CD4+ T cell responses after cells were cultured for 10–12 days and then restimulated for 12-hours with HCoV-NL63 or SARS-CoV-2 spike peptide pools. TNF-α+IFN-γ+CD4+ T cells (shown in the gated box) in response CCC or SARS-CoV-2 spike peptides are shown for a representative vaccine recipient (A) and for 9 vaccine recipients (B) before and after vaccination. (C and D) CD4+ T cell responses after cells were either untreated or cultured for 10–12 days with HCoV-NL63 or SARS-CoV-2 peptide pools (labels shown in rows) and then restimulated with different peptide pools for 12 hours (labels shown in columns) to analyze cross-reactive T cell responses. Responses are shown for an participant after vaccination (C) and for 9 individuals before and after vaccination (D). In panels B and D, the peptide pool used for preculturing is shown first, followed by the peptide pool used for the 12-hour stimulation. NT, not treated; NL63, HCoV-NL63; 229E, HCoV-229E; OC43, HCoV-OC43; S2N, SARS-CoV-2-N; S2S, SARS-CoV-2-S. *P = 0.0332 and **P = 0.0021, by 2-tailed, paired Student’s t test. Horizontal bars represent the median.