How does the COVID-19 vaccine work?

Mike Bohl, MD, MPH, ALM - Contributor Avatar

Written by Chimene Richa, MD 

Mike Bohl, MD, MPH, ALM - Contributor Avatar

Written by Chimene Richa, MD 

last updated: Sep 10, 2021

5 min read

Measles, tetanus, and flu shots—all of the vaccines you’ve heard about work in similar ways. Vaccines safely expose your immune system to a germ in a way that doesn’t cause infection. In the case of the COVID-19 vaccines, like those made by Pfizer and Moderna, the vaccines contain tiny pieces of genetic information that our cells can use as an instruction manual to produce the spike proteins normally found on the outside of the virus. These spike proteins then stimulate our immune systems to mount an immune response against the virus without us ever needing to be exposed.  

If you are exposed to the coronavirus after getting vaccinated, your immune system is already primed and ready to defend you. Let’s take a closer look at how this whole process actually works. 


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How does the COVID-19 vaccine work?

COVID-19 vaccines work by exposing your immune system to some part of the virus in a safe way. In the case of the mRNA vaccines available right now in the United States, the vaccines stimulate the production of the spike proteins that are located on the shell of the virus. In the event of infection, these crown-like spike proteins are what enable the virus to enter your system, so once you've received the vaccine, your body knows how to recognize the virus and mount a response without the virus causing serious infection. 

The virus that causes COVID-19, called the SARS-CoV-2 virus, has an outer crown (or "corona") of protein spikes. 

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These protein spikes are one of the first things that your body detects when it encounters the virus. After your body recognizes them as foreign proteins, it triggers an immune response. This process can take a while, which gives the virus time to reproduce and spread throughout your body.

You might be wondering why you can’t get sick from the vaccine if you’re exposed to the virus’s spike proteins. Because the vaccine exposes your cells to just the proteins and not the rest of the virus, it can’t make you sick. So how does the vaccine accomplish this? In a very clever way, actually.

The COVID-19 vaccine contains the genetic material that codes for those spike proteins. Just like an architect needs the blueprints to build a house, the virus particles need genetic material to create spike proteins. 

Rather than trying to cram the proteins themselves into the vaccine, scientists used blueprints instead, taking advantage of the fact that your cells can recreate the spike proteins. The blueprints (mRNA or DNA, depending on the brand of vaccine) don’t stick around and degrade quickly, so you don’t have viral genetic material floating around forever. Because only the protein spike code is in the vaccine, your body doesn't have the blueprints to build the whole virus meaning there's no risk of infection from the shot (CDC, 2020).

What happens after the COVID-19 vaccine enters my body?

After you get the vaccine, your cells start to produce viral spike proteins. The immune system sees these proteins and recognizes them as foreign proteins that should not be there. It then mounts an immune response and creates antibodies against the spike proteins. This way if you ever come in contact with the real virus, your immune system is ready to attack.

The current recommendations for the Pfizer and Moderna vaccines available in the United States are to get more than one dose. While one dose stimulates your immune system, additional doses bolster the immune response, reducing the chance that you'll develop a serious infection.

Since the vaccines activate an immune response, it’s common to experience side effects like soreness at the injection site, headaches, and fatigue, just like you might experience if you got sick. These symptoms do not mean you have COVID-19. Rather, they are signs that your immune system is working hard against those spike proteins. Since the second dose can give you a stronger response than the first one, some people notice more side effects after the second dose. (Note that, if you don’t experience these side effects, the vaccine is still effective).

No vaccine is 100% effective, but the mRNA vaccines we have available are pretty close. Still, some people have experienced breakthrough infections with COVID-19 after being vaccinated. But the vaccines have proven to reduce the chance of serious disease and the need for hospitalization, too.

As new variants arise, local governments may enact new mask mandates and social distancing regulations to keep people safe. The more people that get vaccinated, the lower the chance for transmission of the virus, and the closer we will get to ending this pandemic. 

Debunking myths about how the COVID-19 vaccine works

There are many myths floating around about how these vaccines work. Let’s dig into some of the most common myths you might encounter. 

Myth: The COVID-19 vaccine changes your DNA

Vaccines, like those developed by Pfizer-BioNTech and Moderna, use what’s called mRNA to pass along the spike protein blueprints. Your DNA is stored in the nucleus of your cells, where mRNA cannot enter. Therefore, it does notinteract or change your DNA in any way. The only purpose of using the viral genetic material is to trigger your body into making the viral spike proteins. After this, the viral mRNA, which is very fragile, breaks down (CDC, 2021). Your immune system then makes antibodies to fight the spike proteins and protect you against COVID-19 infection.

Myth: The vaccine makes you sick with COVID-19

None of the COVID-19 vaccines in use today contain live coronaviruses, so they cannot make you sick. They only provide the information that your cells need to create viral spike proteins, encouraging your immune system to produce antibodies. To get sick with COVID-19, you must be exposed to the virus. 

You may experience side effects of the injection, like injection site soreness, headaches, and fatigue. These are signs that your immune system is working against those spike proteins and not an indication of COVID-19. It takes weeks for your body to develop protection against COVID-19. Therefore, it is possible to get sick with COVID-19 shortly after getting the vaccine if you were exposed to the virus before your body has had time to build up immunity. Also, while the vaccine is effective, it cannot prevent 100% of cases, so even people who are fully vaccinated can catch and transmit coronavirus. The vaccine does, however, reduce the severity of infection if you do catch the virus.

Myth: The COVID-19 vaccine implants a microchip tracker

The COVID-19 vaccine does not contain a microchip or tracking device. A misquoted internet discussion with Bill Gates on Reddit, a popular internet forum, started these rumors. Gates mentioned that having “digital certificates” for health records may be an option in the future—but there was no mention of “microchip implants" (Reddit, n.d.). In a statement given by the Bill and Melinda Gates Foundation to Reuters, “The reference to ‘digital certificates’ relates to efforts to create an open-source digital platform with the goal of expanding access to safe, home-based testing" (BMGF, n.d.; Reuters, 2020).

Myth: The COVID-19 vaccine is made from previously unknown and untested technology

Researchers have been studying mRNA vaccines since the 1990s. Clinical trials of mRNA vaccines and DNA vaccines for HIV, rabies, Zika virus, and influenza have been in the works for years—so it is not as if scientists created this technology specifically for COVID-19 (Pardi, 2018; Montgomery, 1997). In response to the global pandemic, governments provided funding for rapid vaccine development. In addition, one important step of the vaccine development timeline is ensuring that trial participants have been exposed to the virus to verify vaccine effectiveness. While this can take years when dealing with a rare pathogen like measles or polio, coronavirus is so prevalent that the timeline was shortened significantly without compromising on the quality of the study. The vaccine data was then reviewed by the U.S. Food and Drug Administration (FDA), which determined that the vaccines were safe and effective (FDA, 2021c). 


If you have any medical questions or concerns, please talk to your healthcare provider. The articles on Health Guide are underpinned by peer-reviewed research and information drawn from medical societies and governmental agencies. However, they are not a substitute for professional medical advice, diagnosis, or treatment.

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Every article on Health Guide goes through rigorous fact-checking by our team of medical reviewers. Our reviewers are trained medical professionals who ensure each article contains the most up-to-date information, and that medical details have been correctly interpreted by the writer.

Current version

September 10, 2021

Written by

Chimene Richa, MD

Fact checked by

Mike Bohl, MD, MPH, ALM

About the medical reviewer

Dr. Mike is a licensed physician and a former Director, Medical Content & Education at Ro.