Vaccination has a solid track record. Over the years, vaccination campaigns have reduced the incidence and severity of many diseases.
But vaccines still aren’t 100% effective. Their effectiveness can depend on age, missed booster shots, and other factors. About nine recipients out of ten will develop a strong enough immune response.
One immune system in ten however can’t build up the protection required to do the job. Luckily that one person is still protected by all the vaccinated people who do develop immunity. This is called herd immunity. To protect those whose defences can’t do it for them, everyone needs to be vaccinated.