In February each year, the World Health Organization (WHO) makes an assessment of the strains of flu virus that are most likely to be circulating during the following winter in the northern hemisphere. Based on this assessment, WHO recommends which three flu strains the vaccines should contain for the forthcoming winter. Vaccine manufacturers then produce flu vaccines based on the WHO recommendations. These flu jabs are used for the countries in the northern hemisphere, not just the UK.
Production of the vaccine starts in March each year after the WHO announcement. The vaccine is usually available in the UK from September.
How the vaccine is made
The seasonal flu vaccine contains three different types of flu virus (usually two A types and one B type). For most vaccines, the three strains of the viruses are grown in hens’ eggs. The viruses are then killed (deactivated) and purified before being made into the vaccine.
How it protects you
The vaccine causes your body’s immune system to make antibodies to the flu virus.
Antibodies are proteins that recognise and fight off germs that have invaded your blood, such as viruses. If you catch the flu virus later on, the immune system will recognise it and immediately produce the antibodies to fight it.
It may take 10–14 days for your immune system to respond fully after you have had the flu injection.
The antibodies against the flu strains will gradually decrease over time and the flu strains can change from year to year. You need to have a flu jab every year to ensure the best protection against the latest strain of the virus.
Read more about how vaccines work.
Are there any side effects?
The seasonal flu jab does not usually cause side effects. Sometimes, it can cause mild fever and slight muscle aches for a day or so.
The flu jab cannot cause flu as there are no active viruses in the vaccine. However, people sometimes catch other flu-like viruses, or very occasionally could catch flu before the vaccine takes effect.
Allergic reactions to the vaccine are rare.