This field investigation on patients infected with SARS-CoV-2 made it possible to observe the true reality of a SARS-CoV-2 epidemic and provided a good understanding of how to manage clusters or outbreaks. Such results also enable theoretical modeling to be validated or adjusted. Despite a lack of resources at that time (masks, testing), taking adequate and well-adapted measures stopped viral transmission in a few days. However, the results of this investigation should be confronted with those of other observational studies.
All cases in MSFAC were linked to each other within a single chain of transmission. The epidemiological investigation and viral genome sequencings were concordant and compatible with an introduction originating from a previous local circulation in Oise, from the Crépy-en-Valois cluster, which is described elsewhere by Fontanet et al. [12].
Airborne, droplet, and hand-to-hand transmissions were suspected, as 23 cases worked at the same place or together, and an environmental sample was positive for SARS-CoV-2 (despite the late collection date). Viral density in a confined area might have increased the reproductive rate, and viral inoculum at the time of infection could play a crucial role in the expression of the disease and the occurrence of severe cases (2/27, 7% in this cluster), [18].
Aside from ageusia and anosmia, which were not described at the start of the epidemic, clinical signs of SARS-CoV-2 infection are not specific, and COVID-19 mimicked co-circulating flu or colds. The first case identified (Case 7), who was not the index case but the starting point of this alert, was tested 14 days after the onset of symptoms, and 8 days after he deteriorated clinically, as he did not meet the epidemiological criteria of a possible case at that date. A lesson learned from this episode is that epidemiological case definitions are useful to standardize counting of cases everywhere, but they must not replace clinical diagnosis, especially during the emergence of new pathogens, because the infection and symptoms they cause are not well known and have not yet been described. When Case 7 was finally diagnosed, other primary, secondary, tertiary, and quaternary generations of cases had already been infected. The entire chain of transmission had to be identified backwards and forwards as early as possible to prevent an uncontrolled amplification of the outbreak [19, 20].
Deployment of molecular diagnostic tests was crucial. Due to a shortage of swabs and viral transport medium, our testing capability was limited (as was the case throughout France at that time). We defined the cluster area based on active case finding and contact tracing results and tested all symptomatic individuals within that perimeter. This testing strategy, based on medical evaluation of at-risk exposure, symptoms, and RT-PCR quantitative results, made it possible to limit false positives (and inappropriate lockdowns or closings) [21] and false negatives (we collected a second RT-PCR sample if any doubt existed). The effective use of targeted testing according to pre-test probability seems to be a key point in the successful management of outbreaks.
Regarding the possibility of missing asymptomatic cases, serology results confirmed the low percentage of patients who remained asymptomatic (13%), and contact tracing results did not identify any transmission from asymptomatic to symptomatic cases in this cluster. In a meta-analysis, Madewell et al. compared the secondary attack rate around symptomatic (18%) and asymptomatic COVID-19 cases (0.7%, p < 0.001) [22]. Nowadays, questions are being raised about the real role of asymptomatic infected people in the spread of the disease [23,24,25,26,27]. Among SARS-CoV-2 positive individuals, there is a confusion between those who are asymptomatic (= no symptoms at all), which is the case for 20% of COVID-19 patients, CI 95%, 17–25 according to the review of Buitrago-Garcia et al. [28], and those who are pre-symptomatic (= no symptoms at the time of sample collection for RT-PCR) [3, 29]. Indeed, Case 2 probably contaminated the five following cases during her pre-symptomatic stage, and 10 pre-symptomatic contaminations (about 40%) occurred in this cluster, highlighting the need for contact tracing and identifying, testing, quarantining and/or mask wearing for asymptomatic at-risk contact persons [30]. This is a major difference with SARS-CoV-1, where infectiousness started after symptom onset and was proportional to the clinical expression, making it easier to control [31].
Once the cluster area was defined, we placed the MSFAC staff under lockdown to slow down transmission within military settings during the investigation and contact tracing period. Ideally, increasing the use of digital tools and staff for timely contact tracing and early identification of the entire chain of transmission could reduce the duration of a lockdown [20, 32]. Six symptomatic cases (22% of the cases) did not go to the military health facility to be diagnosed and isolated, but no secondary cases occurred around them, probably due to the lockdown. Symptomatic cases that are not isolated are responsible for spreading the outbreak and underline the importance of raising awareness about COVID-19 symptoms within the population [33]. They also justify systematic mask wearing by at-risk populations [34].
Regarding the air base outside the cluster, since no significant viral circulation was identified, a lockdown was not required, but staff turnover, physical distancing, and hand hygiene measures were strongly implemented, without halting all activities.
All cases with a positive RT-PCR result and who underwent serological testing had a positive anti-SARS-CoV-2 serology with neutralizing antibodies, which confirmed a specific immunity against SARS-CoV-2, even if its duration and protective effect against further re-infection with variants are still unknown [10]. A longer follow-up of these patients would be necessary to conclusively establish that immunity.