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How Do COVID Vaccines Work? 

This information was reviewed and approved by Lindsay Sense, RN, BSN, CMSRN (July 2022).

As we learn to live with COVID-19, there may still be questions about vaccines and how many vaccinations a person should receive. Currently, there are three vaccines available in the United States. They are Moderna, Pfizer-BioNTech and Johnson & Johnson. The Moderna and Pfizer BioNTech vaccines use mRNA technology, and the Johnson & Johnson vaccine uses a viral vector process for their vaccine.

mNRA Vaccine

The mRNA vaccine tells the body's mRNA how to make the COVID-19 protein. When the COVID-19 virus infects the body, it will recognize the COVID proteins and start fighting it.

Viral Vector Vaccine

The viral vector process uses a harmless virus to send instructions to the body's cells. Once it tells the cells to recognize the COVID-19 proteins, the body can fight future COVID infections.

While the process of creating these vaccines is different, they both teach the body's cells how to make the protein that is part of the COVID-19 virus. Learn more about the different types of vaccines here.

About Vaccines

All vaccines work by giving you a piece of a contagious germ. Your immune system recognizes the germ as a potentially dangerous invader and starts to protect your body against it. When the immune system encounters that germ a second time, it is ready to fight with a faster and stronger defense. The purpose of the COVID-19 vaccine is to keep the disease from starting or reduce the severity of illness.

Most vaccines require booster shots to work effectively. For example, the measles, mumps and rubella (MMR) vaccine requires two doses.  Data shows that people who get only one dose of the MMR vaccine are four times more likely to catch the measles.

Priming the Pump with a First Dose

The first dose of the COVID-19 vaccine gets the body ready for a possible infection. It makes the body create the first group of antibodies and immune cells to help fight against the COVID-19 virus. This happens within one to two weeks after the first dose in the primary series. The antibodies from the first dose are weak and have a short life, which is why a second vaccine dose is needed.

Greater Immunity with the Second Dose

The second COVID-19 dose of the primary series greatly increases the number of immune cells. It also helps the immune cells make more antibodies, which can fight against the virus. These vaccines reach full usefulness about 10 days to two weeks after the second dose in the primary series.

Some people feel soreness at the injection site, fever, achy muscles and other flu-like symptoms. If you have those symptoms, it does not mean you are getting sick. The immune system causes these symptoms as it responds to the vaccine.  These symptoms are a sign that the body is developing strong protection against the virus.

The Moderna and Pfizer-BioNTech COVID vaccines require two separate shots in the primary series. These shots are given between three to eight weeks apart. These vaccines are about 95% effective at preventing COVID-19 symptoms after receiving both doses.

The Johnson & Johnson (J&J) vaccine requires only one shot in the primary series. It is 66.3% effective at preventing the COVID-19 virus. In most situations, the Centers for Disease Control and Prevention (CDC) prefer the Pfizer or Moderna vaccines over the J&J vaccine. Learn more from the CDC.

What Are Booster Shots?

After getting the primary series of shots, the CDC suggests everyone get booster shots. Booster shots are extra doses of the vaccine given after the primary series. A booster shot is needed because the vaccine’s protection decreases over time. The CDC says people should get booster shots five months after their primary series of Moderna or Pfizer shots to stay protected. Read the CDC's guidelines on staying up-to-date with vaccines.

A Third Primary dose for the Immunocompromised

The Food and Drug Administration (FDA) and CDC recommend a third primary dose for people whose immune systems are compromised. This is in addition to the two shots in the primary series. After the third primary dose, two booster doses are recommended. For more information on COVID-19 vaccines for people with compromised immune systems, please visit this page on the CDC website. 

How Immune Am I?

While the vaccines are shown to be 95% effective, it is unclear if the vaccines prevent 95% of all COVID-19 cases. This is a tricky but important distinction. Some people might develop mild COVID-19, which they could spread to others. Others may carry the virus with no symptoms at all, but still be able to pass it on to others. That is why you should continue to wash your hands often and social distance until your vaccine is fully effective, especially if you’re considered at risk for severe illness.

What about COVID Variants?

The COVID virus can alter or change itself over time. This change produces variants. Some of these variants are more contagious than the original virus. They can also be more life-threatening. Health care companies are paying attention to these new variants and how their vaccine protects against them.

Ongoing Protection

It is important to remember the vaccine is less effective after only one dose. After getting the second shot, it will take two to three weeks for you to be fully protected.

Even once you have the vaccine’s full protection, you can still develop mild COVID-19. You can also transmit the virus to others, including those who haven’t received the COVIDs shots. The CDC is a good resource for this information.

Please continue to use these safety recommendations, even after being vaccinated:

  • Watch for side effects & report any that linger to your doctor
  • Continue getting medical care for chronic conditions
  • Follow health care, government, workplace and business masking guidelines
  • Stay socially distanced, especially from those who are sick or appear sick and if you are not vaccinated
  • Wash hands frequently to protect yourself from germs




The information on our website is medically reviewed and accurate at the time of publication. Due to the changing nature of the COVID-19 pandemic, information may have since changed. and your state’s health department may offer additional guidance.

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