Influenza is a fever-causing illness caused by the influenza virus. The most significant subtypes for humans—Influenza A and B—trigger seasonal epidemics in Finland during winter and early spring. The virus spreads via droplets and surface contact when an infected person coughs or sneezes. Regular handwashing helps prevent transmission.
Influenza symptoms are more severe and abrupt compared to the common cold. Alongside high fever, affected individuals often experience muscle and headache pain, sore throat, and cough. The illness typically lasts 3–8 days.
Secondary bacterial complications can occur. In children, otitis media (middle ear infection) is common, while adults may develop sinusitis or pneumonia. These complications may arise directly from the influenza virus or from subsequent bacterial infections. Influenza also raises the risk of cardiovascular events, such as heart attacks, especially in individuals with heart disease, and can worsen conditions like asthma or COPD.
Rare complications include myocarditis, pericarditis, meningitis, neural inflammation, or muscle inflammation. These secondary infections can be deadly, particularly for the elderly, young children, and individuals with weakened immune systems due to chronic illnesses.
Children account for a significant portion of flu cases. Before childhood flu vaccinations were introduced, around one in four children contracted influenza each winter. The virus spreads easily in daycare settings because children tend to transmit it more efficiently and for longer durations than adults. They also play a key role in community-wide spread.
In a Finnish study, 40% of children under age three who got the flu developed a middle ear infection afterward. Children may also vomit or have febrile convulsions. The illness may require hospitalization, particularly if they develop pneumonia, breathing difficulties, or a decline in overall well-being.
Influenza viruses mutate continuously, which means prior flu infections or past vaccinations may not protect against newly emerged strains. Sometimes, these mutations can lead to more dangerous forms of the virus that swiftly spread through populations, increasing mortality.
The risk of such drastic viral changes is heightened when human and animal influenza viruses mix—commonly through hosts like birds, pigs, or cattle. Among influenza strains, only Influenza A infects both humans and animals. When such mixing occurs, it can lead to global pandemics, which result in significantly more illness and death than typical seasonal outbreaks.
After a pandemic, the newly dominant virus strain usually replaces older types and becomes the new seasonal flu virus. This occurred with the 2009 H1N1 "swine flu" pandemic. Other notable pandemics occurred in 1918, 1957, 1968, and 1977.
Influenza vaccines contain inactivated (non-living) viral components. In Finland, they fall into two main categories:
There are also adjuvanted injected vaccines for individuals over 65; in Finland, these are offered to those over 85 and immunocompromised individuals aged 50 and up.
Because the virus mutates rapidly, vaccines are updated annually. The WHO issues strain recommendations each year, guiding vaccine composition. Current vaccines are either trivalent (two Influenza A strains and one B strain) or quadrivalent (adding a second B strain). Recently, due to the absence of one B strain (Yamagata), WHO now recommends trivalent formulations. During the 2024–2025 flu season, some vaccines may still be quadrivalent due to production lead times.
Effectiveness varies annually, depending on the match between vaccine and circulating strains. Vaccine efficacy against seasonal flu averages 40–70%. It also reduces the risk of flu-related secondary infections. In high-risk groups, vaccination significantly lowers hospitalization and mortality rates.
Influenza vaccination is free for certain groups:
The National Health Institute issues annual recommendations on eligibility. Others may receive the vaccine by prescription at their own expense. The flu vaccine only protects against influenza, not the common cold or run-of-the-mill cold viruses.
Typically, one annual dose is sufficient. However, children under nine receiving their first vaccine may require two doses spaced one month apart. Vaccination is best administered in November, ahead of flu season. Travelers to the Southern Hemisphere should get vaccinated in spring, as their flu season occurs from April to September.
Common side effects include mild local reactions and flu-like symptoms such as headaches, muscle pain, and joint pain. Rare cases include arthritis, nerve inflammation, or facial paralysis post-vaccination. Individuals with severe egg allergies cannot receive the vaccine, as it is produced in fertilized chicken eggs.
FVR is researching more effective flu vaccines—especially for older populations—using:
These innovations could enable faster responses during pandemics. Other goals include developing combination vaccines targeting flu and respiratory viruses like COVID-19 and RSV, and the long-sought universal flu vaccine targeting stable core viral proteins. Such a vaccine might eliminate the need for yearly shots and protect against pandemic strains.
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