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How Do COVID-19 mRNA Vaccines Work?

Updated: Jan 22, 2021

Summary: The mRNA vaccines work by initiating an immune response in our bodies, training our immune systems to protect against future infection from COVID-19.

hand holding vial and withdrawing vaccine

At this time (1/11/2020), Pfizer and Moderna, both of which are mRNA vaccines, are the only two FDA-approved COVID-19 vaccines in the United States. So what exactly are mRNA vaccines, and what do they do?

Many vaccines work by placing a weakened or inactivated germ into our bodies; mRNA vaccines do not work in the same way. Rather, they teach our cells to produce a protein, or a piece of a protein. This protein triggers an immune response, which produces antibodies that protects us from getting infected if the real virus enters our system.

So for COVID-19, the vaccine presents instructions for our cells to generate a harmless piece of “spike protein”, the protein found on the surface of the virus that causes COVID-19. Once the immune cells have produced the spike protein, the mRNA instructions are broken down by the cell, and the piece of spike protein is displayed on the cells’ surfaces. Our immune systems recognize the protein as not belonging there and begins to build an immune response and to make antibodies (proteins that help the body fight against specific diseases), like it would during a COVID-19 infection.

As a result, our immune system learns how to protect against COVID-19 infection, without putting the body through the risk of becoming ill with COVID-19.

View more information about how the mRNA COVID-19 vaccine works here.

Quick Facts and Common Misconceptions

1. Can the mRNA vaccine give someone COVID-19?

No. mRNA vaccines do not contain the live virus that causes COVID-19. It is possible to experience mild symptoms, such as low-grade fever or exhaustion, for a day or two after taking the vaccine, but this is because your body is demonstrating an immune response to produce antibodies, not because you are infected with COVID-19.

2. Will the mRNA vaccine alter someone’s DNA?

No. mRNA never enters the nucleus, where the cell’s DNA is kept. As soon as the cell uses the instructions to produce a piece of spike protein, the cell breaks down the mRNA and gets rid of it.

4. Does the vaccine contain unsafe preservatives?

Though preservatives have been proven safe in vaccines, the new mRNA vaccines are preservative free. This new technology involves fewer ingredients than a traditional “killed virus” vaccine. The vaccine is just the mRNA code to make the COVID “spike protein” wrapped in a small layer of fat to allow it to travel to your cells. The fat layer is made of the same molecules that make up the lining of your cells. The only other ingredients in the vaccine are salts and sugar. The salts and sugar are at your bodies natural levels of each so that the vaccine is easier on your body. You probably have heard how the vaccine must be kept at -70 degrees to keep the mRNA stable. This cold temperature naturally keeps the vaccine preserved so that no preservatives are needed to keep the vaccine sterile.

A more detailed review of the ingredients within the vaccine and their uses can be found here.


Keep checking the Yolo County Coronavirus Vaccine Page for the latest on where Yolo County is in the vaccination process.


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