No 18a - 2026

Cases of mpox clade Ib detected for the first time in Denmark

Since 12 April 2026, several cases of mpox (formerly known as monkeypox) have been identified in Denmark, caused by the mpox virus variant clade Ib. This includes both imported cases and cases acquired in Denmark.

The cases in this period are the first detected cases of mpox in Denmark since June 2025.

Mpox virus

Mpox is caused by a virus, MPXV, which belongs to the same family as smallpox viruses. There are two main groups of MPXV, clade I and clade II. Regardless of clade, the disease is less severe than smallpox. Transmission between humans occurs through close contact, including sexual contact. For further information, see SSI’s disease dictionary.

Mpox virus has been known for many years, primarily in African countries, especially in Central and West Africa. The variants in these regions differed slightly, with the Central African variant referred to as clade I and the West African variant as clade II.

In 2022, mpox spread from Africa to large parts of the world, causing an epidemic in countries outside Africa, including in Europe. Most cases occurred among men who have sex with men (MSM) with many sexual partners. Genetic analyses showed that the epidemic was mainly caused by clade IIb. The epidemic in Europe was controlled through preventive measures and vaccination of high-risk groups. Since then, only sporadic cases have been seen in Europe.

In 2024, an outbreak occurred in the Democratic Republic of Congo and spread to neighboring countries (Burundi, Rwanda, Uganda and Kenya). This outbreak was caused by a new variant, clade Ib, first identified in 2023. There were concerns that clade I could lead to more severe disease than clade II and cause a larger epidemic and spread beyond these countries.

In autumn 2025, the first cases of clade Ib were detected in European countries, Spain, the Netherlands, Italy and Portugal. Similar to the 2022 outbreak, most cases were among MSM with many sexual partners, with transmission occurring in settings such as festivals or clubs with sexual activity.

Symptoms of mpox

Infection with mpox virus often presents with general symptoms such as fever, chills, headache, muscle pain and fatigue. Swollen lymph nodes are also common. Within 1–3 days after symptom onset, a rash with blisters may develop, sometimes limited to the genital or anal area. The blisters may be subtle and resemble ingrown hairs but can also be very painful.

Most cases in Europe since 2022 have been mild and rarely required hospitalisation. Only a few deaths have been reported. No difference in severity has been observed between clade Ib and clade IIb in Europe.

There is no specific treatment for mpox. The disease is self-limiting.

Mpox clade Ib in Denmark

The Danish cases of clade Ib seen since 12 April do not differ epidemiologically from earlier clade IIb cases. They mainly involve MSM, with transmission occurring in sex clubs and similar settings.

All cases have been mild, and in several cases only minor skin changes have been observed.

Whole genome sequencing of clade I cases shows found in Denmark show that there have been multiple introductions originating from the same limited geographical area.

Assessment of the situation

It is not surprising that cases of clade Ib are now being observed in Denmark. Since autumn 2025, clade Ib has been seen in several European countries, both as imported cases and cases acquired within Europe. Transmission occurs in the same groups as clade IIb, and it was therefore expected that clade Ib would eventually be detected in Denmark.

Prevention

As with clade IIb, the most important preventive measures are testing, contact tracing, self-isolation and vaccination. As many cases are mild, broad testing is important, including for atypical skin lesions that do not resemble pox-blisters if cases have relevant exposure.

See further details in the Danish Health Authority’s guidelines on mpox..

It is important to emphasize that vaccination coverage has a direct impact on the clinical presentation of the disease, as vaccination protects against severe outcomes. It is therefore still recommended that individuals in risk groups get vaccinated if they have not yet been vaccinated.

(A. Koch, M. Wessman, Department of Infectious Disease Epidemiology and Prevention, U. Schneider, Virology and Microbiological Preparedness)