Diphtheria strain involved in outbreaks among vulnerable populations across Europe between 2022 and 2025
During 2022, several European countries observed an unusual spike in reported diphtheria cases that mainly affected migrants who had recently arrived in Europe. Available genetic data, now published in the New England Journal of Medicine (NEJM), suggest that transmission occurred along established migration routes to or in Europe. By late 2022, rapid response measures helped to mitigate the outbreak. However, recent data from 2025 indicate that at least one of the bacterial strains detected in the 2022 outbreak is also associated with newly reported cases in the region, affecting additional vulnerable groups including people experiencing homelessness.
Diphtheria is a rare disease in the European region. Vaccination has notably reduced the number of cases worldwide in recent decades, thanks to mass immunisation efforts with a safe and effective vaccine. Between 2016 and 2020, an annual average of 27 diphtheria cases caused by Corynebacterium diphtheriae were reported in the European Union and European Economic Area (EU/EEA). This number went up to 320 cases in 2022, most of which presented with a cutaneous form of the disease. Ten people died from this vaccine-preventable disease in the EU/EEA between 2022 and 2025; most of them had developed a respiratory form of the disease. At the time, the outbreak was limited to groups of migrants who had recently arrived in Europe [1].
Public health interventions – including contact tracing, heightened awareness among clinicians and active searches for secondary cases – helped to curb the number of cases by late 2022.
Following the outbreak and throughout 2023, ECDC monitored the situation through enhanced event-based and indicator-based surveillance, issued a rapid risk assessment with recommendations to mitigate the outbreak, and provided subsequent epidemiological updates [2].
In a multi-country study published in the New England Journal of Medicine on 4 June 2025, authors from ECDC and other European institutions retrospectively performed a detailed analysis and characterisation of the genomic profiles of 363 diphtheria isolates from the countries affected by the 2022 outbreak (Austria, Belgium, France, Germany, Italy, Norway, the Netherlands, Spain, Switzerland and the United Kingdom). The results showed a high degree of genetic similarity between the strains detected during the outbreak, which was driven by four main genotypes. The results also suggested that transmission occurred along migration routes on the way to Europe [3]. Migrants are vulnerable to a number of infections, including those caused by Corynebacterium diphtheriae, due to several factors, including – for example – the poor conditions they may experience during transit through different countries.
Our analysis illustrates that closely related diphtheria clones were detected in multiple European countries, which tells us that people who left their home countries without the disease contracted diphtheria on their way to the destination country where they were diagnosed
explains Andreas Hoefer, expert in Microbiology at ECDC and one of the co-authors of the study.
To date, neither the full extent of circulation nor the exact place of infection has been determined, which is why both public health professionals and healthcare providers need to remain vigilant to avoid further spread of diphtheria in Europe.
In 2022, the outbreak mainly affected migrants who were living in reception centres or other housing for migrants following their recent arrival in Europe. During the study period, there were no documented secondary infections among the wider resident populations in the countries that reported a surge in diphtheria cases.
Diphtheria can present with a broad range of clinical symptoms. Especially in infections with toxin-producing bacteria, respiratory complications are feared – as those can be life threatening
says Prof. Adrian Egli, Director of the Institute of Medical Microbiology at the University of Zurich, Switzerland, and one of the senior authors of the study.
In addition to the analysed isolates from 2022, the study also reports data on 169 reported cases from six countries (Austria, France, Germany, Switzerland, the Netherlands and the United Kingdom) in 2023, indicating that diphtheria cases were not limited to migrant groups but had also been detected among other vulnerable populations, such as people experiencing homelessness and people who inject drugs. Genomic investigation of isolates from 2023 confirmed the continued circulation of the four main genotypes identified during the 2022 outbreak.
Sharing of sequencing data helped shape outbreak response across Europe
Rapid sharing of sequencing data among reporting countries allowed us to define the commonalities of the diphtheria strains across borders
says Prof Sylvain Brisse from Institut Pasteur, one of the senior authors of the study.
The detection of Austria’s first respiratory diphtheria case in 2022 highlighted the critical importance of cross-border information sharing and international collaboration in outbreak response
said Stefanie Schindler, microbiologist at the Austrian Agency for Health and Food Safety (AGES).
