The objective of this study is to identify the sociodemographic factors, places frequented, and behaviors associated with an increased risk of SARS-CoV-2 infection. The study consists of two parts:

• the first component describes the circumstances of transmission among index cases diagnosed as positive for SARS-CoV-2 during the curfew period, particularly when the source of infection is known;

• the second part compares the characteristics, behaviors, and practices of index cases with those of a series of controls matched for age, sex, region, and population density, during the curfew period and the lockdown period.

Summary of Results

Study on the circumstances of infection among index cases during the curfew

• 44% of infected individuals know the source of their infection, 21% suspect a specific event without knowing the source of the infection, and 35% do not know how they became infected.

• The vast majority (97%) of index cases who responded to this questionnaire self-isolated, but only 54% did so upon the first symptoms, and 64% upon learning of contact with an infected case, when symptoms or knowledge of contact with an infected case were the only warning signs.

• In cases of household transmission (35% of transmissions when the source person is known), these adults were primarily infected by their spouse (64% of cases). The fact that children are asymptomatic or have mild symptoms when infected may explain why they are not often identified as the source of the infection.

• For infections outside the household (65% of infections when the source person is known), the primary transmission routes are within the family circle (33%), followed by the workplace (29%), and then among friends (21%). Meals play a central role in these transmissions, whether in family, social, or, to a lesser extent, professional settings. Shared offices are also a significant factor in the workplace.

Study on factors associated with SARS-CoV-2 infection during curfew and lockdown

Increased risk of SARS-CoV-2 infection:

• Occupations (compared to civil service executives who have an average risk):

  • Corporate administrative and sales managers

  • Health and social work professionals

  • Industrial workers

  • Drivers

• Number of people living in the household

• Having children:

  • Cared for by a child care provider

  • In daycare

  • In preschool

  • In middle school

  • In high school

• Carpooling

• Having recently traveled abroad

• Having attended an in-person meeting:

  • Work-related

  • Private (with friends or family)

• Visited:

  • Bars

  • Restaurants

  • Gyms

Reduced risk of SARS-CoV-2 infection

• Occupations (compared to civil service executives who have a moderate risk)

  • School teachers

  • Scientists and university professors

  • Intermediate administrative professions in the civil service

  • Civil servants and public service staff

  • Corporate administrative staff

  • Students

  • Farmers

  • Stay-at-home men/women

• Remote work (compared to someone with an in-person office job)

• Taking the bus or tram

• Exercising outdoors

• Visiting stores (grocery, clothing, etc.)

Study methodology

The individuals who responded to the study were invited to participate in the survey by the French National Health Insurance (Assurance Maladie). Since May 13, 2020, the French National Health Insurance has been piloting a “contact tracing” system and, as part of this effort, has been contacting all infected individuals to help identify the people with whom they may have had risky contact.

Since October 27, the French National Health Insurance has been sending individual emails to people diagnosed with SARS-CoV-2, inviting them to complete the Institut Pasteur’s online questionnaire.

Between October 17 and 29, 30,000 people have already participated. This initiative is carried out with the utmost respect for the protection of data collected to combat the COVID-19 pandemic.

The French National Health Insurance Agency has made its contact resources for reaching these individuals available to the Pasteur Institute because this survey, by deepening our understanding of the contexts of transmission, can help further improve the tools used to combat the epidemic. IPSOS identifies and contacts “controls” matched to index cases based on age, sex, region of residence, population density, and time period (curfew starting October 17, 2020, and lockdown starting October 29, 2020).

Index cases and controls are invited to complete a self-administered questionnaire regarding their sociodemographic characteristics, the places they frequent, and their behaviors. Index cases are asked to describe the circumstances of their infection when known. Two types of analysis were conducted and are presented below.

1. Analysis of the circumstances of infection based on the index cases during the curfew period.

Of the 370,000 emails sent inviting participation in the study, 30,330 (8.2%) questionnaires were returned by individuals who were very likely infected between October 17 and 30, 2020 (curfew period): 25,644 concern non-healthcare worker index cases, and 4,686 concern healthcare workers treated separately because they may have been infected differently.

