Updated Mar. 4, 2021

Messenger RNA vaccines—also called mRNA vaccines—are some of the first COVID-19 vaccines authorized for use in the United States.

New Approach to Vaccines

mRNA vaccines are a new type of vaccine to protect against infectious diseases. To trigger an immune response, many vaccines put a weakened or inactivated germ into our bodies. Not mRNA vaccines. Instead, they teach our cells how to make a protein—or even just a piece of a protein—that triggers an immune response inside our bodies. That immune response, which produces antibodies, is what protects us from getting infected if the real virus enters our bodies.

A Closer Look at How COVID-19 mRNA Vaccines Work

COVID-19 mRNA vaccines give instructions for our cells to make a harmless piece of what is called the “spike protein.” The spike protein is found on the surface of the virus that causes COVID-19.

1. First, COVID-19 mRNA vaccines are given in the upper arm muscle. Once the instructions (mRNA) are inside the immune cells, the cells use them to make the protein piece. After the protein piece is made, the cell breaks down the instructions and gets rid of them.
2. Next, the cell displays the protein piece on its surface. Our immune systems recognize that the protein doesn’t belong there and begin building an immune response and making antibodies, like what happens in natural infection against COVID-19.
3. At the end of the process, our bodies have learned how to protect against future infection. The benefit of mRNA vaccines, like all vaccines, is those vaccinated gain this protection without ever having to risk the serious consequences of getting sick with COVID-19.

COVID-19 mRNA Vaccines Will Be Rigorously Evaluated for Safety

mRNA vaccines have been held to the same rigorous safety and effectiveness standards [332 KB, 24 pages]external icon  as all other types of vaccines in the United States. The only COVID-19 vaccines the Food and Drug Administration (FDA) will make available for use in the United States (by approval or emergency use authorization) are those that meet these standards.

mRNA Vaccines Are New, But Not Unknown

Researchers have been studying and working with mRNA vaccines for decades. Interest has grown in these vaccines because they can be developed in a laboratory using readily available materials. This means the process can be standardized and scaled up, making vaccine development faster than traditional methods of making vaccines.

mRNA vaccines have been studied before for flu, Zika, rabies, and cytomegalovirus (CMV). As soon as the necessary information about the virus that causes COVID-19 was available, scientists began designing the mRNA instructions for cells to build the unique spike protein into an mRNA vaccine.

Future mRNA vaccine technology may allow for one vaccine to provide protection for multiple diseases, thus decreasing the number of shots needed for protection against common vaccine-preventable diseases.

Beyond vaccines, cancer research has used mRNA to trigger the immune system to target specific cancer cells.

Updated Sept. 8, 2021

CDC and its partners are actively monitoring reports of myocarditis and pericarditis after COVID-19 vaccination. Active monitoring includes reviewing data and medical records and evaluating the relationship to COVID-19 vaccination.

Myocarditis is inflammation of the heart muscle, and pericarditis is inflammation of the outer lining of the heart. In both cases, the body’s immune system causes inflammation in response to an infection or some other trigger. Learn more about myocarditis and pericarditis.external icon  Seek medical care if you or your child have symptoms of these conditions within a week after COVID-19 vaccination.

What You Need to Know

• Cases of myocarditis reported to the Vaccine Adverse Event Reporting System (VAERS)external icon  have occurred:
  • After mRNA COVID-19 vaccination (Pfizer-BioNTech or Moderna), especially in male adolescents and young adults,
  • More often after the second dose
  • Usually within several days after vaccination
• Most patients with myocarditis or pericarditis who received care responded well to medicine and rest and felt better quickly.
• Patients can usually return to their normal daily activities after their symptoms improve. Those who have been diagnosed with myocarditis should consult with their cardiologist (heart doctor) about return to exercise or sports. More information will be shared as it becomes available.

Both myocarditis and pericarditis have the following symptoms:

• Chest pain
• Shortness of breath
• Feelings of having a fast-beating, fluttering, or pounding heart

Seek medical care if you or your child have any of these symptoms, especially if it’s within a week after COVID-19 vaccination.

If you have any health problems after vaccination, report them to VAERSexternal icon .

Healthcare Providers: For additional recommendations and clinical guidance, visit Clinical Considerations: Myocarditis after mRNA COVID-19 Vaccines | CDC

Should I Still Get Myself or My Child Vaccinated?

Yes. CDC continues to recommend that everyone aged 12 years and older get vaccinated for COVID-19. The known risks of COVID-19 illness and its related, possibly severe complications, such as long-term health problems, hospitalization, and even death, far outweigh the potential risks of having a rare adverse reaction to vaccination, including the possible risk of myocarditis or pericarditis.

If you or your child has already gotten the first dose of the Pfizer-BioNTech or Moderna vaccine, it’s important to get the second dose unless a vaccination provider or your doctor tells you not to get it.

If you have concerns about COVID-19 vaccination, talk with your or your child’s doctor, nurse, or clinic.