Abstract:
Aim of the Study Despite the availability of an effective vaccine, measles remains one of the most contagious human diseases. Italy experienced a substantial resurgence of measles between 2024 and 2025. Molecular surveillance of circulating measles virus (MeV) variants plays a critical role in documenting the interruption of endemic transmission and in accurately classifying cases as imported, import-related, or endemic. Genotyping relies on the sequence analysis of the 450-nucleotide carboxyl-terminal region of the nucleoprotein gene (N450). As the National Reference Laboratory (NRL) for measles and rubella at the Istituto Superiore di Sanità (ISS), in collaboration with the national MoRoNet laboratory network, we performed a comprehensive molecular epidemiological analysis of measles cases reported in Italy during the 2024-2025 biennium. This study specifically aimed to assess the contribution of imported strains to sustained viral circulation through the use of distinct sequence identifiers (DSIds) submitted to the WHO Measles Nucleotide Surveillance (MeaNS) database. Ultimately, this work seeks to demonstrate the value of high-resolution genomic surveillance in tracking measles transmission pathways and supporting evidence-based strategies toward measles elimination in Italy.
Methods Used: Urine, oral fluid, and dried blood spot samples from suspected measles cases were collected nationwide. Measles virus RNA was detected by real-time RT-PCR, and positive samples were genotyped by sequencing the 450-nucleotide carboxyl-terminal region of the nucleoprotein gene (N450). Sequences were analysed using distinct sequence identifiers (DSIds) and named strains in the WHO MeaNS database and integrated with available epidemiological data.
Results and Conclusions: Of 967 and 502 laboratory-confirmed measles cases reported in Italy in 2024 and 2025, 632 (65.4%) and 314 (62.5%) were successfully genotyped, respectively. Only genotypes D8 and B3 circulated during this period, with a marked increase in B3 detections observed in 2025. Overall, 25 distinct DSIds for genotype D8 and 28 for genotype B3 were identified. The most prevalent variants were D8 DSId 8491 (167 cases) and B3 DSId 6418 (78 cases in 2025). DSId 5963 (D8) demonstrated the longest sustained circulation, persisting for approximately 24 months and linked to multiple importations, predominantly from Romania. Notable importation events included DSId 9705 (Indonesia) and DSId 6418/9716 (Morocco).
The use of distinct sequence identifiers (DSIds) provided significantly higher resolution than genotyping alone, enabling precise tracking of viral lineages, as different DSIds predominate in specific geographic regions. No evidence of endemic transmission was documented at the regional level. However, incomplete genotyping coverage in several high-incidence areas limited full reconstruction of transmission chains.
Conclusions: The 2024–2025 measles resurgence in Italy was primarily driven by multiple independent importations rather than sustained endemic transmission. The systematic application of DSIds proved essential for high-resolution tracking of international linkages and for distinguishing imported from locally transmitted cases. These findings underscore the critical importance of strengthened laboratory capacity and expanded molecular surveillance coverage to accurately document interrupted transmission and support the verification of measles elimination in Italy.
Biography:
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