Vaccines can stop most people from getting sick with COVID-19, but not everyone.
Even after someone takes all of the recommended doses and waits a few weeks for immunity to build up, there is still a chance that they can get infected. Vaccines do not provide full (100%) protection, so ‘breakthrough infections’ – where people get the virus, despite having been fully vaccinated – will occur.
If vaccinated people do get sick, they are likely to have milder symptoms, in general 'It is very rare for someone vaccinated to experience severe illness or die.