People experiencing displacement such as migrants and refugees as well as unhoused people are among those at increased risk for exposure, say experts. 

A total of 536 diphtheria cases, including three deaths, have been recorded across Europe since the start of 2022, making it the largest diphtheria outbreak the continent has experienced over the last 70 years, according to a study published in the New England Journal of Medicine (NEJM) on June 4.

The outbreak actually began in 2022 among migrants and in 2023 spread to other vulnerable populations in several European countries.  The contagion is reported to be the result of contaminations occurring during migratory travel or in destination European countries, and not in the countries of origin. 

Diphtheria is a highly contagious bacterial infection that can attack the respiratory tract and spread throughout the body, causing a sore throat, fever, and other symptoms. 

Patient samples taken from ten countries showed that 98 percent of the cases were in males with an average age of 18. Almost all had recently migrated. 

Infection came 'as a result' of the migration journeys

"The outbreak, which mainly affected migrants from Afghanistan and Syria, was not the result of people being infected in their countries of origin, but during their migration journeys or in their places of accommodation in European countries," said a joint statement from France's public health agency and the Pasteur Institute.  

The Institut Pasteur, which is one of the authors of the study, released a statement on the findings, underscoring the importance of ensuring adequate immunization levels, especially among vulnerable populations.  

"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," Isabelle Parent du Châtelet, unit manager at national public health agency Santé publique France said.  

Unusual surge in cases in 2022

In 2022, there was an "unusual surge" in the bacteria that causes diphtheria -- Corynebacterium diphtheriae -- in several European countries, notably Germany. The cases were noted to be particularly among recently arrived migrants.

That year 362 cases were recorded by the European Centre for Disease Prevention and Control. 

Most diphtheria patients were reported to have traveled through the Western Balkan migration route. COVID-19-related travel restrictions in many European countries led to overcrowding in migrant camps, straining hygiene and medical services. While one strain of the disease, known as cutaneous diphtheria is generally milder and less likely to cause systemic illness, it often goes undiagnosed due to the need for microbiological testing, Health experts believe that this possibly explains why the outbreak went undetected in transit countries.  

Potentially fatal 

Diphtheria is a serious bacterial infection that primarily affects the respiratory system, causing severe throat inflammation and breathing difficulties. If left untreated, diphtheria can pose life-threatening complications. 

Diphtheria spreads easily through close contact, especially among people who live in close quarters in shared spaces and or share respiratory secretions such as saliva. The infection can also spread through exposure to open sores or ulcers.  

The disease is commonly transmitted through coughing or sneezing which release droplets that others can inhale. People who are not fully vaccinated against diphtheria face a significantly higher risk of infection.  

According to the World Health Organization, among unvaccinated people, diphtheria can be fatal in about 30 percent of cases. For children who are unvaccinated, the risk can be higher.  

Risk for vaccine-preventable diseases 

Low immunization rates in the countries of origin of migrants and refugees, combined with the risky and often unhygienic conditions during the journey to Europe, put many at increased risk of exposure to vaccine-preventable diseases such as diphtheria. 

Studies have found that migrants and refugees generally have lower vaccination rates compared to people born in Europe. 

One major factor behind this disparity is low immunization rates which may be because of limited access to vaccines in countries of origin. However, challenges continue after arrival in Europe, found the report's authors. The transient and often unpredictable nature of migration makes it difficult to complete vaccine schedules that require multiple doses over a prescribed period of time. 

In addition, migrants with irregular or undocumented status may avoid accessing health services due to fear of deportation or being reported to authorities, further reducing access to vaccination. 

The study's authors said that after studying the outbreaks in 2022 and 2023, the data suggested that "a number of actions are needed in Europe to reduce the risk of such outbreaks in the future, including improvement in awareness among migrants, their physicians, and relevant personnel with whom migrants are in contact; thorough vaccination protocols for migrants, local populations, and medical and social care personnel; clinical monitoring of persons who are at risk; rapid diagnosis in symptomatic persons and screening of contacts, with laboratory confirmation of cases; antimicrobial-susceptibility testing to define appropriate antimicrobial treatment and prophylaxis; and whole-genome sequencing of toxigenic strains and sharing of sequencing data to inform local and regional dissemination."

They added that although the World Heath Organization (WHO) has not yet recommended booster vaccinations for older people, their data suggests that it might be a good idea as immunity from the childhood vaccinations tend to wane as a person ages.