CD4 and COVID-19: A CD4+ T-cell count < 200 cells per μL (p < 0.001), older age (p = 0.001), a vector-based primary vaccination series (p < 0.001), and increased time since the last COVID-19 vaccination dose or SARS-CoV-2 infection (p < 0.001) were associated with lower S1-specific IgG antibody levels over time (S2 Table).