CD4 and COVID-19: To this end, naive BALB/c mice were prime–boost intranasally vaccinated with SARS-CoV-2 receptor-binding domain (RBD) proteins, a core part of the SARS-CoV-2 spike protein and COVID-19 vaccines45, and thus gained CD8+ and CD4+ T cell memory specific to the SARS-CoV-2 RBD (Extended Data Fig. 9a,b).