Most vaccine candidates target the spike glycoprotein (S), eliciting antibodies that block the interaction with the ACE2 receptor in the human cell. Researchers studied the immune response to the SARS-CoV-2 spike (S) glycoprotein, which binds to angiotensin-converting enzyme 2 (ACE2). They recruited a cross-sectional cohort of 41 adults who had recovered from mild-to-moderate Covid-19 disease and 27 controls in Australia. Results showed that antibodies against S and the receptor-binding domain (RBD) were seen in all infected participants compared to controls and were correlated with neutralizing activity and ACE2RBD binding inhibition. To test cross-reactivity, researchers utilized SARS-CoV and HCoVHKU1 viruses. Response to the SARS-CoV RBD was minimal, contrasting a large response to HCoV-HKU1 in infected participants. Furthermore, researchers found populations of S, RBD and S-RBD-binding B-cells. Most S-binding B-cells were IgG+ with phenotype of CD21 and CD27, but other combinations were also seen. Cross-reactivity between Covid-19 and SARS-CoV was also seen with IgG+ B-cells. Additionally, Tnem and cTFH levels for Sspecific protein were high in comparison to RBD-specific protein, and a boosting of cross-reactive T-cells was observed with SARS-CoV-2 via cTFH levels. Furthermore, CCR6+ and CXCR3+ cTFH had Th17 and Th1-like cytokines. Overall, neutralizing activity was seen to be correlated with S-specific and RBD-specific antibodies, B-cells, and S-specific cTFH (with Th17 and CCR6+CXCR3-). Neutralization, S-Anbitody and S-cTFH responses were correlated with symptom severity. This data can be used to inform upcoming vaccine trials that utilize S protein.