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A Project of The Annenberg Public Policy Center

How is mRNA in vaccines delivered to cells?

Messenger RNA, or mRNA, vaccines work by instructing a small number of a person’s cells to make specific proteins. In the case of the approved mRNA vaccines for COVID-19, the cells make spike protein — one component of the virus that causes COVID-19.

For mRNA vaccines to work, it’s not enough to just put mRNA molecules into a vial and then inject them into a person’s muscle. One innovation that made the current mRNA vaccines possible was the use of lipids to encircle the mRNA molecules.

These fatty structures — called lipid nanoparticles — protect the mRNA from being broken down prematurely. They also help the mRNA cross the cell membrane and get into cells. The approved mRNA vaccines for COVID-19 each use a blend of four types of lipids.

Once the lipid nanoparticles make it past the cell membrane, they release the mRNA into the cell’s interior. Called the cytoplasm, this region encompasses the inside of the cell excluding the nucleus, in which the cell’s DNA resides. The mRNA is processed and used to create the spike protein. The body then mounts an immune response to the spike protein, preparing the immune system to respond to the virus that causes COVID-19 in the future.