In this cluster, the index case infected 12 other people. All but one were symptomatic. Transmission was limited solely to the chalet environment, with a very high attack rate. While the reproduction number (R0) of COVID-19 ranges between 2 and 3 (each case generating an average of 2 to 3 other cases), the number of secondary cases in this cluster was 4 to 6 times higher. Hence the term “super-spreading” for this cluster. This phenomenon had already been described during other emerging coronavirus epidemics, such as the Middle East Respiratory Syndrome (MERS-CoV) outbreak in 2015 and the Severe Acute Respiratory Syndrome (SARS-CoV) outbreak in 2003. The two viruses, SARS-CoV and SARS-CoV-2—the latter being responsible for the current pandemic—share the same cellular receptor. The hypothesis is that “super-spreaders” emit larger quantities of the virus over a longer period than other individuals carrying the virus. This complex combination of factors—related to the host (viral load, symptoms), the virus, individual behavior, and the environment (proximity and airflow dynamics)—likely plays a role in the number of infections caused by a single super-spreader. Early identification of such events is crucial for limiting transmission.

One interesting finding concerns a child who was co-infected with SARS-CoV-2 and other respiratory viruses and who had attended three schools while symptomatic. Seventy-three children who had been in contact with the child and were experiencing symptoms were tested, but none tested positive for SARS-CoV-2, although infections with other respiratory viruses, including influenza, were confirmed. Among all other individuals who had been in contact with the infected child and were monitored for two weeks, none developed symptoms. The child infected with SARS-CoV-2 had continued his normal activities and social life, as his symptoms were mild.

It is now known that, in cases of SARS-CoV-2 infection, children are more often asymptomatic than adults, and those who are symptomatic present milder clinical forms of the disease. The situation described here suggests that children, being less likely to become infected and developing less severe forms of the disease, may play a lesser role than adults in the transmission of this new virus. However, since the study is an outbreak report, it is not intended to demonstrate that the rate of SARS-CoV-2 transmission by children is lower than that of adults. The initial widespread testing was conducted by analogy with influenza, where children are frequent sources of transmission, particularly to adults. We did not observe the same dynamics during this pandemic.

This observation, arising from a specific situation, sparked keen interest in the scientific community and the international press, particularly during discussions about reopening schools, as the role of children in transmission was then poorly documented. Today, there is a growing body of evidence in the international literature supporting the moderate role of children in the epidemic dynamics compared to adults.

Furthermore, this investigation revealed the occurrence of a tertiary case detected in a symptomatic patient whose deep respiratory specimen tested positive, while all nasopharyngeal swabs taken from this individual at the same time were negative. The discrepancy between upper and lower respiratory tract results underscores the importance of deep swabs for confirming the diagnosis, particularly in cases of viral pneumonia.

The chain of transmission was interrupted thanks to a coordinated response. This investigation required collaboration with health authorities in five countries (France, the United Kingdom, Spain, Switzerland, and Australia) and support from the national focal points of the EWRS and the International Health Regulations (IHR), underscoring the importance of international reporting systems. The second generation of SARS-CoV-2 cases in this cluster was detected within 24 hours of the initial notification. A large-scale contact tracing and follow-up operation was launched the very next day, requiring the rapid and coordinated mobilization of about 100 professionals over the weekend and the collaboration of multidisciplinary teams at the local and national levels. A testing campaign was conducted in Les Contamines-Montjoie on Sunday with healthcare workers and allowed for direct communication with the affected populations. Experience gained during previous outbreaks of emerging diseases, such as SARS, MERS-CoV, A(H1N1) pdm09, and Ebola, facilitated rapid and coordinated action.