How does the COVID-19 vaccine work?

On December 15, 2020, five health care workers at University of Utah Health became the first in Utah to receive the COVID-19 vaccine. In the coming weeks and months, the vaccine will become available to high-risk populations and eventually to the general public. To keep you informed, here is expert information on the vaccine and how it works.

Find this and other resources about the COVID-19 vaccine via U of U Health here.

Pharmaceutical companies, such as Pfizer (jointly with BioNTech) and Moderna, have created effective new vaccines that rely on genetic material called messenger RNA (mRNA) to prevent people from getting ill from COVID-19.

Scientists have been researching and working with mRNA vaccine technology for decades. However, the COVID-19 pandemic gave them the unique opportunity to put their knowledge and tools to the test and develop the first working mRNA vaccine. The Pfizer/BioNTech mRNA vaccine is the first COVID-19 vaccine authorized for use in the United States.

Despite the quick turnaround for these vaccines, they are still being held to the same rigorous standards for safety and efficacy. We break down how it works, its effectiveness and more to help you decide if the mRNA vaccine is right for you and your loved ones.

What is an mRNA vaccine?

Many traditional vaccines put a weakened or inactivated germ or virus into your body to trigger an immune response. However, mRNA vaccines are different. Instead, they teach your cells how to make a small, harmless piece of the virus to trigger an immune response inside our bodies.

Advantages of using mRNA vaccines

Traditional vaccines typically require growing large amounts of infectious viruses and then inactivating them—a process that can take weeks or months. However, mRNA vaccines can be quickly designed, tested and mass produced. mRNA vaccines are also safer because they do not contain live viruses.

How do mRNA vaccines work?

The mRNA vaccines for COVID-19 are injected into the muscle of your upper arm. Once the mRNA is inside your body, your muscle cells will follow the genetic instructions that tell them how to make the spike protein (a small, harmless piece of the COVID-19 virus). The spike protein does not cause infection or cause people to become sick from COVID-19.

After the spike protein is made, your immune system will recognize that it doesn’t belong there. As a result, your immune system will begin making T-cells and antibodies that recognize the spike protein.

This process teaches your body how to recognize and destroy the virus if you are ever infected with COVID-19 in the future.

Click on the infographic to view a larger version.

How effective are mRNA vaccines against COVID-19?

The Pfizer/BioNTech and Moderna vaccines are about 95% effective in preventing COVID-19 infections. More data will become available as time goes on and as more people are vaccinated.

What are the possible side effects of an mRNA vaccine?

Volunteers from both the Pfizer and Moderna vaccine trials have reported the following side effects:

  • Mild pain, swelling or redness at the injection site where you get the vaccine
  • Mild fever
  • Chills
  • Feeling tired
  • Headache
  • Muscle and joint aches

A small number of people who received the vaccine reported worse, but short-lived, fevers and aches. These side effects can be managed with fever-reducing medications, like Tylenol.

No harmful side effects have been reported in clinical trials. However, the possibility of rare, harmful side effects could still exist. More information will become available once we know more.

Media Contacts

Doug Dollemorescience writer, University of Utah Health
Office: 801-587-2596