Clinical characterisation and outcomes of human clade IIb mpox virus disease - a European multicentre observational cohort study (MOSAIC).

Background: The global mpox outbreak which started in May 2022 was caused by a novel clade IIb variant of the mpox virus (MPXV). It differed from the traditional Western and Central Africa disease in transmission patterns and clinical presentation.

Methods: To address the need for detailed clinical and virologic data, we conducted an observational cohort study (MOSAIC) during May 2022-July 2023 in individuals with confirmed MPXV infection enrolled in six European Countries. Case-management decisions were left to the attending physician. Participants were monitored for up to six months for clinical signs/symptoms and clinical and virologic outcomes through hospital visits, phone interviews, and self-administered questionnaires. Outcomes included time-to-lesion resolution, clinical status, and virus clearance.

Results: The 518 participants not receiving any specific treatment ("untreated") were diagnosed a median 5 days from symptom onset; 90% were managed as outpatients. Lesions were mostly cutaneous (88%) as and peri-genital (74%). By Day 14 from the first PCR-positive sample, 39% had resolved lesions. Time-to 95% unculturable virus was longest in cutaneous lesions (52 days). A putative systemic antiviral was available for 57 participants, 44% as in-patients, 34% and 58% had resolved lesions by D14 from the first PCR-positive sample and from treatment start, respectively. Time-to 95% unculturable virus was 60 days in skin and oropharynx. No death or recrudescence occurred by Day 180.

Conclusion: MOSAIC provides comprehensive insights into the clinical and virologic characteristics of mpox caused by the clade IIb variant. The study forms the basis of clinical characterisation for ongoing mpox outbreaks.