Undetected circulation of monkeypox virus in Portugal: Evidence for a 50-day gap before first detection.

As 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.