In France, enhanced surveillance for mpox (formerly known as monkeypox) was implemented on May 20, 2022, coinciding with the diagnosis of the first case in the country. It was essential to learn everything possible about this emerging disease as quickly as possible to guide management and prevention. This enhanced surveillance, based on a case definition and standard operating procedures, involved supplementing the mandatory reporting of orthopoxviruses (including mpox) with an epidemiological investigation of all cases using a standardized questionnaire.

The first cases detected in Europe in early May 2022 occurred predominantly among MSM, a situation different from that seen in African countries where the disease is endemic. The objectives of this investigation were multiple: to clarify the clinical presentation of cases, the circumstances of transmission—particularly the nature of the link between a case and its index case (source case)—the profile of cases (gender, age, sexual practices, occupation, etc.), and to detect early any potential spread of the outbreak to other population groups. The data collected thus made it possible to adjust the case definition and the recommended course of action, but above all, to develop prevention messages, define their target audiences, and determine those for preventive vaccination.

The goal was to establish a communication plan that included strategies to reach particularly vulnerable groups. The messages were disseminated by community organizations and by Santé publique France via the Sexosafe website, through posters in gay social venues, broadcasts on niche radio stations, and other channels. The questionnaire completed during case investigations also helped identify high-risk contacts of cases for regional contact tracing and prioritization for post-exposure vaccination. This follow-up also provided better documentation of transmissibility through daily contacts (home, educational settings, healthcare settings).

In addition to the epidemiological investigation, the National Reference Center (CNR) for Orthopoxviruses sequenced strains from certain cases to determine the monkeypox virus clade, to monitor the emergence of new viral lineages or sublineages, to detect the emergence of mutations conferring resistance to the antiviral treatment TECOVIRIMAT or mutations facilitating escape from the smallpox vaccine proposed for the prevention (or post-exposure) of mpox, or in cases of suspected reinfection or clusters.

Demographic and behavioral profiles were examined among a subset of 2,216 men (aged 15 years or older) who reported an onset of mpox symptoms between May and July 2022. Among these men, 96% reported having had sex with other men. We observed a decrease between May and July 2022 in the proportion of reported mpox cases in the Île-de-France region (from 78% to 37%) and in the proportion reporting travel to other countries (from 43% to 21%) among all cases. We also observed, during this period, a decrease in the median number of sexual partners (from 4 to 2), in the proportion of cases frequenting MSM social venues (from 60% to 46%), and in the proportion of men living with HIV (from 31% to 22%) among all cases. Finally, we observed a decrease in the proportion of pre-exposure prophylaxis users (77% to 58%) among HIV-negative cases.

The changes in the demographic and behavioral profiles of the cases observed in our study could be interpreted as an initial circulation of the monkeypox virus in the Île-de-France region in May 2022, among a MSM population with high risks of exposure and infection (numerous sexual contacts), followed by nationwide spread of the virus in July 2022, among a MSM population at lower risk (fewer sexual contacts). It is also likely that the population affected by the outbreak changed its behavior and reduced the number of sexual partners following prevention messages disseminated through specialized media and via organizations.

This change in behavior among MSM in 2022 has been observed by other research teams internationally, particularly in the United States (Clay et al. 2023; Copen et al. 2024; Delaney et al. 2022; Phillips et al. 2023).

Enhanced surveillance of mpox cases has enabled the identification of populations most at risk of exposure to the virus, such as men who have sex with men and sex workers, and their involvement in the fight against the outbreak through community-based organizations and HIV advocacy groups. Targeted prevention messages, including those promoting vaccination, disseminated primarily by these organizations, helped control the mpox outbreak in 2022. The behavioral changes observed thus argue for a more systematic involvement of stakeholders, particularly organizations, in the fight against epidemics, as well as for broadening communication targets as an epidemic spreads.

Following our study, the 2023 edition of the “Rapport au sexe” (ERAS) survey, conducted among MSM by Santé publique France in partnership with ANRS – Emerging Infectious Diseases, included questions on behavioral changes during the first months of the 2022 mpox outbreak. The questions included a reduction in the number of sexual partners and anonymous sexual partners, as well as a reduction in visits to social venues frequented by MSM. The results will provide insights to better understand whether the shift in the demographic and behavioral profiles of mpox cases observed between May and July 2022 is primarily due to behavioral changes among MSM with high sexual activity—who may have had fewer sexual partners to reduce their risk of monkeypox infection—or to the spread of the outbreak to less exposed populations.

As of early 2024, while the number of monthly cases reported in France is relatively low, surveillance through mandatory reporting, reinforced by epidemiological investigation of cases (using a simplified questionnaire), remains relevant. This is all the more important given that a major outbreak of Clade I monkeypox virus—usually considered more virulent than Clade II—is currently raging in the Democratic Republic of the Congo (following the global Clade IIb outbreak in 2022–2023). In this context, it is also requested that any sample testing positive for monkeypox virus be sent to the National Reference Center for Orthopoxviruses for clade determination, in order to detect early any potential introduction into France of a Clade I monkeypox virus strain.