Understanding How COVID-19 Vaccines Work

Important things to know

COVID-19 vaccines are safe and effective.
Side effects may occur after vaccination, but they are normal.
It usually takes two weeks for the body to be fully vaccinated to build up protection (immunity) against the virus that causes COVID-19.
Vaccines will be widespread in the coming months. Find a COVID-19 vaccine.
People who have been fully vaccinated can start doing some things that they stopped doing due to the pandemic.

The immune system – the body's defense against infection

To understand how COVID-19 vaccines work, it is helpful to first examine how our bodies fight disease. When germs like the virus that causes COVID-19 enter our bodies, they attack and multiply. This invasion, known as infection, causes disease. Our immune system uses various tools to fight infection. Blood contains red blood cells that carry oxygen to tissues and organs, and white or immune cells that fight infections. Different types of white blood cells fight infection in different ways:

Macrophages are white blood cells that swallow and digest germs and dead or dying cells. The macrophages leave behind parts of the invading germs, so-called "antigens". The body identifies antigens as dangerous and stimulates antibodies to attack them.
B lymphocytes are defensive white blood cells. They produce antibodies that attack the pieces of virus left behind by the macrophages.
T lymphocytes are another type of defensive white blood cell. They attack cells in the body that are already infected.

The first time a person becomes infected with the virus that causes COVID-19, it can take several days or weeks for their body to manufacture and use all of the germ control tools needed to overcome the infection. Once infected, the person's immune system remembers what it learned to protect the body from the disease.

The body holds a few T-lymphocytes called "memory cells" that come into action quickly when the body encounters the same virus again. When the known antigens are detected, B lymphocytes produce antibodies to attack them. Experts are still learning how long these memory cells will protect a person from the virus that causes COVID-19.

How COVID-19 vaccines work

COVID-19 vaccines help our bodies develop immunity to the virus that causes COVID-19 without us having to get the disease.

Different types of vaccines work in different ways to provide protection. However, with all types of vaccines, the body will have a supply of “memory” T-lymphocytes as well as B-lymphocytes, which will remember how to fight this virus in the future.

It typically takes a few weeks after vaccination for the body to produce T lymphocytes and B lymphocytes. Therefore, it is possible that a person could become infected with the virus that causes COVID-19 shortly before or shortly after vaccination and then get sick because the vaccine did not have enough time to provide protection.

Sometimes the process of building immunity after vaccination can cause symptoms such as fever. These symptoms are normal and are signs that the body is building immunity.

Types of vaccines

There are currently three main types of COVID-19 vaccines that are approved, recommended, or carried out in large clinical trials (phase 3) in the United States.

Below is a description of how any type of vaccine causes our bodies to recognize us and protect us from the virus that causes COVID-19. None of these vaccines can give you COVID-19.

mRNA vaccines Contains material from the virus that causes COVID-19 and provides instructions to our cells on how to make a harmless protein that is unique to the virus. After our cells make copies of the protein, they destroy the genetic material in the vaccine. Realizing that the protein shouldn't be there, our bodies build T lymphocytes and B lymphocytes to remember how to fight the virus that causes COVID-19 if we are infected in the future.
Protein subunit vaccines This includes harmless parts (proteins) of the virus that cause COVID-19 instead of the whole germ. After vaccination, our bodies recognize that the protein shouldn't be there and build T lymphocytes and antibodies that remember how to fight the virus that causes COVID-19 if we are infected in the future.
Vector vaccines Contain a modified version of a virus other than the one that causes COVID-19. The shell of the modified virus contains material from the virus that causes COVID-19. This is called a "viral vector". Once the viral vector is in our cells, the genetic material instructs the cells to make a protein that is unique to the virus that causes COVID-19. With these instructions, our cells make copies of the protein. This causes our bodies to build up T lymphocytes and B lymphocytes which will remember how to fight this virus if we are infected in the future.

Some COVID-19 vaccines require more than one shot

To be fully vaccinated, you will need two shots of some COVID-19 vaccines.

Two recordings: If you are given a COVID-19 vaccine that requires two shots, you are considered fully vaccinated two weeks after your second shot. Pfizer BioNTech and Moderna COVID-19 vaccines require two shots.
One shot: If you are given a COVID-19 vaccine that requires a shot, you are considered fully vaccinated two weeks after the shot. Johnson & Johnson's Janssen COVID-19 vaccine only requires one shot.

If it has been less than two weeks since your shot, or if you still need your second shot, you are NOT fully protected. Take all preventive measures until you are fully vaccinated (two weeks after your last shot).

The bottom line

Vaccination against COVID-19 and By following CDC's recommendations to protect yourself and others, you will provide the best protection against COVID-19.

Vaccination is one of many steps you can take to protect yourself and others from COVID-19. Protecting against COVID-19 is vital as COVID-19 can cause serious illness or death in some people.

To stop a pandemic all available tools must be used. Vaccines work with your immune system so your body is ready to fight the virus if you are exposed.

After you are fully vaccinated against COVID-19, there are some things you may be able to do that you have stopped due to the pandemic. However, we are still learning how vaccines affect the spread of COVID-19. After you have been fully vaccinated against COVID-19, you should continue to take precautions in public places or when you are with unvaccinated people from more than one household.