Is the K variant more worrying this year?
Dr Gérald Kierzek: Above all, this variant makes the flu much more contagious. Result: more cases, and therefore potentially more hospitalizations. On the other hand, it does not really seem more virulent than other A(H3N2) viruses, except in fragile people, in whom severe forms are more frequent.
Concretely, the K variant is a subtype of the A(H3N2) virus which has accumulated several mutations. This allows it to be transmitted more easily and to partially evade immunity. We are therefore observing a very early epidemic in 2025, with mechanically more complications and deaths among the elderly or vulnerable, even if the severity of the virus itself does not seem fundamentally different.
Are masks and barrier gestures really useful?
Yes, clearly. Health authorities are insisting this winter on getting back to good habits: wearing a mask as soon as you have symptoms, washing your hands regularly and ventilating the rooms. An interministerial text published in November 2025 also specifies that the mask is again compulsory or strongly recommended in certain health establishments, when the virus is circulating a lot, to protect the most vulnerable.
In practice, wearing a mask in transport, crowded enclosed places, when visiting vulnerable people or if you yourself are sick is strongly recommended with this very contagious variant.
Is the current vaccine effective against this variant?
The 2025-2026 vaccine was designed before the complete appearance of the K variant. It is therefore possible that its effectiveness will be a little less good against this H3N2 than if the match was perfect.
But the first data, particularly from the United Kingdom, are rather reassuring: the vaccine continues to protect well against serious forms and hospitalizations. We are talking about an effectiveness of up to 75% in children and around 30 to 40% in adults for severe forms. It’s not perfect, but it’s far from negligible.
Is there still time to get vaccinated?
Yes. Vaccination remains recommended every year for those aged 65 and over, pregnant women, people suffering from chronic illnesses, immunocompromised people, nursing home residents and those close to vulnerable people.
Among those over 65, reinforced vaccines such as Efluelda or Fluad are now preferred. They reduce hospitalizations by 15 to 25% compared to traditional vaccines.
Even as the holidays approach, getting vaccinated remains useful. It takes 10 to 14 days for protection to be optimal, but even late vaccination provides a start of protection and reduces the risk of serious illness, for yourself and your loved ones.
How long are we contagious? Can you get the flu twice?
With the flu, you are contagious about 24 hours before the first symptoms, and then generally for 5 to 7 days after their appearance in a healthy adult. The peak of contagiousness is in the first 3 to 5 days. In children or immunocompromised people, this can last up to 10 days or more.
After an infection or vaccination, we are in principle protected for the season, but this protection is neither total nor absolute. Reinfection remains possible, especially with a variant like K which partly escapes immunity. On the other hand, the risk of serious form is significantly reduced.
As the holidays approach, the message is therefore simple: vaccination when eligible, and barrier gestures targeted around vulnerable people, masks, ventilation, and avoid close contact in the event of symptoms.