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. 2022 Feb;27(6):2001953.
doi: 10.2807/1560-7917.ES.2022.27.6.2001953.

Early chains of transmission of COVID-19 in France, January to March 2020

Juliette Paireau  1   2 , Alexandra Mailles  2 , Catherine Eisenhauer  1 , Franck de Laval  3   4 , François Delon  3   4 , Paolo Bosetti  1 , Henrik Salje  1   5 , Valérie Pontiès  6 , Simon Cauchemez  1
Affiliations

Early chains of transmission of COVID-19 in France, January to March 2020

Juliette Paireau et al. Euro Surveill. 2022 Feb.

Abstract

IntroductionSARS-CoV-2, the virus that causes COVID-19, has spread rapidly worldwide. In January 2020, a surveillance system was implemented in France for early detection of cases and their contacts to help limit secondary transmissions.AimTo use contact-tracing data collected during the initial phase of the COVID-19 pandemic to better characterise SARS-CoV-2 transmission.MethodsWe analysed data collected during contact tracing and retrospective epidemiological investigations in France from 24 January to 30 March 2020. We assessed the secondary clinical attack rate and characterised the risk of a contact becoming a case. We described chains of transmission and estimated key parameters of spread.ResultsDuring the study period, 6,082 contacts of 735 confirmed cases were traced. The overall secondary clinical attack rate was 4.1% (95% confidence interval (CI): 3.6-4.6), increasing with age of index case and contact. Compared with co-workers/friends, family contacts were at higher risk of becoming cases (adjusted odds ratio (AOR): 2.1, 95% CI: 1.4-3.0) and nosocomial contacts were at lower risk (AOR: 0.3, 95% CI: 0.1-0.7). Of 328 infector/infectee pairs, 49% were family members. The distribution of secondary cases was highly over-dispersed: 80% of secondary cases were caused by 10% of cases. The mean serial interval was 5.1 days (interquartile range (IQR): 2-8 days) in contact tracing pairs, where late transmission events may be censored, and 6.8 (3-8) days in pairs investigated retrospectively.ConclusionThis study increases knowledge of SARS-CoV-2 transmission, including the importance of superspreading events during the onset of the pandemic.

Keywords: SARS-CoV-2; cluster; contact tracing; secondary clinical attack rate; superspreading; transmission.

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Conflict of interest statement

Conflict of interest: None declared.

Figures

Figure 1
Figure 1
Distribution of COVID-19 cases (n = 735) and contacts (n = 6,028) by age group, France, 24 January 2020–30 March 2020
Figure 2
Figure 2
Description of contact tracing data for COVID-19 cases over time and regions, France, 24 January 2020–30 March 2020 (n = 735 index cases; 6,028 contacts; 248 secondary cases)
Figure 3
Figure 3
Secondary clinical attack rate and factors associated with the risk of a contact of a COVID-19 case to become a case. France, 24 January 2020–30 March 2020 (n = 735 index cases; 6,028 contacts; 248 secondary cases)
Figure 4
Figure 4
Chains of transmission for COVID-19 cases, distribution of sex and distribution of age. France, 24 January 2020–30 March 2020 (n = 418 cases)
Figure 5
Figure 5
Summary statistics of the transmission chains: chain sizes, generations, secondary cases arising from a COVID-19 case and serial intervals. France, 24 January 2020–30 March 2020 (n = 418 cases)
See this image and copyright information in PMC

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