A virus can mutate in a way that makes it stronger or easier to spread. That’s what happened with the Delta variant. This variant still has a lot in common with previous variants. So if you’ve had COVID-19 or the vaccine, the antibodies you have aren’t completely useless. However, they may not be able to fight this variant as effectively as they would the previous one, resulting in re-infection, and (for some people) getting sick enough to be hospitalized. Getting vaccinated is your best protection, even for those who have had COVID-19.