The objective of the study is to identify socio-demographic factors, behaviors and practices associated with infection with SARS-CoV-2 to help determine where and how patients mostly get infected with SARS-CoV-2.

It is a case-control study including :

• cases identified by the nationwide system of positive SARS-CoV-2 tests (COVID-19 diagnosis information system, SIDEP) (currently limited to qRT-PCR) and invited to participate by the National Health Insurance (CNAM) which hosts the data from the nationwide test system;
• controls included via Ipsos, a polling institute with access to personal data from a panel from which they will include controls matched on age (divided into 10-year categories), gender and geographic area (departement);
• cases will be offered to invite a person they live with to participate in the study offering another case-control analysis inside a household. These participants will be required to report the result of the test as recommended by contact tracing guidelines to determine whether they are cases or controls.

Data will be collected via a self-administered online questionnaire. Some of the participants will be called for a complementary telephone questionnaire to measure reliability of online retrieved data and explore more specific aspects of SARS-CoV-2 transmission.

The objective of the study is to identify socio-demographic factors, behaviors and practices associated with infection with SARS-CoV-2 to help determine where and how patients mostly get infected with SARS-CoV-2.

The study will focus on sociodemographic data (age, gender, location of residence, socio-economic level, etc.), behaviors and practices (adherence to hand-washing, mask-wearing, work habits, meetings, etc.), the places they visit (public transportation, bars, restaurants, sport facilities, etc.).

It is a case-control study including :

• cases identified by the nationwide system of positive SARS-CoV-2 tests (SIDEP) (currently limited to RT-PCR) and invited to participate by CNAM, which hosts the data from the nationwide test system, through a nationwide email campaign sent once a week; cases will be asked if they can tell where they most probably were infected, leading to a detailed description of the circumstances of the contamination, including whether the suspected contamination took place within the household or not.
• controls included via Ipsos, a polling institute with access to personal data from a panel from which they will include controls matched on age (divided into 10-year categories up to 68 years old included as expected adherence to an online questionnaire quickly drops beyond that age), gender and geographic area (departement); invitations will be sent every week via email to adapt the profile of recruited controls to the age, gender and geographic profiles of the cases that participated in the previous week;
• cases will be offered to invite a person they live with to participate in the study offering another case-control analysis inside a household. Cases will be recommended not to invite the person that they suspect was responsible for their contamination, if a intra-household contamination is suspected. They will indicate the email address of that person to allow sending of an invitation via email. These participants will be required to report the result of the test as recommended by contact tracing guidelines to determine whether they are cases or controls.

Analysis will also include descriptive analysis:

• Estimation of the proportion of cases infected within the household or outside
• Among the cases with a extra-household contamination suspicion, proportion of cases for whom infection circumstances are known and description of these circumstances
• Proportion of cases for whom no specific circumstance of contamination could be evidenced.

Questionnaires will focus on the 10 days preceding :

• symptom onset (symptomatic cases)
• day of testing (asymptomatic cases)
• day of symptom onset of the first case within a household (people from the same household as cases) or testing of the first case if they are asymptomatic
• questionnaire completion (Ipsos-recruited controls)

Participants will be offered at the end of the online self-administered questionnaire to indicate a phone number. Among those agreeing to do so, a random sample of case-control pairs and cases of particular interest (eg with specific contamination circumstances) will be called within two weeks following the online questionnaire for a more detailed telephone questionnaire to measure reliability of online retrieved data and explore more specific aspects of SARS-CoV-2 transmission.

Inclusion Criteria:

All participants:

• Aged 18 years or more
• Agreeing to participate in the study

Index cases:

• Cases identified by the National Health Insurance (CNAM) in the nationwide database SIDEP (COVID-19 diagnosis information system)
• Positive SARS-CoV-2 RT-PCR on nasopharyngeal or throat swab (or another sample which state of the art considers that it indicates an active infection if positive)

"Distant" controls:

• Selected by Ipsos among Ipsos' panel on age, gender and location of residence criteria (to allow for matched case-control analysis)
• Health Care Workers

Close subjects:

• People who live in the same household as a case and who have been invited to participate by that case
• People who have had a RT-PCR test on nasopharyngeal or throat swab (or another sample which state of the art considers it indicates an active infection if positive) following diagnosis of the index case (following contact tracing recommendations), classified as:
• close cases if test is positive
• close controls if test is negative

Exclusion Criteria:

For all participants:

• People under judicial protection measures ("sauvegarde de justice", "curatelle", "curatelle renforcée", "tutelle", ie various forms of curatorship under French law)

For related cases, related controls and "distant" controls:

• People with a history of SARS-CoV-2 infection confirmed by RT-PCR or positive testing of anti-SARS-CoV-2 antibodies with serology (for related cases: before the current episode)

Index cases: People with positive SARS-CoV-2 test identified by the National Health Insurance (CNAM) in the nationwide database SIDEP (COVID-19 diagnosis information system), which is a system that collects data from tests performed in all testing facilities in France. Only people who have given their e-mail address to the CNAM will be contacted.

Close subjects: Invited by index cases with whom they live. "Distant" controls: Selected by Ipsos' panel on age, gender and location of residence criteria (to allow for matched case-control analysis) and Health Care Workers.