Opinion of the Commission on Zoonoses and Viral Infections of Animals on H5N1

Opinion of the Commission on Zoonoses and Viral Infections of Animals on H5N1

Highly pathogenic avian influenza subtype H5N1 is a zoonotic and potentially life-threatening viral infection caused by influenza viruses of the influenza A genus. H5N1 belongs to the subtypes of avian influenza that mainly affect wild birds (especially waterfowl) and are regularly transmitted from there to domestic poultry. Since its first appearance in Scotland in 1959, the virus has spread worldwide and established itself in wild bird populations, which serve as a permanent reservoir.

In recent years, there have been repeated outbreaks of H5N1 in wild birds and domestic poultry in Europe. Viruses are repeatedly introduced into poultry farms, particularly during the migratory bird season. In 2025, H5N1 infections were also confirmed in wild birds and poultry farms in several European countries, including Germany, Hungary and Denmark. In Germany, the situation has recently worsened significantly: following detections in wild birds in all federal states, there has been an increasing number of cases in poultry farms. Several hundred thousand chickens, turkeys, ducks and geese have already had to be culled to contain the spread. Farms in northern Germany are particularly affected, where large-scale protection and surveillance zones have been set up.

While the H5N1 virus has traditionally circulated mainly in birds, a widespread infection in dairy cows in the United States was described for the first time in 2024. This outbreak represents a new epidemiological situation. In early 2024, infections with the H5N1 virus were detected in lactating animals in dairy herds in several US states. By the end of the year, over 600 farms had been affected. Clinically, the infected animals primarily showed a decline in milk production and a change in milk consistency. The pathogen was detected in the mammary gland and in raw milk samples. The pasteurization of H5N1-contaminated milk leads to the complete inactivation of these viruses.

Molecular analyses have shown that the H5N1 viruses from the cattle herds have mutations that enable them to bind to mammalian receptors and therefore also to human receptors. This means that they differ in part in their host specificity from classic H5N1 strains, which prefer to bind to avian receptors. Two genetically different virus lines have been identified in the USA: the previously dominant strain B3.13 and a newly described isolate D1.1. These findings indicate several independent transmissions from wild birds to cattle. On-farm spread occurs mainly via contaminated milking utensils. To date, there is no evidence of efficient animal-to-animal transmission within dairy herds. However, various studies show that these H5N1 viruses can be transmitted repeatedly between mammals.

In connection with the cattle outbreaks, a small number of human infections were also documented in the USA, predominantly among employees on dairy farms with close contact with animals. The courses of illness here were mild, mostly with localized symptoms such as conjunctivitis. Human-to-human transmission has not yet been observed. Accordingly, the US health authorities as well as the WHO and WOAH currently assess the risk for the general population as low.

A general vaccination against H5 viruses in humans is currently not indicated and is not recommended by the STIKO. However, the recommendation for annual seasonal influenza vaccination has been extended to people who come into regular and direct contact with poultry or wild birds [1]. Although this vaccination does not protect against avian influenza viruses such as H5N1, it can reduce the risk of double infection with a seasonal and an avian influenza virus and thus make it more difficult for novel, potentially pandemic reassortants to emerge. For people who are at particular risk of possible contact with large quantities of the virus, a vaccine to protect against H5 viruses would certainly be desirable, but is not currently available in Germany. An H5 vaccine approved in the EU has already been used in Finland in recent years for people who are particularly at risk.

The discovery of new H5N1 isolates in mammals, particularly in dairy cows, marks a new situation in the epidemiology of this virus. For the first time, it has been shown that H5N1 viruses can circulate efficiently in previously atypical hosts. This underlines the considerable adaptive potential of the virus and highlights the urgency of regular monitoring and preventive measures in susceptible farm animals to prevent further adaptations to mammals.

H5N1 continues to pose a significant threat to poultry populations. Outbreaks regularly lead to high animal losses and considerable economic damage. Measures taken to date, such as bans on keeping poultry indoors, hygiene rules and the culling of affected flocks, primarily serve to minimize the exposure of livestock. These strategies have so far been sufficient for localized, controllable outbreaks, but are reaching their limits in view of the increasingly endemic situation and the ongoing risk of introduction from wild birds.

A sensible next step is therefore the targeted use of suitable vaccines to reduce animal losses, economic damage and the spread of the virus. In the EU, vaccinations against H5N1 were approved in 2023 under strict monitoring requirements (EU Delegated Regulation 2023/361). Two H5N1 vaccines have already received EU approval. These vaccines have not yet been approved in Germany, but the manufacturers are currently preparing corresponding applications. Several countries, including France, Italy, the Netherlands and Germany, have introduced or are currently preparing limited vaccination programs to test the efficacy in various poultry species.

The available H5N1 vaccines, based on recombinant H5 antigens, vector vaccines or mRNA constructs, show a good protective effect against clinical disease and significantly reduce virus shedding. According to EU regulations, the vaccines available to date must be dead vaccines, i.e. they themselves do not pose a risk of infection. Vaccination in risk areas can significantly reduce the risk of mass culls.

However, according to the EU directive, vaccination strategies must be closely combined with strict surveillance measures in the sense of virological monitoring in order to keep an eye on the development of undetected viral infections under the vaccination blanket. Vaccination programs do not replace the need for biosecurity measures and epidemiological surveillance, but must be closely interlinked with them.

In practice, however, this comprehensive monitoring represents a considerable organizational and economic challenge, as it places high demands on personnel, laboratory capacity and financial resources. Although rapid antigen tests are available, they are less sensitive. Therefore, more practical testing strategies need to be developed, such as adapted testing frequencies in fattening poultry flocks or the supplementary use of environmental, water or pooled samples in combination with serological tests.

The current situation impressively illustrates the close link between animal, environmental and human health, as enshrined in the One Health concept. Only through coordinated surveillance, research and prevention measures can the risk to livestock be limited and the zoonotic potential of newly emerging avian influenza viruses be identified at an early stage.

In summary, the Commission and the Executive Board of the GfV support

• the STIKO vaccination recommendation for people with contact to wild birds/poultry
• calls for changes to immunization options against highly pathogenic avian influenza viruses in poultry
• the introduction of practicable testing strategies for the continuous monitoring of vaccinated livestock and
• continuity in the area of research and development on animal diseases with zoonotic potential.

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[1] Epidemiological Bulletin 29/2025, pages 11-22, retrieved on 10.11.2025: https://www.rki.de/DE/Aktuelles/Publikationen/Epidemiologisches-Bulletin/2025/29_25.pdf?__blob=publicationFile&v=6