We noticed a dramatic increase of toxigenic C. diphtheriae in Germany in the summer of 2022, and initially informed our Austrian and Swiss colleagues in the informal European diphtheria network
said Prof Andreas Sing from the German Reference Laboratory for Diphtheria at the Bavarian Health and Food Safety Authority (LGL), and part of the EU Reference laboratory for Diphtheria and Pertussis (EURL-DIPE).
Analysis of the sequences also provided insight into the susceptibility of the bacterial strains to the commonly used antimicrobial treatment options. This, in turn, helped to shape the public health actions and outbreak responses, including identification and screening for erythromycin resistance.
Silvia Funke, ECDC expert on vaccine-preventable diseases, concludes that
The study shows how important it is to ensure that everyone’s vaccination status against diphtheria is up to date, including among vulnerable people such as migrants, people experiencing homelessness, people who inject drugs or unvaccinated people. This also means that clinicians need to be vigilant of and pay attention to diphtheria symptoms, especially when their patients are in vulnerable groups or have any link to these populations.
The likelihood of developing diphtheria is generally very low for people who have completed a full diphtheria vaccination series and have an up-to-date immunisation status, which might include a booster dose 10 years after receiving the last dose.
At the end of April 2025, Germany reported a diphtheria outbreak caused by a C. diphtheriae sequence type that was originally linked to the 2022 outbreak and was also detected in a few cases in 2023. According to the information from Germany, diphtheria cases were reported among people experiencing homelessness, an older adult and an unvaccinated child [4].
Following this recent outbreak signal from Germany, ECDC is in close contact with EU/EEA Member States and will update its risk assessment and recommendations based on the new data.
References/notes to editors:
[1] Data from the ECDC Surveillance Atlas of Infectious diseases, data accessed on 3 June 2025.
[2] Diphtheria is a notifiable vaccine-preventable disease caused by bacteria Corynebacterium diphtheriae and Corynebacterium ulcerans and is spread by droplets from the respiratory tract of an infected person (e.g. by coughing or sneezing). If the disease affects the skin, it can be spread by contact with wounds or lesions of an infected person (cutaneous diphtheria). Humans are the only significant reservoir for Corynebacterium diphtheriae. People infected with diphtheria bacteria may develop symptoms or may have no symptoms but be carrying the bacteria and able to infect others.
Vaccination against diphtheria is part of routine national immunisation programmes in Europe (in combination with vaccines against other diseases). According to WHO/UNICEF, immunisation coverage estimates for 2023 for the diphtheria, tetanus and pertussis vaccines in the EU/EEA varied across Member States, ranging from 78% (Romania) to 99% (Greece, Hungary, Luxembourg, and Portugal).
See also ECDC Rapid Risk Assessment from 2022, ECDC Epidemiological Update 5 Dec 2022 and Epidemiological Update 18 August 2023
[3] Hoefer et al. ‘Corynebacterium Diphtheriae Outbreak Among Migrant Populations in Europe’. This article was published on 4 June 2025, at NEJM.org. DOI: 10.1056/NEJMoa2311981
[4] ECDC Communicable disease threats report, 10–16 May 2025, week 20. Available from: https://www.ecdc.europa.eu/en/publications-data/communicable-disease-threats-report-10-16-may-2025-week-20
See also ‘Epidemiologisches Bulletin’ (in German) from the Robert Koch Institut from 30 April 2025. Available from: https://www.rki.de/DE/Aktuelles/Publikationen/Epidemiologisches-Bulletin/2025/18_25.pdf?__blob=publicationFile&v=4
[5] A migrant, as defined by the European Migration Network, is ‘a person who either: (i) establishes their usual residence in the territory of an EU/EFTA Member State for a period that is, or is expected to be, of at least 12 months, having previously been usually resident in another EU/EFTA Member State or a third country; or (ii) having previously been usually resident in the territory of the EU/EFTA Member State, ceases to have their usual residence in the EU/EFTA Member State for a period that is, or is expected to be, of at least 12 months.’ Migrants are therefore a highly heterogeneous group, and it is difficult to generalise about their health and social needs.
