This is now the fifth iteration of the COVID vaccines, which have been updated regularly to keep up with the rapidly evolving virus, SARS-CoV-2.
...
For adults aged 65 to 74, a booster is recommended every 12 months, but they’re eligible every six months. For adults over 75, a shot is recommended every six months.
Adults aged 18 to 64 are eligible every 12 months, unless they have a severe immune deficiency. Many conditions can cause immunodeficiency, including genetic disorders, infections, cancer, autoimmune diseases, diabetes and lung disease, as well as having received an organ transplant. For this group, it’s recommended they receive a shot every 12 months, but they’re eligible every six.