IFNG and COVID-19: PBMCs isolated from patients who recovered from COVID-19, from patients with COVID-19, and from healthy controls were exposed in vitro to LPS, diphtheria/tetanus/pertussis vaccine (DTaP), or quadrivalent flu vaccine (FLU), and the T cell–mediated response was evaluated in terms of number of IFN-γ–secreting cells, as indicated by the number of spots observed per 1 × 106 plated PBMCs (Figure 3, A–F).