There are two flu vaccines available: one has four different viruses in it (the quadrivalent one) and one with three different viral strains (the trivalent one). The trivalent one also includes an additional ingredient (called the adjuvant) which helps boost the recipient’s immune response.
Therefore, older people over 65 who tend not to give such a strong immune response compared to younger people should have the trivalent vaccine.
Although you might think the quadrivalent one is the ‘better’ one, one of the four viruses it contains does not normally make older people ill, only younger people. It is therefore better for younger people to have the quadrivalent vaccine and older people to have the trivalent one with the adjuvant that helps the immune system work better. The ideal might be for everyone to get a quadrivalent one with adjuvant but that is not available. The pharmacist – or other clinic – will check your age and give you the right one.