No 1/3 - 2026

Infectious diseases 2025

• New RSV vaccination program for pregnant women
• Intense respiratory infection season and an unusual influenza outbreak in late summer
• Mpox subclade Ib: local transmission in several European countries among men who have sex with men. No cases have yet been detected in Denmark
• Sharp increase in findings of wild birds infected with highly pathogenic avian influenza as well as outbreaks in poultry farms

New vaccination program for pregnant women

From 1 October 2025, an RSV vaccination program for pregnant women came into effect. The purpose is to protect the newborn child during the first months of life, when the risk of severe disease is highest. From 2021–2023, there were on average 1,741 hospital admissions per year among infants, of whom 30% received intensive care. Pregnant women are offered the vaccination by their general practitioner in connection with the pregnancy check-up in week 32, and the offer applies up to and including week 36 of pregnancy. By the end of 2025, 63 percent of pregnant women had been vaccinated. This creates an expectation of an effect in the form of a reduced number of hospital admissions caused by RSV already in the current season.

This season’s vaccination program ends on 31 January 2026, and the next season’s program starts on 1 May 2026.

Surveillance of respiratory infections

SSI conducts year-round surveillance of detected cases and hospital admissions not only for the three well-known viruses – influenza, RSV, and SARS-CoV-2 – but also for the bacterial infection Mycoplasma pneumoniae, while surveillance of severe acute respiratory infections (SARI) also includes Streptococcus pneumoniae infections.

The 2024/25 respiratory season was intense, and already during the summer and autumn of 2024 an increased occurrence of both SARS-CoV-2 and M. pneumoniae was observed. At the end of November, the number of influenza and RSV cases also began to rise, and for influenza both the number of detected cases and the number of hospital admissions were at a high level for several weeks. This led to peaks in the burden of infections, hospital admissions, and SARI admissions (admissions with severe respiratory infection) in February 2025. There was very high uptake of the vaccination programs against influenza and COVID-19, at 76% and 75% respectively among those aged 65 years and older. By comparing vaccinated with unvaccinated individuals, it was observed that the vaccines effectively protected against hospital admission and death. In the current 2025/26 season, influenza was at a high level at the end of December and COVID-19 at a medium level.

In August, a very unusual increase in the number of detected influenza cases was observed. Examination of the viruses showed that the individuals were infected with almost identical viruses, and telephone interviews with those infected revealed that they had all attended the same music festival. The influenza outbreak could be identified very quickly because SSI conducts year-round surveillance of detected influenza cases and also operates so-called sentinel surveillance, in which selected physicians submit samples. A questionnaire survey among the participants showed that nearly 5,000 (40%) had experienced influenza-like symptoms in connection with the festival. Although it cannot be concluded with certainty that everyone who experienced influenza-like symptoms was infected with influenza, the study shows that this was a very large outbreak, but that the many infected individuals did not lead to widespread transmission in the population after the festival.

If you would like to help SSI gain more knowledge about infectious respiratory diseases, it is possible to participate in the Influmeter project , which contributes to the overall disease surveillance of respiratory infections. Influmeter is a digital surveillance project in which citizens report once a week whether or not they have influenza-like symptoms. It is a voluntary self-reporting system that provides a unique overview of disease spread in Denmark – including among people who rarely contact the healthcare system.

Local transmission of Mpox subclade Ib in Europe

During 2025, local transmission of Mpox (formerly called monkeypox) subclade Ib was observed in several European countries among men who have sex with men. This represents a change from earlier, as subclade Ib, apart from a few travel-related cases, has been limited to Central Africa. The Mpox outbreak in Europe with onset in 2022, which primarily occurred among men who have sex with men, was predominantly subclade IIb. The European Centre for Disease Prevention and Control (ECDC) has assessed that the overall risk of infection with Mpox subclade Ib in Europe is moderate for men who have sex with men and low for the general population in Europe.

