What Actually Happens When COVID-19 Vaccines Enter the Body?

The COVID-19 pandemic, caused by the SARS-CoV-2 virus, has changed the way people live around the world. As of October 14, 2021, more than 716,000 people have died in the United States alone. Health experts agree that COVID-19 vaccines are an important way to end the pandemic.

But getting a vaccine can be scary for both children and adults. In addition, there is a lot of information about how the COVID-19 vaccines work, but some of it can be difficult to understand.

As a pediatrician, infectious disease physician, and scientist who studies germs such as bacteria and viruses, I’ve paid close attention to the pandemic and the development of the COVID-19 vaccines.

COVID-19 vaccines have been shown to be safe and effective. But it’s understandable to have questions.

Vaccines are made to look like invaders

The most important thing to understand about vaccines is that they teach your body how to prepare to fight an infection without having to deal with the infection itself. In this way, vaccines help prepare your body for invasions by germs that could otherwise make you very sick.

All three COVID-19 vaccines available in the US target what is called the spike protein of the SARS-CoV-2 virus or coronavirus. SARS-CoV-2 is a round virus, with bumps everywhere – a kind of baseball covered with golf tees. The bumps are the spike proteins.

In a real coronavirus, spike proteins allow the COVID-19 virus to enter the cells so that the virus can make more copies of itself. It does this by sticking to certain types of proteins called receptors on human cells, especially lung cells. In this way, the virus can enter and infect healthy cells.

The vaccines from Pfizer-BioNTech, Moderna and Johnson & Johnson all work in the same way by instructing the body’s cells to make the spike protein. The Pfizer and Moderna vaccines carry these instructions on a molecule called mRNA. This single-stranded molecule looks like a long piece of tape with instructions on one side to make a protein.

The Johnson & Johnson vaccine, on the other hand, relays the instructions to cells through DNA molecules. It uses a virus called adenovirus, which cannot make copies of itself, to carry the DNA from the spike proteins into human cells. This DNA is copied into mRNA, which then translates the instructions into proteins, in this case the coronavirus spike protein.

The three COVID-19 vaccines encode the coronavirus spike protein – two use mRNA as a template (Pfizer and Moderna) and one uses DNA as a template (J&J).

So the main difference between the three vaccines is that the Pfizer and Moderna injections instruct your body for the spike protein via mRNA, while the Johnson & Johnson injection directs it via DNA. After that, all three vaccines work the same way.

What happens when the vaccine enters your body?

Once a COVID-19 vaccine is injected, the mRNA or DNA is gobbled up by tissue cells and special immune cells that live in muscles, skin and organs called dendritic cells. Dendritic cells guard all parts of the body like sentries, looking for signs of invading germs, such as the coronavirus.

Once the DNA or mRNA is in the dendritic or tissue cells, the cells use the instructions to make spike proteins. This process usually takes less than 12 hours. After the spike proteins are made and ready to “display” to the immune system, the mRNA or DNA is broken down by the cell and eliminated.

It’s important to know that although your cells have made their own spike proteins, they don’t have enough information to make copies of the entire virus. But the spike proteins can trigger the body’s immune system to bolster its defenses so it’s ready when the entire coronavirus invades.

When the tissue cells and dendritic cells recognize the spike proteins as unwanted visitors, the cells place portions of the spike proteins on their exterior for other cells to see. The dendritic cells also emit “danger” signals at the same time to let other cells know that the spike protein is a threat. The danger signals are like flashing neon yellow signs pointing to the displayed spike protein piece and saying, “This isn’t right!”

These warning signs then activate your body’s immune response.

What happens once the immune system kicks in?

Thanks to that process, the body is now highly alert and ready to learn to fight off invaders — in this case, the spike proteins made after being injected with the COVID-19 vaccine.

Immune cells in the body, called B cells and T cells, recognize the warning signs of an outside invader. Thousands of these cells are rushing to the area to learn more about this new threat so they can help provide protection.

B cells are specialists in building ‘traps’ called antibodies that will remove any invading spike proteins. Different B cells make many specialized antibodies that recognize different parts of a virus or bacteria. And B cells will act like a factory and continue to make antibodies against the perceived threat, even after it’s gone, to protect the body for a long time.

This artistic image shows an antibody (right) attacking a coronavirus particle, with wave-tee-shaped spike proteins (in bright pink) on its outer surface.

One type of T cell, called helper T cells, helps the B cells make antibodies when danger signals are present. Another type of T cell is there to check whether other cells in the body are infected with the virus. When that type of T cell discovers an infected cell, it removes the infected cell, so it can’t make more copies and pass the infection on to other cells.

Why does my arm hurt?

Since all of these important processes take place in your body, you may see some physical signs that there is a battle going on under the skin. If your arm hurts after you get the injection, it’s because immune cells like the dendritic cells, T cells, and B cells race to the arm to inspect the threat.

You may also develop a fever or other signs of illness. All this means that your body is doing exactly what it is supposed to do. This is a safe and natural process that takes place when the body learns to fight the spike proteins. For example, if you do come into contact with the real coronavirus, your body has learned to protect you against it.

Written by Glenn J Rapsinski, Pediatric Infectious Diseases Fellow, University of Pittsburgh.

This article was first published in The Conversation.