In 2022, Western Europe experienced its largest outbreak of diphtheria in 70 years. At the time, the outbreak, which affected more than 320 people, was limited to migrants who had recently arrived in Europe.

What followed has been a stress test of Europe’s adult immunization systems.

Recent data indicate at least one 2022 strain persists in newer cases, and the tally has kept climbing: From the beginning of 2022 up to April 30, 2025, 536 cases — including at least three deaths — were reported in EU/European Economic Area (EEA) countries.

Increasingly, infections have appeared in vulnerable resident groups, including people experiencing homelessness, people who use or inject drugs, and those who are unvaccinated.

“To me, that’s the main significance of these events: that diphtheria didn’t disappear, it wasn’t completely eradicated — one of the same strains spilled over into resident populations. It’s a reminder that this is an important disease and one that we shouldn’t underestimate,” Sylvain Brisse, PhD, director of the Biodiversity and Epidemiology of Bacterial Pathogens Unit and National Reference Centre for diphtheria at the Institut Pasteur, Paris, France, and who was involved with the analysis of cases in 2022, told Medscape Medical News.

From Border Cluster to Community Circulation

Investigators concluded that most European exposures in the 2022 outbreak originated during migrant transit along the western Balkan route. Infected travellers then carried the bacterium onward, with further cases detected in migrant reception centers in Europe, where crowding, poor hygiene, and low vaccination rates favored further spread.

Public health measures — contact tracing and alerts to clinicians — helped bring the initial wave under control in Austria, Belgium, France, Germany, Italy, Norway, the Netherlands, Spain, Switzerland, and the UK. But newer reports in 2023-2025 (more than 200 cases across the EU/EEA) underscore wider circulation of Corynebacterium diphtheriae beyond reception centers.

Brisse emphasized the equity imperative: “It’s a reminder that we should better care for the health of migrant populations and especially for vaccination among those coming from countries that don’t or can’t vaccinate or vaccinate at low rates. In our study, many migrants didn’t know if they’d been vaccinated and didn’t have vaccination records. This is particularly true among those coming from Syria or Afghanistan where the public health system isn’t functioning properly.”

The Underimmunized Adults Behind the Gap

Research shows adult and adolescent migrants are underimmunized for routine vaccines. Missed childhood doses, interrupted records, and marginalization from health systems combine with heightened exposure risks — malnutrition, overcrowding, and unsanitary living conditions — to widen immunity gaps. The result, researchers say, is “a growing under-immunized adult population group who could be targeted by specific vaccination initiatives in order to improve vaccine coverage for certain key infections.”

Despite European Centre for Disease Prevention and Control guidance on life-course “catch‑up” vaccination, progress has been slow.

The World Health Organization (WHO) Immunization Agenda 2030 (IA2030) calls for at least 90% coverage of essential childhood/adolescent vaccines and explicitly stresses including migrants in vaccination programs, while acknowledging a basic problem: “As they often move across borders, it is not even clear who is responsible for vaccinating them, and they may be marginalized and overlooked.”

On the ground, system barriers remain.

In Norway, Professor Jörn Klein’s work with healthcare providers highlighted four structural challenges to vaccinating adult migrants:

• lack of a consistent, structured municipal approach
• no clear division of responsibilities among healthcare professionals
• out‑of‑pocket costs for migrants
• language barriers

Norway has since established a national adult immunization program launching later this year, extending vaccination rights to everyone in the target group living in or temporarily residing in the country — including migrants. Its initial scope covers annual influenza, COVID, and pneumococcal vaccines for those aged 65 or older.

“The new adult program is narrow in scope at launch and in some cases uses co-payment, which may limit uptake among lower-income groups — including some labor migrants,” said Klein, who is responsible for microbiology, epidemiology, and infectious disease control at the University of South-Eastern Norway, Notodden, Norway. “It’s a meaningful step but does not yet close adult migrant gaps. Expanding scope, removing selective fees, and standardizing municipal pathways would materially improve equality.” He added that using certified translators, offering weekend clinics for labor migrants, and collecting better data would help close gaps by enabling targeted action.

At the Clinic: Catch-Up Protocols and Preparedness

Franziska Badenschier, MScPH, epidemiologist who oversees diphtheria surveillance at Germany’s Robert Koch Institute, stressed the importance of checking migrant vaccination status during medical examinations prior to admission to reception centers.

“If vaccination status cannot be determined because no vaccination certificate or other evidence is available, medical staff should assume that the person has not been vaccinated in accordance with STIKO [the German Standing Committee on Vaccination] recommendations,” she said.

STIKO recommends catch‑up vaccination for tetanus, diphtheria, pertussis, and poliomyelitis when needed. In the recent outbreak, however, vaccination status was often impossible to ascertain, so local teams offered catch‑up vaccinations on‑site.

That ad hoc approach underscores a wider European challenge: delivering life‑course catch‑up vaccination to underimmunized adults with uncertain records.

At the regional level, IA2030’s 2024 progress report found most performance indicators — especially routine coverage and the reduction of zero‑dose children — were off track.

Europe has faced increased measles outbreaks due to inadequate immunization and primary care systems and barriers to running high‑quality catch‑up campaigns — the same conditions that complicate diphtheria control. With Europe experiencing its largest measles resurgence in decades in 2024 and its worst diphtheria outbreak in 70 years, experts argue that adult migrants must be fully integrated into life‑course vaccination — including newer vaccines such as HPV — to align with national schedules. “This all illustrates why catch-up and equitable access that includes migrants are not optional if IA2030 targets are to be met,” Klein said.

Brisse noted that infection resurgence also exposes treatment bottlenecks: There is “very suboptimal” availability and stockpiles of the diphtheria antitoxin, for example. “Several countries don’t have stock, and very few companies globally produce it — it’s a technology from the 18th century and not very safe but very efficacious. If we had [more] outbreaks, it would be very difficult to treat people.”

Taken together, the past 3 years have shown how quickly diphtheria can move from border clusters to community circulation when adult immunity gaps persist and preparedness lags. For clinicians and public health teams, the priorities are integrating adult migrants into life‑course and catch‑up vaccination pathways, reducing practical barriers to access, and ensuring availability of antitoxin alongside rapid detection and response. Until those pieces are in place, Europe will remain vulnerable to outbreaks wherever vaccination gaps persist.

Brisse, Klein, and Badenschier reported having no relevant financial relationships. 

Sophie Cousins, MIPH, is a global health journalist who has reported from more than 20 countries.