Denmark is well prepared to identify infected individuals through robust diagnostics and surveillance. Since 2022, 224 people have tested positive for subclade IIb in Denmark, the vast majority in 2022 (192), and only 18 cases in 2025. No one in the country has tested positive for subclade Ib. A total of 6,111 people in risk groups for Mpox infection have been vaccinated against the disease since mid-2022.

Sharp increase in avian influenza

Prompted by a sharp increase in findings of avian influenza (HPAI) in wild birds in Denmark and neighboring European countries, and outbreaks in poultry flocks in Denmark, the Danish Veterinary and Food Administration raised the threat level for avian influenza from medium to high at the end of October 2025. This has the consequence that rules are activated to prevent wild birds from coming into contact with domestic birds and infecting them with avian influenza. No transmission to humans has yet been observed in Europe, but the increasing number of reports of transmission to mammals globally continues to raise concern about whether the virus may adapt to infect humans more efficiently. The situation is therefore closely monitored. In Denmark, 12 people were tested and all were negative. Testing typically took place in connection with the development of influenza-like symptoms following breaches of personal protective equipment during culling in poultry flocks.

Foodborne disease outbreaks 2025

Two unusually large disease outbreaks with an estimated 900 laboratory-confirmed cases of Campylobacter infection took place during the summer of 2025. Campylobacter jejuni can cause diarrhea, abdominal pain, vomiting, fever, and in rare cases serious complications such as arthritis or the neurological disorder Guillain–Barré syndrome. In these outbreaks, around half of the diagnosed cases were hospitalized. By comparing patient samples with findings of Campylobacter jejuni in food production, it was shown that the outbreaks were caused by Danish chicken meat.

Campylobacter is the most common cause of foodborne bacterial infection in Denmark, and chicken meat remains the most common source of Campylobacter infections.

In August 2025, SSI identified a new outbreak of invasive listeriosis. Based on interviews with patients, as well as trace-back investigations and product analyses, it was possible to identify ready-to-eat fish cakes as the source of the outbreak. A total of 11 people aged between 30 and 95 years became ill. All patients were hospitalized. The fish cakes were recalled, which likely prevented additional cases.

Listeriosis is a serious foodborne infection that can lead to sepsis and meningitis in individuals who are immunocompromised. A general increase in infections with Listeria monocytogenes in the EU has prompted the Commission to introduce stricter requirements for producers of ready-to-eat foods from July 2026. The amended EU rules introduce zero tolerance for Listeria bacteria in food. This means that there will be a requirement for absence of Listeria monocytogenes throughout the entire shelf life, unless the producer can document that the bacteria cannot grow to a level harmful to health. Both companies and authorities will thus have better tools to prevent future outbreaks.

November and December 2025 were characterized by two Salmonella outbreaks in Denmark. One outbreak involved 29 registered cases with the somewhat unusual type Salmonella Strathcona, where the source of infection is known to be Italian small tomatoes, which over several years have caused a larger European outbreak. The other was a larger outbreak with 47 cases of Salmonella Typhimurium, where the source of infection is still under investigation at the time of writing.

New surveillance systems established for marine bacteria and tularemia

Tularemia, also known as rabbit fever, is caused by the bacterium Francisella tularensis. With the revision of the “Executive Order on Notification of Infectious Diseases” in autumn 2023, it became possible to monitor tularemia via MiBa, and clinical microbiology departments were required to submit isolates to SSI for further characterization. As of 31 October 2025, 32 cases of tularemia had been confirmed in Denmark. There was a predominance of men, and a seasonal pattern was observed, with the number of cases and samples increasing during the summer months and peaking in August and September, EPI-NEWS 49b/2025.

In 2025, infections with “sea bacteria” could also for the first time be continuously monitored via MiBa. Sea bacteria consist of specific species of the bacteria Vibrio and Shewanella. The bacteria thrive best in brackish water at high temperatures, and infections in Denmark typically manifest as ear or wound infections in connection with bathing in inner waters during the summer. Infections are especially common in summers when bathing water temperatures remain high, which was not the case in 2025.

