Corynebacterium diphtheriae image. CDC

PARIS, France - The largest diphtheria outbreak to strike Western Europe in 70 years has been affecting vulnerable people such as migrants and the homeless since 2022, new research said Wednesday.

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.

For unvaccinated people, it can be fatal in around 30 percent of cases, and is deadlier for children, according to the World Health Organization.

In 2022, there was an unusual surge in the bacteria that causes diphtheria -- Corynebacterium diphtheriae -- in several European countries, particularly among recently arrived migrants, according to a study in the New England Journal of Medicine.

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

Contact tracing and screening helped tamp down the outbreak, but rare infections have continued to be recorded, the researchers said.

A total of 536 cases, including three deaths, have been recorded across Europe since the start of 2022.

Patient samples from 10 countries showed that 98 percent of the cases were in males with an average age of 18. Almost all had recently migrated.

"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 genetic similarities between the strains seen in people from different countries suggests that there was a "recent point of contact, outside the country of origin" at the source of the outbreak, the statement added.

The exact areas affected by the outbreak remain unclear.

But a genetic link between the 2022 strain and the one detected in Germany this year indicates that "the bacteria continues to circulate quietly in Western Europe," the statement said.

Vaccination is very effective at fending off diphtheria, and the researchers emphasized the importance of immunization programs for the general public.

They also called for European nations to do more to ensure their most vulnerable people avoid contracting the disease.

That included raising awareness of the symptoms among doctors and those in contact with migrants and the homeless, as well as increasing access to vaccines and antibiotic drugs.

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