Cordeiro, RitaBatista, Fernando da ConceiçãoPelerito, AnaDe carvalho, IsabelLopo, SílviaNeves, RaquelRocha, RaquelPalminha, PaulaBorrego, Maria JoséNuncio, MSGomes, João Paulo2026-02-102026-02-102025-12-15Glob Epidemiol. 2025 Dec 15:11:100238. doi: 10.1016/j.gloepi.2025.100238. eCollection 2026 Jun2590-1133http://hdl.handle.net/10400.18/10877As mpox continues to spread globally, proactive monitoring and preparedness are crucial to minimize impact and enhance response strategies. Using a mathematical model combining a negative binomial distribution with Richards' logistic curve, we reconstructed the hidden phase of mpox transmission in Portugal, offering insights into the timing and dynamics of the initial outbreak. The analysis of 950 PCR-positive and 986 negative cases suggested that symptom onset occurred between March 24 and April 2, 2022, with March 27 identified as the most probable date. This study delineates the likely period of silent circulation of MPXV in Portugal, providing a clearer understanding of early outbreak dynamics and surveillance performance. Possible imperfections in early diagnostic testing and limited awareness of mpox may have contributed to delayed recognition of the outbreak. By demonstrating how retrospective mathematical modelling can estimate undetected transmission periods, our findings highlight the value of such approaches in epidemic reconstruction and underscore the importance of strengthening early surveillance systems to detect undiagnosed transmission of mpox in non-endemic countries.Highlights: Mpox circulated in Portugal ∼50 days before the first official case detection. Symptom onset estimated at March 27, 2022, via retrospective modelling. Mathematical modelling revealed hidden phases of mpox community transmission. Incomplete onset data was addressed using statistical imputation methods. Findings emphasize need for stronger early surveillance in non-endemic regions.engMpoxFirst CaseMathematical ModellingSurveillanceSymptom OnseInfecções Sistémicas e ZoonosesPortugaljournal article10.1016/j.gloepi.2025.10023841551841