The advisory function has had a busy year

SSI provides advice to healthcare professionals on vaccination programs and – in accordance with current guidelines – the provision of vaccines and immunoglobulin for the prevention of infection following exposure. The number of inquiries increased from 7,161 in 2024 to 7,630 in 2025.

Among the reasons for the increase is that more Danish holiday travelers are suspected of having been exposed to rabies when bitten or scratched by animals abroad or by bats in Denmark, and are therefore offered preventive treatment. In 2023, the number of treatments was 281; in 2024, a total of 340 people in Denmark received treatment for possible infection – the vast majority, 296 people, after travel abroad, especially to Thailand and Turkey. The trend continued upward to 534 people in 2025, which is the highest number recorded at SSI. The increase underscores the need to strengthen the focus on rabies prevention in connection with travel vaccination counseling for people traveling to countries where the disease is endemic, EPI-NEWS 40/2025.

Poliovirus detected in wastewater in Germany

Polio is an acute viral infection that can cause permanent and disabling paralysis. On 14 November 2025, the German health authorities reported the detection of wild poliovirus type 1 (WP1) in a wastewater sample in Hamburg, while no cases of polio have been detected in humans in Germany. Europe therefore remains a polio-free region. A high vaccination coverage such as in Denmark, where 96% are vaccinated in the first year of life, can minimize the risk of polio outbreaks.

Detection of wild poliovirus in wastewater is an unusual but generally not unexpected event. WP1 is known to circulate only in Afghanistan and Pakistan, with 9 and 30 detected cases respectively in 2025. In recent years, WP1 has also been detected outside these two countries: in environmental samples in Iran (2019) and in confirmed cases in Malawi (2021) and Mozambique (2022).

Lowest number of people infected with HIV since 1990

This year’s HIV report shows that the positive trend from recent years continues, with an overall decline in first-time HIV diagnoses. Among men who have sex with men, the number is also declining and is – as last year – the lowest since 1990, EPI-NEWS 49a/2025.

A total of 202 HIV cases were reported, including 103 first-time diagnoses. Half of the new diagnoses concerned people born in Denmark and half people born abroad. As last year, people already diagnosed abroad constituted a large proportion, including many from Ukraine.

The decline in new cases among men who have sex with men is mainly attributed to early initiation of treatment and preventive medication (PrEP), which remains a key tool in the Danish response.

SSI assists in establishing a new network for mortality surveillance in Latin America and expands with a new participating country in Europe

A new network for mortality surveillance – AMoMO (Americas Mortality Monitoring) – has just been launched by the Pan American Health Organization (PAHO) in collaboration with WHO and national representatives from 14 countries in the region. The AMoMO network aims to strengthen the region’s preparedness for future pandemics and other public health threats.

The network is inspired by EuroMOMO’s methods, principles, and organization, which were developed through European collaboration and are coordinated by SSI.

EuroMOMO is a joint European mortality surveillance system used to register and analyze fluctuations in mortality across countries to provide an early picture of excess mortality – such as during pandemics and seasonal epidemics of influenza and COVID-19. Latvia joined the network in 2025, which now consists of 27 countries and regions. The system is used as a basis for public health assessments and decision-making.

SSI becomes coordinator for two EU reference laboratories

The EU has designated EU reference laboratories (EURLs) in relation to infectious diseases, and two of the new EURLs will be coordinated by SSI. EURL-PH-AMR is the reference laboratory for antimicrobial resistance and started in 2025. EURL-PH-FWDB, the reference laboratory for food- and waterborne bacteria, will begin work in January 2026. Both have been designated for a 7-year period.

The role of the reference laboratories is to strengthen preparedness and countries’ ability to manage serious cross-border health threats. The reference laboratories are to coordinate a network of national laboratories and support them with services such as reference diagnostics, test protocols, quality programs, training, and technical assistance. In addition, the reference laboratories are to provide advice to ECDC, for example in connection with outbreaks.

With these words, we wish all readers of EPI-NEWS a Happy New Year.

(P. Valentiner-Branth, Department of Infectious Disease Epidemiology and Prevention)