Distribution of anti-diphtheria toxin antibodies in the Italian population by age, sex, and geographic region

Objectives: Diphtheria incidence has dropped markedly thanks to global vaccination programs, yet some cases still occur even in highly vaccinated countries. We aimed to assess the seroprevalence of anti-diphtheria antibodies in the Italian population by age, sex, and geographical area. Methods: Samples were collected from subjects (6–90 years) consulting for routine blood tests. Antibody levels were measured using an immuno-enzymatic assay (seroprotection threshold: 0.01 IU/ml). Results: Overall, 3,282 serum samples were collected in 2019–2020 from healthy individuals aged 6–90 years residing in 13 Italian regions. Overall, 81.4% of participants showed protective antibody levels. Protection decreased progressively with age: 90.7% among children aged 6–12, 85.4% in those aged 13–24, 78.6% in the 25–39 age group, and 73.2% among 40–64-year-olds. A slight increase was observed in individuals aged ≥ 65 (75.2%). High protection (≥0.1 IU/ml) was most prevalent in the youngest group (78.4%) and declined in older groups, reaching its lowest level in those aged 40–64 (44.7%). Conversely, low protection (<0.1 IU/ml) was highest in participants aged ≥ 65 (30.1%). Males showed significant higher protection than females in the 25–39, 40–64, and ≥ 65 age groups. Geographically, protection levels were higher in Northern (93.4%) and Central Italy (85.6%) than in Southern regions and the Islands (73.8%), for almost all age groups (except 6–12 years). Conclusions: Protection against diphtheria in older age groups remains suboptimal. Booster vaccination strategies should be strengthened, as insufficient immunity in adults and the elderly poses a potential risk for severe disease outcomes.