COVID-19 spike antibody levels vs CD4⁺ T cells. Spike antibodies by the LIPS (A) and Elecsys (B-D) assays after the third (n = 155 in panel A; n = 231 in panel B), fourth (n = 159 in panel C), and fifth (n = 77 in panel D) vaccine doses. Points including spike antibody levels of >24 999 U/mL are shown above the 24 999 level.