Given evidence that antibodies, CD4+ T cells, and CD8+ T cells can each participate in protective immunity against COVID-19, we measured acute and memory SARS-CoV-2 spike–specific antibodies, CD4+ T cells, and CD8+ T cells in the blood of subjects who received a low-dose (25 μg) or standard-dose (100 μg) mRNA-1273 COVID-19 vaccine. Here, CD4 is linked to COVID-19.