Eurosurveillance - Volume 24, Issue 17, 25 April 2019

Measles is still endemic in Italy and outbreaks are frequent. From 2016 to 2018, more than 7,000 measles cases were reported to the national integrated measles and rubella surveillance system, the largest outbreak since implementation of this system.

We aimed to describe the characteristics and spatiotemporal distribution of measles cases in Italy and explore determinants of incidence at municipality level.

We performed a retrospective observational study, mapping by municipalityall measles cases reported to the national surveillance system with symptom onset between 1 September 2016 and 31 July 2018. We also analysed measles–mumps-rubella (MMR) vaccination coverage (VC) data (2000–2017) for the first and second dose, collected from the Ministry of Health. We used regression analysis to explore factors associated with measles incidence at municipality level.

We analysed 7,854 cases, 3,927 (50%) female. Median age was 26 years; 475 cases (6%) were younger than 1 year. The outbreak occurred in two epidemic waves. The first started in central/northern regions (end of 2016), the second (mostly within 2018) was concentrated in southern regions. In 2016 and 2017, national VC was below 95% for both MMR doses. In 2017, only one region reported VC above 95% for the first dose. At municipality level, incidence was associated with higher urbanisation, less deprivation and fewer adults.

The spread of measles between September 2016 and July 2018 in Italy indicates the need to improve VC and to explore further how societal and other parameters might be linked to incidence.

Measles elimination is based on 95% coverage with two doses of a measles-containing vaccine (MCV2), high vaccine effectiveness (VE) and life-long vaccine-induced immunity. Longitudinal analysis of antibody titres suggests existence of waning immunity, but the relevance at the population-level is unknown.

We sought to assess presence of waning immunity by estimating MCV2 VE in different age groups (2–5, 6–15, 16–23, 24–30 and 31–42 years) in Berlin.

We conducted a systematic literature review on vaccination coverage and applied the screening-method using data from a large measles outbreak (2014/15) in Berlin. Uncertainty in input variables was incorporated by Monte Carlo simulation. In a scenario analysis, we estimated the proportion vaccinated with MCV2 in those 31-42 years using VE of the youngest age group, where natural immunity was deemed negligible.

Of 773 measles cases (median age: 20 years), 40 had received MCV2. Average vaccine coverage per age group varied (32%–88%). Estimated median VE was  > 99% (95% credible interval (CrI): 98.6–100) in the three youngest age groups, but lower (90.9%, 95% CrI: 74.1–97.6) in the oldest age group. In the scenario analysis, the estimated proportion vaccinated was 98.8% (95% CrI: 96.5–99.8).

VE for MCV2 was generally high, but lower in those aged 31-42 years old. The estimated proportion with MCV2 should have led to sufficient herd immunity in those aged 31-42 years old. Thus, lower VE cannot be fully explained by natural immunity, suggesting presence of waning immunity.