"Protection from the vaccine takes about two weeks to develop. Risk groups should not wait," said Hanna Nohynek, chief physician at THL.
The early onset is part of a wider global trend. Several countries across Europe and elsewhere have already reported similar developments.
In Finland, laboratory-confirmed influenza infections have been increasing throughout November. THL noted that influenza A viruses had undergone genetic changes over the summer, with a mutated subtype of H3N2 now dominant among tested samples.
This new subtype differs structurally from the virus strain used in the current seasonal flu vaccine.
"If the circulating virus differs from the vaccine strain, the vaccine's protection may be weaker than expected," said Nohynek. "Still, vaccination remains the best available protection."
In addition to the mutation, other factors affecting vaccine effectiveness include age, chronic illnesses, treatments that suppress the immune system, and the time since vaccination.
THL recommends influenza vaccination for several population groups, including all children under seven, people aged 65 and over, pregnant women, those with chronic illnesses, conscripts beginning their military service, healthcare and pharmaceutical personnel, and people living in institutional settings. Close contacts of individuals highly vulnerable to influenza are also advised to get vaccinated.
Influenza viruses are known for their rapid genetic shifts. Multiple strains typically circulate during each season. The presence of several virus types and their potential mismatch with the vaccine formula is a recurring challenge in public health planning.
In addition to vaccination, THL advised the public to take standard preventive measures against respiratory infections. These include staying home when ill, maintaining good hand hygiene, coughing or sneezing into one's sleeve, and using masks when needed.
Indoor ventilation and social distancing were also recommended, especially in crowded or enclosed spaces.
THL also reminded the public that several other respiratory viruses are currently in circulation. These include viruses responsible for common colds and other flu-like illnesses.
Most flu cases can be treated at home with rest and symptom relief. Antiviral medications may be prescribed in certain cases but are most effective when started early.
HT