Although swine influenza viruses have changed in recent years, they have not combined with influenza viruses that affect humans or birds. In order to identify potential risks for humans and animals in a timely manner, it is important to continue monitoring swine influenza viruses. These findings are from research conducted by RIVM, Royal GD Animal Health, Erasmus Medical Centre (Erasmus MC) and Wageningen Bioveterinary Research (WBVR).
From 2022 to 2025, RIVM, Royal GD, Erasmus MC and WBVR investigated which swine influenza viruses were circulating at Dutch pig farms and whether these viruses were mutating. This is relevant because humans and pigs can also be infected with each other’s influenza viruses, as well as with avian influenza viruses. If a pig is infected with multiple influenza viruses at the same time, these viruses may combine into a new influenza virus variant. Although this is very unlikely, if such a virus were able to infect humans easily, it could cause many cases of influenza at the same time, affecting many people at once. For that reason, it is important to know which influenza viruses are circulating in the Netherlands – including among pigs.
Research on samples from pigs
Assisted by veterinarians, Royal GD collected samples from pigs, including nasal mucus and saliva. Researchers at Royal GD tested these samples for influenza viruses. Between 1 July 2023 and 16 September 2025, they tested 2237 samples taken from pigs. They found flu viruses in 484 of these samples.
Research of genetic material
The research partners conducted further testing to identify genetic characteristics of some of the viruses found in the samples. They then shared genetic data with each other using an online platform, which contains data from 251 swine influenza viruses.
The research showed some exchange of genetic information between different viruses, indicating that the viruses are evolving. Similarities with human influenza viruses were also found a number of times, indicating that viruses had been transmitted from humans to pigs. No new variants were found in which a swine influenza virus had combined with a human influenza virus or an avian influenza virus.
Protection from medicines and vaccines
25 viruses were studied to assess how effectively they responded to medicines that are given to people who have severe influenza. These antiviral medicines also proved effective against the swine influenza viruses that were tested. This research also helps to understand vaccine effectiveness in protecting against these flu viruses. This information makes it possible to adapt vaccines for pigs, as well as vaccines that protect humans in the event of a possible pandemic.
Comparison with other countries
Since the viruses keep changing over time, it is important to continue monitoring swine influenza viruses. It is also useful to compare the viruses found in the Netherlands to those found in other European countries, since viruses also spread across national borders. The information obtained from the research is shared with the World Health Organization (WHO) for the purpose of vaccine development in the context of pandemic preparedness.