Diphtheria in Switzerland, 1877–2022: long-term epidemiology and the impact of mid-20th-century vaccination programs

BACKGROUND: Diphtheria was a major cause of childhood morbidity and mortality in Switzerland until the mid-20th century. While mortality declined already before vaccination due to improvements in living conditions and medical care, recurrent epidemic waves persisted. The role of vaccination programmes introduced in the 1930s and 1940s has not been systematically quantified for Switzerland. DATA AND METHODS: We compiled and digitised multiple historical time series on diphtheria morbidity, hospitalisations and mortality in Switzerland from 1877 to 2022, including cantonal incidence data from 1910 onwards. Using Bayesian negative binomial models, we estimated counterfactual incidence and mortality trajectories in the absence of vaccination. We compared observed trends with model predictions, focusing on the early mandatory programme in Geneva (from 1933) and on differences between cantons with mandatory versus recommended/facultative vaccination following the nationwide recommendation in 1943. RESULTS: Between 1877 and 2022, 35,271 diphtheria deaths were recorded, with 99.5% occurring before widespread vaccination in the 1940s. Incidence and mortality declined in several stages but were punctuated by major epidemic waves around World War I, 1930, and World War II. After the introduction of mandatory vaccination in Geneva, observed incidence and mortality were consistently lower than predicted counterfactual levels. Following the nationwide vaccination recommendation, diphtheria declined rapidly in all cantons, but reductions were earlier and more pronounced in cantons with mandatory vaccination, which also experienced fewer and smaller residual outbreaks. DISCUSSION: Our findings show that while pre-vaccination declines reduced diphtheria burden, sustained containment was only achieved through vaccination.