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What Happens to Your Body After the COVID-19 Vaccine

The COVID-19 vaccine sparks your immune system into action, and the CDC explains how.
FACT CHECKED BY Emilia Paluszek

The immune system is your body's "defense against infection," according to the CDC, and it plays a key role after you're vaccinated. With so many questions about whether or not the vaccine is safe, the agency has revealed how exactly it works, and why you'll want one. Read on to see what happens to your body when you get the COVID-19 vaccine, according to the CDC—and to ensure your health and the health of others, don't miss these Sure Signs You've Already Had Coronavirus


The Goal of the Vaccine is to Spring Your Immune System Into Action

woman getting over a cold

"To understand how COVID vaccine work, it helps to first look at how our bodies fight illness," says the CDC. "When germs, such as the virus that causes COVID-19, invade our bodies, they attack and multiply. This invasion, called an infection, is what causes illness. Our immune system uses several tools to fight infection. Blood contains red cells, which carry oxygen to tissues and organs, and white or immune cells, which fight infection." Keep reading to see how they fight infection.


The White Blood Cells Fight Infection in Different Ways

Red and White Blood Cells in a Vein

"Different types of white blood cells fight infection in different ways," says the CDC:

  • "Macrophages are white blood cells that swallow up and digest germs and dead or dying cells. The macrophages leave behind parts of the invading germs 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 the virus left behind by the macrophages.
  • T-lymphocytes are another type of defensive white blood cell. They attack cells in the body that have already been infected." These come into play later—keep reading.


Your Immune System Can Remember What it Learned When Fighting the Infection

Ill woman coughing at bed.

"The first time a person is infected with the virus that causes COVID-19, it can take several days or weeks for their body to make and use all the germ-fighting tools needed to get over the infection. After the infection, the person's immune system remembers what it learned about how to protect the body against that disease," says the CDC. "The body keeps a few T-lymphocytes, called memory cells, that go into action quickly if the body encounters the same virus again. When the familiar antigens are detected, B-lymphocytes produce antibodies to attack them. Experts are still learning how long these memory cells protect a person against the virus that causes COVID-19."


So How Do the COVID-19 Vaccines Work?

covid-19 vaccine

"COVID-19 vaccines help our bodies develop immunity to the virus that causes COVID-19 without us having to get the illness. Different types of vaccines work in different ways to offer protection, but with all types of vaccines, the body is left with a supply of 'memory' T-lymphocytes as well as B-lymphocytes that will remember how to fight that virus in the future."


It Takes at Least Two Weeks For Your Vaccine to Work

Young woman taking a vaccine from her doctor.

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


You May Have Side Effects

A man experiencing discomfort in his upper arm

"Sometimes after vaccination, the process of building immunity can cause symptoms, such as fever. These symptoms are normal and are a sign that the body is building immunity," says the CDC. Dr. Anthony Fauci, the chief medical advisor to the President and the director of the National Institute of Allergy and Infectious Diseases, felt a pain in his arm. Others have had chills or fatigue. One study says women feel stronger side effects then men.

RELATED: Dr. Fauci Just Said When We'd Get Back to Normal


How the Different Types of Vaccines Work

Moderna and Pfizer Covid-19 vaccine

There are three main types of vaccines now available or being tested, says the CDC:

  • "mRNA vaccines"—like those from Moderna and Pfizer—"contain material from the virus that causes COVID-19 that gives our cells instructions for 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 from the vaccine. Our bodies recognize that the protein should not be there and build T-lymphocytes and B-lymphocytes that will remember how to fight the virus that causes COVID-19 if we are infected in the future.
  • Protein subunit vaccines include harmless pieces (proteins) of the virus that cause COVID-19 instead of the entire germ. Once vaccinated, our immune system recognizes that the proteins don't belong in the body and begins making T-lymphocytes and antibodies. If we are ever infected in the future, memory cells will recognize and fight the virus.
  • Vector vaccines"—like the kind from Johnson & Johnson—"contain a weakened version of a live virus—a different virus than the one that causes COVID-19—that has genetic material from the virus that causes COVID-19 inserted in it (this is called a viral vector). Once the viral vector is inside our cells, the genetic material gives cells instructions to make a protein that is unique to the virus that causes COVID-19. Using these instructions, our cells make copies of the protein. This prompts our bodies to build T-lymphocytes and B-lymphocytes that will remember how to fight that virus if we are infected in the future."


Most COVID-19 Vaccines Require More Than One Shot

Woman with face mask getting vaccinated, coronavirus, covid-19 and vaccination concept.

"All but one of the COVID-19 vaccines that are currently in Phase 3 clinical trials in the United States use two shots," says the CDC. "The first shot starts building protection. A second shot a few weeks later is needed to get the most protection the vaccine has to offer." So get vaccinated when it becomes available to you, and to protect your life and the lives of others, don't visit any of these 35 Places You're Most Likely to Catch COVID.

Alek Korab
Alek Korab is a Co-Founder and Managing Editor of the ETNT Health channel on Eat This, Not That! Read more about Alek