Measles Immunity Status of Greek Population After the Outbreak in 2017-2018: Results From a Seroprevalence National Survey

Accurate data on susceptibility rates against measles in general population of Greece are scarce. Many studies estimate the vaccination coverage but none has calculated the nationwide immunity rate, involving all age groups, against measles virus. The purpose of our study was to determine the immunity status to measles, especially after the latest outbreak in 2017-2018. In total, 3,972 leftover blood samples were collected from a nationwide laboratory network using a geograph-ically stratified sampling strategy and were tested for the presence of measles specific IgG anti-bodies. The overall crude seroprevalence was calculated 89.6% and the adjusted 89.8% (95% CI: 88.8% - 90.8%). There was no statistically significant difference in seropositivity among sexes (p=0.783). Higher immunity rates and antibody titer were found in older age groups ≥ 41 years old (94.9%, 95% CI: 93.7% - 95.9%, and 730.0 IU/l) in comparison to younger individuals 1-40 years old (83.4%, 95% CI: 81.6% - 85.7%, and 616.5 IU/l). ). Comparing the seroprevalence among Nomenclature of Territorial Units for Statistics (NUTS 2), a statistically significant difference was estimated among them (<0.001). The two regions where higher measles incidence was observed during the 2017-2018 outbreak, Eastern Macedonia and Thrace and Western Greece, were among the four regions with the lower seropositivity (84.6%, 95% CI: 79.9% -89.4% and 85.9%, 95% CI: 81.4% - 90.4%, respectively). Our study showed a measles immunity gap that affects younger age groups and makes a new measles outbreak likely. Enforcement of vaccination campaigns and ad-dressing vaccine hesitancy could bridge it and achieve the required target for herd immunity.