Credit NIAID
#18,853
It is not often that we find ourselves talking about seasonal influenza A in August, but Denmark's SSI is reporting an unusually early appearance of H1N1 which is linked to a large (60K) music festival (Smukfest in Skanderborg held Aug 3rd-10th).Beyond the early appearance of flu (which the SSI compares to a `super-spreader event'), the SSI notes unspecified genetic changes in the virus:
The current outbreak is caused by a variant of influenza A H1N1, which is similar to the viruses that circulated in the 2024/25 season, but with some unique changes.
While it isn't revealed what those `unique changes' might entail, I imagine we'll see a more detailed follow-up (perhaps in Eurosurveillance or on the ECDC website) in the days ahead.
Whether this particular variant has `legs', and will spread further across Europe, remains to be seen. There is no mention of increased severity of this virus, but how well it matches this year's vaccine is obviously a concern.
For now, the only details we have come from the following (translated) statement by Denmark's SSI.
Unusually early flu outbreak can be traced to major festival
The Statens Serum Institut (SSI) has detected a major outbreak of influenza A, which is very likely linked to this year's Smukfest in Skanderborg. This is the first time in 15 years of monitoring that such an early influenza outbreak has been registered in Denmark.
Last edited on August 27, 2025
A number of cases of influenza A H1N1 can probably be traced back to Smukfest in Skanderborg earlier in August.
By examining samples from infected people and interviewing them, the Statens Serum Institut (SSI) has been able to demonstrate that several of the sick were infected with an almost identical virus, and that they had all participated in the festival in week 32.
“Everything points to the outbreak being linked to Smukfest. All those infected with the identical virus had been at the festival, while the other cases had a different source of infection. Therefore, we can very likely link the outbreak to this particular event,” says Ramona Trebbien, section manager at the Statens Serum Institut.
Unique find in 15 years of monitoring
Influenza is a disease that typically only causes major infections in the winter months. It is therefore unusual to see a major outbreak in mid-August.
“This is the first time in the 15 years that we have monitored influenza transmission year-round that we have seen an outbreak at this time of year. It is very unusual, and we are following the development,” says Ramona Trebbien.
The outbreak was identified quickly, among other things, because SSI monitors influenza cases nationally and runs a so-called sentinel surveillance, where selected doctors and laboratories submit samples year-round.
In week 33, influenza A cases nationally increased to 115 from 19 cases the week before. Likewise, 15 out of 56 samples from the sentinel surveillance were positive for influenza, which confirmed the signal of an actual outbreak.
Keeping an eye on developments
The current outbreak is caused by a variant of influenza A H1N1, which is similar to the viruses that circulated in the 2024/25 season, but with some unique changes. SSI is now monitoring the incidence and can add that there has been no further increase in infections this week.
“The weather plays a role in how the infection develops, so we are monitoring whether the good weather can help break the chains of infection,” says Ramona Trebbien.
SSI emphasizes that there is no reason to warn against attending festivals in general, but that the Smukfest episode shows that large events can in rare cases act as super-spreading events if the circumstances are right.
For most healthy people, influenza is an unpleasant but transient illness that typically goes away on its own after a week or so with fever, muscle aches and cough.
However, for young children, pregnant women, the elderly and people with chronic diseases, influenza can in some cases be serious and cause complications. If you have an underlying medical condition or become very ill, you should contact your doctor for advice and possible treatment. If you experience symptoms of influenza, it is recommended that you stay home, rest and drink plenty of fluids.
How to reduce the spread of infection
If you want to avoid getting infected – and infecting others – there are a few tips you can follow:You can read more about influenza and prevention of the disease on SSI's website .
- Avoid close contact with people you know are sick.
- Stay home from work, school, institutions and the like if you are sick.
- Cough and sneeze into a disposable tissue or, if necessary, your sleeve.
- Wash your hands frequently with soap and water – especially after coughing, sneezing or blowing your nose. You can also use hand sanitizer.
- Avoid touching your eyes, nose or mouth without washing your hands first. This will reduce the risk of infection, even if you have contracted the virus on your hands.
- Keep common items and surfaces clean with regular cleaning
