Fig. 1: Induction of immune responses by currently available COVID-19 vaccines.

a | mRNA vaccines (including BNT162b2 and mRNA-1273) are delivered to bystander cells inside lipid nanoparticles at the injection site. The mRNA encodes a modified version of the spike protein which is translated by ribosomes, secreted by the bystander cell and in turn taken up and processed by antigen-presenting cells (APCs; in this image dendritic cells (DCs)). b | Adenoviral vector vaccines (including ChAdOx1, Ad26.COV2.S and Sputnik V) contain cDNA encoding a full-length spike protein. While most vaccines use the same adenoviral vector for each vaccine dose, Sputnik V uses two different human adenoviral vectors to prevent immune reactions against the vector. cDNA is transported to the nucleus where it is transcribed to mRNA and subsequently translated into spike protein in the cytoplasm. This spike protein is then taken up and processed by APCs. c | Protein-based vaccines (including NVX-CoV2373) consist of the spike protein and an adjuvant which is directly processed by APCs. d | Attenuated virus vaccines (including CoronaVac and BBIBP-CorV) contain whole inactivated virus particles and adjuvants which are directly processed by APCs. In the lymph nodes, APCs will present processed peptides and thus activate T cell responses (including CD4⁺ and CD8⁺ responses) and B cell responses, and in turn antibody responses. The precise immune reaction and strength of activation depends on the vaccine type. Detailed information for each vaccine is summarized in Supplementary Table 1.