Mpox outbreak in France: epidemiological characteristics and sexual behaviour of cases aged 15 years or older, 2022

In France, enhanced surveillance of mpox (formerly known as monkeypox) was implemented on 20 May 2022, at the same time as the first case was diagnosed in this country. We had to learn everything about this emerging disease, and as quickly as possible, to guide management and prevention. This enhanced surveillance, supported by a case definition and procedures document, meant that in addition to the mandatory declaration of orthopoxvirus cases (including mpox), there would be an epidemiological investigation of all cases using a standardised questionnaire. 

The first cases detected in Europe in early May 2022 occurred primarily among MSM, which is a different picture from that seen in African countries where the disease is endemic. The investigation had multiple objectives: to determine the clinical presentation of the cases, the circumstances of contamination and in particular what type of link there was between a case and its index case (source case), to identify the profiles of the cases (sex, age, sexual practices, profession, etc.), and to detect any spread of the epidemic to other population groups. The resulting findings meant that the case definition and procedures to follow could be adjusted, but more importantly, that information about prevention could be developed alongside a definition of the public targeted, as well as information on preventive vaccination. 

The aim was to set out a communication plan that included strategies to reach particularly vulnerable groups. The messages were relayed by community associations and by Santé publique France via the Sexosafe website, with the campaign displayed in places where gay men socialise and messages broadcast on partner radio stations, etc. The questionnaires completed during the investigation were used to identify any contacts at risk for each case. As a result, contact tracing could be carried out in the regions, with a primary focus on post-exposure vaccination. This scheme also improved documentation of transmissibility through day-to-day contacts (home, education and care settings). 

In addition to the epidemiological investigation, the National Reference Centre (CNR) for Orthopoxvirus sequenced the strains of certain cases to determine the mpox viral clade, monitor the appearance of new lineages or sub-lineages of the virus, and search for mutations that are resistant to the antiviral treatment TECOVIRIMAT or mutations that may inhibit the smallpox vaccine proposed for mpox prevention (or post-exposure), or even where there is a suspected re-infection or cluster.

Demographic and behavioural profiles were studied in a subset of 2,216 men (aged 15 years or older), whose mpox symptoms developed between May and July 2022. Of these men, 96% said they had sex with other men. Between May and July 2022, we saw a decrease in the proportion of mpox cases reported in the Île-de-France region (from 78% to 37%) and in the proportion who reported travelling to other countries (from 43% to 21%) among all cases. During this period, we also observed a fall in the median number of sexual partners (4 to 2), the proportion of cases visiting spaces where MSM meet (60% to 46%), and the proportion of men living with HIV (31% to 22%) among all cases. Finally, we noted a decrease in the proportion of users of pre-exposure prophylaxis (77% to 58%) among HIV seronegative cases.

The change in the demographic and behavioural profiles of the cases observed during our study could be interpreted as the mpox virus initially circulating in the Île-de-France region in May 2022 among an MSM population at a high risk of exposure and infection (numerous sexual contacts), followed by the virus spreading nationally in July 2022 among an MSM population at lower risk (fewer sexual contacts). It is also likely that the population affected by the epidemic has changed its behaviour and reduced numbers of partners in response to prevention messages broadcast by specialised media and associations.

This change in the behaviour of MSM in 2022 was noted by other 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 cases of mpox has meant that we can identify the populations most at risk of exposure to the virus, such as MSM and sex workers, and we can involve them in the fight against the epidemic, via community or HIV associations. Targeted prevention messages, including those promoting vaccination, especially when delivered by associations, helped to control the mpox epidemic in 2022. The behavioural changes observed in this respect are an argument for routinely getting stakeholders, especially associations, involved in the fight against epidemics, as well as for broadening the target audience of a campaign as an epidemic spreads.

Following our study, the 2023 version of the survey “Rapport au sexe” (ERAS) included questions on behavioural changes during the first few months of the mpox epidemic in 2022. This survey is conducted by Santé publique France in partnership with the ANRS Emerging Infectious Diseases research agency with the aim of understanding sexual behaviours among MSM and the preventive measures used in the population. Respondents were asked whether they had reduced their number of sexual partners or anonymous sexual partners, and whether they were visiting spaces where MSM meet less often. The findings will help us to understand whether the changing demographic and behavioural profiles of mpox cases observed between May and July 2022 can be attributed to intentional changes in the behaviour of MSM in light of mpox (e.g. MSM who chose to have fewer sexual partners to reduce their risk of infection), or rather to the spread of the epidemic to less exposed populations.

As we enter 2024, while the number of monthly cases reported in France is relatively low, surveillance through mandatory reporting of cases backed up by epidemiological investigation (via a simplified questionnaire) remains ongoing. This strategy is ever more important given that the global epidemic of mpox clade IIb in 2022–2023 was followed by a major epidemic of clade I monkeypox virus, which is usually considered more virulent than clade II, in the Democratic Republic of Congo. For this reason, any positive monkeypox virus sample should be sent to the Orthopoxvirus CNR for the clade to be determined, so that any instance of a strain of monkeypox virus clade I introduced to France can be detected early.