A study published in the New England Journal of Medicine (NEJM) reveals that the largest diphtheria epidemic in Western Europe for 70 years, which broke out in 2022 among migrants and in 2023 spread to other vulnerable populations in several European countries, is the result of contaminations occurring during migratory travel or in destination European countries, and not in the countries of origin. However, the geographical area and conditions of these initial contaminations are still unknown. A genetic link has also been established between the strain that circulated during the 2022 epidemic and an epidemic that occurred in Germany in 2025, suggesting that the bacterium has been circulating silently in Western Europe. While demonstrating the effectiveness of vaccination programs for the general population, this research, carried out by an international team including researchers from the Institut Pasteur and epidemiologists from Santé publique France, underlines the importance of maintaining a high level of vigilance and public health support dedicated to diphtheria (vaccination, screening, clinical examination) among vulnerable populations in Western Europe.
In 2022, several European countries observed an unusual spike in cases of infection with the bacterium responsible for diphtheria (Corynebacterium diphtheriae), mainly among migrants who had recently arrived in Europe. Three hundred sixty-two cases were recorded in Europe that year by the European Centre for Disease Prevention and Control (ECDC). By the end of 2022, rapid intervention measures such as contact tracing and screening of secondary cases had mitigated the epidemic. In 2023, 123 cases were reported in Austria, France, Germany, Switzerland, the Netherlands and the UK (March 2025 - Emerging Infectious Diseases). Despite this decline, rare infections have been observed in these countries among migrant populations and other vulnerable populations, particularly the homeless, since 2022 and up to the present day. A total of 536 cases, including at least three deaths, have been reported in Europe since the start of the 2022 epidemic.
A clearer epidemiological picture, but the origin and scope are still unknown
The researchers analyzed 363 isolates from 362 patients in ten European countries. The data reveal that 98% of patients were male, with a median age of 18, 96% of whom had recently migrated to Europe. The majority of infections (77%) were cutaneous, with 15% of more severe respiratory forms.
The study shows that the epidemic, which has mainly affected migrant populations from Afghanistan and Syria, is not the result of initial contamination in these countries of origin, but rather contamination during migratory journeys or in reception centers in European countries. Genomic analysis of the bacterial strains circulating during the epidemic showed a very high degree of genetic proximity between the strains observed in people from different countries, implying the existence of a recent point of contact, outside the country of origin, which led to contamination. The most likely hypothesis is that this point of contact is one or more places frequented by migrants on their journey from their country of origin or in the destination countries. The exact scale of the epidemic remains difficult to determine due to the limitations inherent in screening these vulnerable populations.
Significant results for public health
"This study highlights the crucial importance of cross-border epidemiological surveillance and international collaboration in epidemic response," says Prof. Sylvain Brisse of the Institut Pasteur, one of the study coordinators. "The rapid sharing of sequencing data between countries has made it possible to define the common characteristics of diphtheria strains and adapt the health response."
In view of the major unknowns that persist with regard to this epidemic, public health experts are calling for vigilance and the strengthening of public health measures aimed at vulnerable populations in Western Europe: raising awareness of disease symptoms among doctors and, more broadly, people in contact with the vulnerable populations concerned, checking (and updating if necessary) vaccination status, appropriate antibiotic therapies, etc. Overall, the results highlight the need to step up surveillance of infectious diseases in vulnerable populations, improve access to healthcare and vaccination for migrant populations, and maintain heightened vigilance in the face of emerging antibiotic resistance.
Isabelle Parent du Châtelet, unit manager at Santé publique France states that "the study shows how important it is to ensure that diphtheria immunization levels are up to date, particularly for vulnerable population groups, such as migrants, and that diphtheria poses a risk especially among homeless people, injecting drug users, unvaccinated individuals and elderly people with pre-existing illnesses, as well as people with professional ties to these groups. It also means that clinicians need to be aware of and pay attention to the common symptoms of diphtheria, especially when their patients have a professional or other link with these vulnerable populations."
Collaborative research on an international scale
This study was conducted with financial support from the Institut Pasteur and Santé publique France as part of the missions of the National Reference Center (CNR) for diphtheria. The center was recently appointed as one of the members of the European Reference Laboratory for Diphtheria and Pertussis. The study is part of a concerted approach to diphtheria monitoring and control in Europe, led by the European Centre for Disease Prevention and Control (ECDC) and an ad hoc consortium of research and public health institutions in the countries concerned.
