CD4 and infection: The percentage of spike-specific CD4+ T cells one month after the third vaccine dose was not associated with breakthrough infection (p = 0.72 for the combined cohort; Figure 4B), but a higher percentage of spike-specific CD8+ T cells at this time point was associated with remaining SARS-CoV-2-naive (p = 0.046 for the combined cohort) (Figure 4C).