62% of respondents were women, and 72% were aged 29–58 (only adults were eligible for this study). 55% come from metropolitan areas with more than 100,000 inhabitants, with a significant proportion of residents from the Auvergne-Rhône-Alpes and Île-de-France regions (22% and 21%, respectively).

44% of infected individuals know the source person who infected them, and most are well aware of their risky behavior (failure to wear a mask or maintain physical distancing, lack of isolation measures for the source person within the household, etc.); 21% suspect a specific event without knowing the source of the infection; and 35% do not know how they became infected.

The vast majority (97%) of index cases who responded to this questionnaire self-isolated, but only 54% did so upon the first symptoms, and 64% upon learning of contact with an infected person, when these were the only warning signs.

In cases of household transmission (35% of infections where the source is known), the primary source of infection for these adults was their spouse (64% of cases). The fact that children are asymptomatic or have mild symptoms when infected may explain why they are not often identified as the source of the infection. It was found that only 51% of household source cases self-isolated, and when they did, only 52% did so as soon as symptoms began.

For infections outside the household (65% of infections when the source is known), the primary transmission routes are within the family circle (33.1%), followed by the workplace (28.8%), and finally among friends (20.8%). Meals play a central role in these transmissions, whether in family, social, or, to a lesser extent, workplace settings. Shared offices are also significant in the workplace.

2. Case-control study covering the curfew and lockdown periods.

For this study, responses to the self-administered questionnaire from 3,426 cases and 1,713 controls matched for age, sex, region, population density, and period (curfew or lockdown) were analyzed.

Compared to public service executives who have an average risk, administrative and sales executives, industrial workers, drivers, and intermediate health and social work professionals had a higher risk of SARS-CoV-2 infection during the curfew or partial lockdown. Living in a larger household, particularly with children in daycare or school, attending an in-person professional meeting, carpooling, frequenting bars, restaurants, and gyms, and attending a private social or family gathering were also associated with an increased risk.

Compared to civil service executives, school teachers, university scientists or professors, civilian employees and civil service staff, corporate administrative employees, students, farmers, homemakers, and those in intermediate administrative roles in the public sector had a lower risk of SARS-CoV-2 infection during curfew or partial lockdown. Working remotely (compared to someone with an in-person office job), taking the bus or tram, exercising outdoors, and visiting stores (grocery, clothing, etc.) were all associated with a reduced risk of SARS-CoV-2 infection during the curfew or partial lockdown.

Of all the circumstances analyzed, private gatherings accounted for the largest share of infections (19%) during the study period.

These results should be interpreted with great caution: they pertain only to the curfew and lockdown periods and may be subject to significant bias due to the selection of the study population—which represents only a small fraction of all infections—and the possibility that some responses were influenced by the respondent’s knowledge of their own sick or healthy status.

Nevertheless, these results are consistent with data from the literature for those findings already reported in other studies, and align with what we know about SARS-CoV-2 transmission. The locations and circumstances of transmission are likely to evolve over the course of the epidemic, and this study could serve as a tool for monitoring changes in transmission patterns over time. It would be particularly interesting to accompany the reopening of certain public or private venues with this type of surveillance to determine whether such reopening is associated with an increased risk of SARS-CoV-2 transmission.

According to Arnaud Fontanet, Director of the Emerging Diseases Epidemiology Unit at the Pasteur Institute and professor at the Conservatoire national des arts et métiers: “This study highlights the high risk of SARS-CoV-2 infection during meals and private gatherings. It will be very important to minimize this risk during the gatherings that will accompany the end-of-year holidays.”

According to Daniel Lévy-Bruhl, head of the Respiratory Diseases and Vaccinations Unit at Santé publique France, this type of study and these initial results are important for better understanding the transmission mechanisms of this disease. Sharing this knowledge also helps refine recommendations for preventing transmission and enables everyone to adopt the appropriate preventive measures depending on the situation.