CD4 and COVID-19: Because Spike also induced both CD4+ and CD8+ T cell responses, and since most individuals (70%) have already received at least one dose of Spike protein-based COVID-19 vaccines, it is likely that Spike-specific memory CD4+ and CD8+ T cells have been developed and often boosted and re-boosted following multiple exposures to various SARS-CoV-2 variants over time [248,249].