Flu vaccines have an excellent safety record. They are the best protection we have against an unpredictable virus which can cause severe illness and deaths each year among at-risk groups. It is important to have a flu vaccine every year because the flu virus is very variable and changes over time. Each year there are different strains around, and a new vaccine has to be prepared to deal with them. Vaccination from previous years is not likely to protect people against current strains of flu.
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Each year’s flu vaccine is made to give the best protection against the strains of flu that are expected to circulate in the coming season. The trivalent vaccine protects against three of the flu virus strains which are most likely to be around. The quadrivalent vaccine protects against four flu virus strains. However, decisions about what to put in the flu vaccine have to be made six months before the flu season starts.
Every February in the Northern Hemisphere, the World Health Organization (WHO) reviews the types of flu that have been circulating in all parts of the world and chooses the ones which will go into the vaccine for the following autumn. This allows time for the vaccine to be made – but it also gives the flu virus time to change before vaccination starts in the autumn. This means that sometimes the flu vaccine may not be a good match for all the strains of flu that are circulating. Read more about the WHO recommendations for the 2022-23 season.
Researchers are investigating ways to create a flu vaccine that protects against all the many different varieties of flu. If they are successful, it will mean that people will only need a single flu vaccine to give them lifelong protection, instead of having a yearly vaccine. However, it will be several years before we find out if it is possible to do this.
The viruses for flu vaccines are usually grown in hen’s eggs. This is a slow process and can lead to something called ‘egg adaptation’. The flu virus strain starts to adapt to the conditions in the egg, leading to changes in the virus. This is another reason that the flu vaccine may not always match the circulating strains of flu. For flu seasons since 2018-19, a new vaccine has been developed using cell culture instead of eggs. This vaccine is less likely to adapt as it has been developed using cells that are very similar to human cells.
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