Individuals with a combination of history of COVID and vaccination appear to have exceptionally strong protective immunity that has been described as "bulletproof." As published in Nature [1] and analyzed by JAMA [2], individuals who had been infected (even those with mild disease) and vaccinated had over 50 times more neutralizing antibody activity than those who were unvaccinated. This level of protection was more robust even than that seen in fully vaccinated individuals.

[1] https://www.nature.com/articles/s41586-021-03696-9 [2] https://jamanetwork.com/journals/jama/fullarticle/2782139