During 2022, an unusually high number of infections with Corynebacterium diphtheriae were reported in several European countries. This mainly affected refugees who had recently arrived in Europe. Infections within the resident population in the countries affected by the increase in diphtheria cases were not documented at the time.

A study just published in the renowned New England Journal of Medicine (NEJM) now traces the transmission routes for the first time. A European research consortium, of which AGES is a member, analysed the genomic profiles of 363 bacterial isolates. These came from patients from the ten countries that reported an increase in diphtheria cases between January and the end of November 2022 and analysed the sequencing data together: Germany (118 isolates), Austria (66 isolates), the United Kingdom (59 isolates), Switzerland (52 isolates), France (30 isolates), Belgium (21 isolates), Norway (8 isolates), the Netherlands (5 isolates), Italy (3 isolates) and Spain (1 isolate). "The discovery of the first respiratory diphtheria case in Austria after almost 30 years in 2022 has shown the importance of cross-border information sharing and international cooperation in controlling outbreaks," says co-author Stefanie Schindler, microbiologist at AGES.

Diphtheria transmissions along the travel routes of migrants

The diphtheria strains reported in 2022 show a high degree of genetic identity. This indicates a common source of infection or that there are specific locations along the travel routes of migration to European countries where persistent diphtheria transmission occurs. Almost all of the 362 patients (98%) were male, with an average age of 18 years. The vast majority (96%) of them had recently travelled from their country of origin to the countries where diphtheria was diagnosed. Of the 266 patients for whom information on their country of origin was available, 222 (83%) were from Afghanistan or Syria. Most patients followed a migration route along the Western Balkans to their countries of destination. A total of 28 transit countries were reported.

Of the 346 patients for whom clinical data were available, 268 (77%) contracted cutaneous diphtheria, which causes skin sores, and 52 (15%) contracted a respiratory form of the disease that affects the throat.

Information from the European Centre for Disease Prevention and Control

Literature

Andreas Hoefer, Helena Seth-Smith et al. On behalf of the 2022 European Diphtheria Consortium. Corynebacterium diphtheriae outbreak among migrant populations in Europe. NEJM. June 4, 2025. DOI: 10.1056/NEJMoa2311981