The investigative work carried out by Santé publique France epidemiologists in Phase 2 involves identifying and tracking contacts. When a case is confirmed, a two-step investigation is launched. The first phase consists of what is known as a prospective investigation, or “contact tracing,” which involves identifying all individuals who have been in direct contact with the patient since the onset of the illness.

Once these individuals are identified, the level of risk of potential transmission must be assessed based on various exposure criteria: proximity of contact, duration, frequency, etc. These individuals are then monitored for 14 days following their last contact with the patient to watch for any early symptoms.

If the risk level is high, active monitoring will be implemented, meaning home quarantine, daily calls, and self-monitoring (taking temperature twice a day), along with the requirement to report to the emergency medical services (15) upon the onset of initial symptoms.

The second phase of the investigation is retrospective. This is an epidemiological investigation aimed at tracing the source of the patient’s infection. The initial questions focus on identifying the most obvious causes: has this person returned from a high-risk area (such as Northern Italy or China)? Have they been in contact with anyone who has visited one of these areas?

If no obvious exposure is identified, we will interview the patient to review their entire schedule for the previous 15 days and attempt to identify all potentially sick individuals with whom they may have come into contact during that period. Particular attention will be paid to anyone who is tired, weak, feverish, etc. It may be necessary to contact the patient’s family members (spouse, children, brothers, sisters, etc.) if the patient is unable to answer. The goal is to gather as much information as possible.

If these inquiries still yield no results, efforts will be made to determine whether the patient has attended events where people from diverse backgrounds gather.

Information regarding the patient (specific health status, test results, etc.) remains confidential. It is important to remember that everyone interviewed benefits from speaking with us, for the sake of their own health.

All these investigations are conducted under the coordination of Santé Publique France and the Regional Health Agencies (ARS). They involve epidemiologists and public health medical inspectors. In France, we have the capacity to mobilize several hundred of them to carry out this type of investigation.

In the case of an investigation into a patient with a “low-key” lifestyle—meaning few outings and a small social circle—one or two people are assigned to the case. Conversely, an investigation into a patient who is very socially active may require numerous interviews, thus mobilizing half a dozen professionals, and sometimes more, to complete the investigation.

At the European level, the European Centre for Disease Prevention and Control (ECDC) is responsible for pooling information from the various national agencies. At the global level, the World Health Organization (WHO) is responsible for this work.

Generally speaking, investigations are conducted by contacting people by phone. In more exceptional cases, teams from Santé publique France or the Regional Health Agencies may visit the site in person. In the case of the Les Contamines “cluster,” for example, officials from both organizations went to a school to conduct a case-by-case risk assessment. We have to adapt, because every situation is different.

If, during the investigation, our agents discover that they are dealing with contacts who are sick at the time of the investigation, an ambulance team takes over to transport them to the hospital, where they will be tested and, as a precaution, admitted while awaiting results to prevent any risk of potential future transmission.

In the case of face-to-face interviews, our agents are required to follow certain protective measures: if the patient has symptoms, they will be asked to wear a mask, before the agent also protects themselves with a different type of mask.

All these procedures have been implemented based on our expertise in epidemiological surveillance, which is one of the primary missions of Santé publique France. This knowledge has been further enriched by the 2009 H1N1 influenza outbreak, as well as the 2012 MERS-CoV outbreak—which was less transmissible but presented certain challenges identical to those of COVID-19—and the 2014–2016 Ebola outbreak in West Africa, and the return of people from high-risk areas.

The simplest preventive measures are often the most effective:
Wash your hands, avoid crowds, and wear a mask as soon as the first symptoms appear. This is the best way to protect your loved ones.