Flu can be highly unpredictable and is highly contagious so anyone can get it when it is circulating in the community and no one is completely immune, meaning there is a chance you can catch it. Past success in avoiding flu won’t guarantee you’ll be protected in future.
If you have never been exposed to the virus and you choose not to be vaccinated, your body will have no antibody protection to help with fighting the virus should you be exposed. You may be lucky enough to only develop a mild illness but it is also feasibly possible that you get very sick indeed. It is entirely up to you as to whether you choose between playing the odds or taking a positive step to protect yourself.
The flu jab also offers wider community protection if enough people get vaccinated. There are plenty of people who are unable to get the flu jab and herd immunity protection means flu cannot spread as easily within a population.
This year in particular, the added benefit of the flu jab is to avoid confusion with coronavirus. The flu jab reduces the likelihood of you contracting flu, which during the flu season would avoid confusion and a misdiagnosis with Covid-19.