hybrid immunity
This is the immunity acquired from both having the vaccine and experiencing a bout of COVID-19. It is thought to be a stronger immunity than that produced by having only one of two immune-boosting events.
There have been studies which show that those who have both vaccination and infection have, as a result, a strong and durable protection against severe disease, whereas those with just one option have considerably less protection. It seems that people who have hybrid immunity may find that their immunity level is not actually increased with a booster shot. This may affect the rolling out of vaccinations.