Primary objective The overall objective is to assess the long-term amplitude and durability of the immune responses after mpox virus (MPXV) infection.

Primary endpoint The main outcomes are: the presence of neutralizing antibodies in the serum and the presence of detectable mpox virus-specific memory B cells. Neutralizing antibodies titers will be measured with a semi-automated cell-based assay using live mpox virus. Memory B cells will be measured by flow cytometry and other techniques.

Secondary objectives

1. Assess the amplitude and the stability of the MPXV antigen-specific B-cell memory response after infection, against the immunodominant antigens A27, A33, B5, L1, D8, and H3.
2. Assess the breadth of the B-cell repertoire and its neutralizing capacity in a subset of patients
3. Assess the humoral immunity using different serological techniques, for the detection and quantification of anti-MPXV antibodies, and assess the seroneutralization capacities of each antibody.

Secondary endpoints

1. The presence of MPXV antigen-specific B-cell memory. Memory B cells will be measured by flow cytometry and other techniques.
2. The breadth and neutralizing capacity of the B-cell repertoire in a subset of patients.
3. The humoral immunity using serological techniques.

Single inclusion visit with data collection and blood sample. No further follow-up of participants.

A total of 60 participants previously enrolled in the main cohort ANRS 0758s MOSAIC are planned to be included, distributed among France (n = 35), Italy (n = 20), and Switzerland (n = 5).

Inclusion Criteria:

• Written informed consent, signed by the participant and the investigator prior to any study-related procedure
• Person previously enrolled in the ANR0258s-MOSAIC cohort (see Appendix 19.2 for MOSAIC inclusion criteria) between July 2022 and July 2023
• Person with a blood sample taken within 8 months following mpox infection
• ≥ 18 years old

Exclusion Criteria:

• Individuals who, in the investigator's opinion, are unlikely to comply with study procedures or whose medical condition is incompatible with study participation
• For France only: person not affiliated to or not benefiting from a social security scheme in accordance with article L1121-11 of the French Public Health Code (Aide Médicale d'Etat [AME] does not constitute a social security scheme)
• Pregnant or breastfeeding women
• Individuals under guardianship or curatorship, or deprived of liberty by judicial or administrative decision