FIG. 1.

COVID‐19 vaccine delivery systems.

(A) mRNA vaccines.

1. The mRNA is surrounded by a lipid nanoparticle.
2. The lipid nanoparticle assists with cell entry.
3. mRNA is released into the cytoplasm.
4. Ribosomes and cellular proteins are used to translate the mRNA into the spike protein.
5. The spike protein gets expressed on the cell surface and/or secreted into the serum.
6. The spike proteins expressed on the cell surface by the MHC receptors can activate T cells, which can activate the immune system, for additional T cells, B cells, and the production of antibodies against the spike protein.
7. Antigen‐presenting cells can engulf secreted spike proteins, which can also activate the immune system.

(B) Adenoviral vector vaccines.

1. The adenovirus contains DNA, which includes genetic material to produce the spike protein.
2. The adenovirus is taken up by the human cell.
   1. The adenovirus enters the cytoplasm.
   2. The adenovirus releases its DNA into the nucleus.
   3. Transcription of the DNA to mRNA occurs in the nucleus.
   4. mRNA is transferred into the cytoplasm.
3. Ribosomes and cellular proteins are used to translate the mRNA into the spike protein.
4. The spike protein gets expressed on the cell surface and/or secreted into the serum.
5. The spike proteins expressed on the cell surface by the MHC receptors can activate T cells, which can activate the immune system, for additional T cells, B cells, and the production of antibodies against the spike protein.
6. Antigen‐presenting cells can engulf secreted spiked proteins, which can also activate the immune system.

(C) Weakened live attenuated virus vaccines.

1. Weakened live attenuated virus containing the mRNA of the spike protein
2. The attenuated virus binds to the ACE2 for cell entry.
3. mRNA is released into the cytoplasm.
4. Ribosomes and cellular proteins are used to translate the mRNA into the spiked protein.
5. The spike protein gets expressed on the cell surface and/or secreted into the serum.
6. The spike proteins expressed on the cell surface by the MHC receptors can activate T cells, which can activate the immune system, for additional T cells, B cells, and the production of antibodies against the spike protein.
7. Antigen‐presenting cells can engulf secreted spiked proteins, which can also activate the immune system.