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.
Keywords: Clade IIb; Lesion resolution; Mpox; Observational cohort; viral load.
© The Author(s) 2025. Published by Oxford University Press on behalf of Infectious Diseases Society of America.