This study is to evaluate the long-term immunogenicity of a boost dose of MVA-BN vaccine

Mpox is an endemic zoonosis in Africa, caused by the MPXV virus of which there are two clades: I (former Congo Basin) and II (former West Africa). Since 2022, clade II has emerged globally via sexual transmission, primarily among men who have sex with men (MSM), resulting in a declaration of public health emergency (PHEIC) by the WHO.

In 2023, a clade I epidemic emerged in East Africa with a high case fatality rate (3-5%). In August 2024, the WHO again declared a PHEIC after the spread of clade I to African countries with no previously reported cases and outside Africa, raising fears of higher mortality and transmission.

A 3rd generation vaccine, MVA-BN (Imvanex® /Jynneos®), initially developed against smallpox, was approved in 2022 to prevent mpox. In France, the HAS recommends post- and pre-exposure vaccination for populations at risk: MSM, trans people with multiple partners, sex workers and certain professionals. For people born before 1980 (history of smallpox vaccination), a single dose is recommended as primary vaccination, while immunocompromised subjects require 3 doses.

Data show vaccine effectiveness of 20-80% in post-exposure prophylaxis (PEP) and ~80% in pre-exposure but neutralizing antibodies become undetectable after one year. Since the summer of 2024, the HAS has recommended a booster dose 2 years after the primary vaccination, on the basis of immunogenicity studies showing an increase in seroconversion to 98.7% one month after administration, but underlines the need to have other data, in particular on the durability of this response.

A study is proposed in MSM on HIV PrEP (pre-exposure prophylaxis), a priority population for structured medical monitoring, to evaluate the immunogenicity and safety of the MVA-BN booster in this context.

Inclusion Criteria:

• Men aged over 18 years
• Have received two doses of MVA-BN vaccine as an initial schedule
• Eligible for a booster dose of MVA-BN (according to the HAS recommendation)
• Be eligible and wish to initiate PrEP-HIV treatment or be followed for PrEP-HIV treatment
• Covered by social security (excluding AME)

Exclusion Criteria:

• History of mpox (virologically confirmed)
• Be under guardianship or curatorship
• Be subject to a judicial protection measure
• Have a contraindication to vaccination against mpox

The study population will consist of Men who have Sex with Men (MSM), taking HIV pre-exposure prophylaxis (HIV PrEP), eligible for MVA-BN vaccination.