children (0–17 years of age) in the households, Finland, April 2020. In adults, the average numbers of reported contacts were 2.4 (all contacts, A) and 0.75 (physical contacts, B). In children, the average numbers of reported contacts were 3.5 (all contacts, A) and 1.9 (physical contacts, B).

FIGURE 2.

Mean numbers of daily contacts between the eight 10-year age classes in April 2020 and under normal conditions, Finland. A, All contacts (April 2020); B, physical contacts (April 2020); C (all contacts, Polymod data); D (physical contacts, Polymod data). The respondent’s age class (years) is on the top row. The rows correspond to contactees age classes (years). The numbers are posterior expectations of the mean numbers of contacts . See eTables 1; http://links.lww.com/EDE/B806 and 3; http://links.lww.com/EDE/B806 for the 95% posterior intervals of the mean numbers as given in the April 2020 matrices.

FIGURE 3.

Age-class-specific relative reductions in the mean numbers of daily contacts when comparing April 2020 (strong social distancing) with the normal conditions as based on the Polymod survey data. The posterior means and 95% credible intervals are shown. Black = all contacts; red = physical contacts.

FIGURE 4.

Relative reductions in the mean numbers of daily contacts (during vs. before social distancing) between the eight 10-year age classes. A, all types of contacts; B, physical contacts. The respondent’s age class (years) is on the top row. The rows correspond to the contactees’ age classes (years). The numbers are posterior expectations.