Estimating measles susceptibility and transmission patterns in Italy: an epidemiological assessment.

BACKGROUND

Identifying measles transmission patterns and the most susceptible populations is crucial for anticipating and preventing outbreaks. The aim of this study was to assess the current epidemiology of measles in Italy to provide key metrics to anticipate and prevent future transmission risks.

METHODS

In this epidemiological assessment study, we analysed measles epidemiological data from the National Integrated Measles and Rubella Surveillance System coordinated by the Department of Infectious Diseases of the Istituto Superiore di Sanità in Italy from Jan 1, 2013, to Dec 31, 2022. We analysed individual case records to assess the proportion of transmission that occurred in different settings; we also used pairs of measles cases to estimate the generation time and the proportion of transmission episodes between population groups defined by age and vaccination status. All suspected cases meeting the measles case definition were included in our analysis. Data, complemented with 2023 and 2024 incidence records, were used in a catalytic model to estimate the age-specific proportion of individuals susceptible to measles and the effective reproduction number (R) in Italy in 2025 at the national level and for each Italian region.

FINDINGS

During the study period, 14 946 measles cases (7426 females and 7520 males) were reported to the National Integrated Measles and Rubella Surveillance System. The mean generation time estimated from 795 measles infection episodes was 11·7 days (95% credible interval 11·3-12·0). 707 (88·9% [bootstrap 95% CI 86·8-91·1]) of 795 infection episodes originated in unvaccinated individuals, 265 (33·3% [30·1-36·7]) of 795 infection episodes involved individuals aged 20-39 years, and only 12 (8·5% [4·3-13·5]) of 141 transmission episodes with available information on setting were linked to school contacts. We estimated that, in 2025, 9·2% (95% prediction interval 8·9-9·3) of the Italian population remains susceptible to measles, including 11·8% (11·8-11·8) of individuals younger than 20 years. Despite marked geographical heterogeneities, immunity gaps among individuals aged 20-40 years (ie, those born in the 1980s and 1990s) were consistently observed across all regions. The average R estimated for 2025 ranged from 1·31 to 1·78 across regions, consistent with reproduction numbers associated with national epidemics and local outbreaks between 2013 and 2019. Higher R values were found in regions with a large fraction of susceptible adults (eg, Emilia-Romagna 1·78 [1·34-2·27]) or more than 15% susceptible individuals younger than 20 years (eg, South Tyrol 1·53 [1·11-2·02]).

INTERPRETATION

Unvaccinated adults contribute substantially to measles transmission in Italy. Heterogeneous immunity gaps exist across regions, with some regions showing persistently low vaccine uptake in children and other regions showing a high proportion of susceptible adults. These results emphasise the need for tailored vaccination strategies, including catch-up campaigns for adults. By integrating routine surveillance data with modelling techniques, this study presents a resource-efficient approach to quantifying immunity and transmission risks, providing a scalable framework for countries aiming to refine their immunisation policies.

FUNDING

NextGenerationEU-MUR PNRR Extended Partnership Initiative on Emerging Infectious Diseases.

TRANSLATION

For the Italian translation of the abstract see Supplementary Materials section.