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Early chains of transmission of COVID-19 in France

Our take —

The objective of this study, available as a preprint and thus not yet peer-reviewed, was to describe the secondary clinical attack rate and its variation across age-group, geographic region, and other factors in France during the beginning of the pandemic (January-March 2020). They estimated an attack rate of 4.1% for secondary cases, and a tertiary attack rate of 2.3%. They also estimated the rate was higher at the beginning compared to the end of the study period, and that older individuals aged 75+ were more likely to transmit COVID-19 than younger individuals. However, they used individual-based surveillance that did not identify asymptomatic cases and only provided confirmatory testing to cases with clinical diagnosis from symptoms, and also had difficulty contacting individuals hospitalized. Therefore, secondary and tertiary transmission were likely underestimated as the symptomatic extremes (e.g., mild disease and severe disease) were missing in their analysis.

Study design

Prospective Cohort

Study population and setting

The study sought to describe the secondary clinical attack rate and transmission among PCR-confirmed COVID-19 cases identified early in the pandemic in France. From January 24, 2020 to March 30, 2020, the study included any clinical COVID-19 cases identified in the hospital or in the community. Presumed cases were isolated after diagnosis. Cases were administered a questionnaire including sociodemographic characteristics, clinical presentation, and history of exposure. Contacts of confirmed cases were identified through standard contact tracing methods, and only contacts with COVID-19 symptoms were given a PCR test for SARS-CoV-2 infection. Their contacts were then also traced. Possible exposure was grouped into 3 categories (negligible, low, moderate/high) and only contacts with low to moderate/high exposure were followed up. Due to limits in staffing that reduced the timeliness of entry into the web database, the database of contacts was not exhaustive and represented only a sample of all contact tracing. Contact patterns were described using an age-specific contact matrix using French population-based data from another study (COMES-F in 2012). The clinical secondary attack rate and risk factors for secondary transmission were also estimated using logistic regression.

Summary of Main Findings

During the study period, 735 confirmed cases were successfully traced resulting in identification of 6,028 individual contacts. Cases had an average of 8.3 contacts traced (median: 4, range: 0 – 146). Of the 6,028 contacts, 248 were secondary cases, with an estimated clinical attack rate of 4.1% (95% CI: 3.6 – 4.6). The tertiary clinical attack rate (that is, the secondary attack rate among secondary cases) was 2.3% (95% CI: 1.4 – 3.6). This decreased over time, with an estimated attack rate of 6.8% in the first week of the pandemic, and 1.8% in weeks 12-13. There was also geographic and age-based variation, with individuals 75+ years of age having the highest estimated attack rate (6.2%, 95% CI; 4.3 – 8.7). The attack rate was also highest among family member contacts (7.9%, 95% CI: 6.6 – 9.3), and coworkers/friend (3.4%, 95% CI: 2.5 – 4.4). Estimating chains of transmission, 329 connections were identified between cases, with the largest cluster identified in Oise, France, with 39 cases across 5 familial generations identified, beginning with a pair of cases identified 25 February 2020.

Study Strengths

The study made use of extensive contact tracing efforts across France in order to identify chains of transmission across the 735 indexed cases and their 6028 contacts. Based on their comprehensive questionnaire, they were able to examine differences between contacts and their modeling was able to identify a large family cluster in northern France.


The study was limited by the human resources available to them—the authors note that they were not able to input and follow-up with every contact in their web database due to the limits of their staff. This highlights the importance of public health departments being well-resourced during critical periods. Additionally, they noted that the workload differed by region, and areas with higher case burdens were less likely to be able to successful trace all contacts compared to less affected regions. Additionally, their testing protocol only included symptomatic cases, which means this study was not reflective of asymptomatic cases or a population-based surveillance strategy that would be more representative of the general population. Additionally, they reported difficulty in identifying contacts from hospitalized individuals, and thus the findings may not have reflected more severe cases of disease.

Value added

The study shows the importance of extensive contact tracing efforts and describes a high attack rate during the early pandemic.

This review was posted on: 7 April 2021