Vaccines: How They Work, How They’re Made, and Why Get Vaccinated

Your immune system is your body’s defense against foreign invaders like viruses, bacteria, and parasites. It can adapt and learn to recognize certain invaders to prevent you from getting sick. Vaccines help us train our immune systems to better protect us. Vaccines have been used for over 200 years, and doctors and researchers continue to find new ways to boost our immune systems.1

Keep reading to learn more about how vaccines have been developed over time, the different types of vaccines, and why it’s important to get vaccinated.

What Is a Vaccine and Types of Vaccines

A vaccine is a liquid injection, nasal spray, or pills you take to build your body’s immune system against a foreign invader. Vaccines can protect you against both bacteria and viruses (pathogens). There are several types of vaccines that contain pathogens or pieces of them — known as antigens — that activate your immune system.2

Examples of types of vaccines include:

  • Inactivated vaccines — Dead version of the pathogen
  • Live-attenuated vaccines — Alive, weakened version of the pathogen
  • mRNA vaccines — Use messenger RNA (mRNA), which provides instructions for making a pathogen protein
  • Toxoid vaccines — Contain a toxin made by the pathogen
  • Recombinant, subunit, conjugate, and polysaccharide vaccines — Contain only specific pieces of a pathogen, such as casings, proteins, or sugars
  • Viral vector vaccines — Use a harmless virus containing DNA from the pathogen to introduce the DNA into your cells

How Are Vaccines Made?

When making a vaccine for a pathogen, researchers first have to determine which antigens to include. It’s important to choose an antigen that will properly activate your immune system and create a strong enough response so you stay immune for a long time.

Once the antigen is selected, it’s mixed with other extensively studied and tested ingredients to ensure the vaccine is safe.3 The following are ingredients commonly included in vaccines:

  • Stabilizers — Stabilize the vaccine by preventing the antigen from breaking down or sticking to the inside of the glass vial; examples include gelatin, sugars, proteins, and amino acids
  • Preservatives — Prevent contamination once the glass vial is opened for multiple vaccine doses; the most common preservative is 2-phenoxyethanol, which is widely used in baby care products
  • Adjuvants — A substance that helps activate your immune system to respond to the vaccine; adjuvants are typically very small amounts of aluminum salts commonly found in our diets
  • Surfactants — Substances that help prevent vaccine ingredients from separating in the vial; surfactants are commonly found in processed foods
  • Diluent — A liquid (typically sterile water) that’s used to dilute the vaccine
  • Residuals — Very small amounts of substances from the manufacturing process; they can include yeast, antibiotics, or egg proteins

Once a vaccine has been developed, it’s tested in clinical trials to ensure it’s safe and effective at protecting against infection. Vaccines must be approved by the U.S. Food and Drug Administration (FDA) before they can be released to the public.

The History of Vaccines

For centuries, people looked for a way to prevent infection with smallpox (variola virus).4 Some groups noticed that intentionally exposing healthy people to those who were infected could help prevent illness — this process was known as variolation. Historians believe different civilizations around the world practiced variolation as early as 200 BCE.

The world’s first vaccine was administered in 1796 by Dr. Edward Jenner. He treated an 8-year-old boy named James with pus collected from a woman infected with cowpox. James had a mild reaction for several days, then he fully recovered. Two months later, Dr. Jenner treated him with matter from pus from a smallpox sore — James never became infected with smallpox because he was immune. The term vaccination comes from the Latin word vacca, meaning cow.

As doctors learned more about different bacteria and viruses, they also began researching vaccines for them. From the 1800s onward, many vaccines were developed:

  • 1937: Yellow fever
  • 1939: Pertussis (whooping cough)
  • 1945: Influenza
  • 1952-1955: Polio
  • 1969: Hepatitis B virus
  • 1971: Measles, mumps, and rubella (MMR)
  • 1978: Pneumococcal pneumonia
  • 1985: Haemophilus influenzae type b (Hib)
  • 1995: Chickenpox
  • 1995: Hepatitis A virus
  • 1999: Rotavirus
  • 2006: Human papillomavirus (HPV)
  • 2006-2017: Shingles
  • 2019: Malaria
  • 2019: Ebola
  • 2021: SARS-CoV-2 (COVID-19)
  • 2021: Monkeypox
  • 2023: Respiratory syncytial virus

How Do Vaccines Work?

To learn how vaccines work, it’s important to first understand how your immune system works.

Your immune system is a complex system made up of many types of cells that all work together to fight off foreign invaders and prevent you from getting sick. There are two parts to your immune system — the innate immune system and the adaptive immune system. The innate immune system is your first line of defense against invaders. Examples include your skin, stomach acid that breaks down bacteria and viruses, or mucus in your nose that traps particles.5

Your adaptive immune system is named for its ability to “adapt” to fight off invaders. It’s made of white blood cells (WBCs), known as T cells and B cells. T cells are responsible for creating inflammation and activating your immune system to attack cells infected with bacteria or viruses. B cells are responsible for creating antibodies or specialized proteins that bind to bacteria or viruses or their toxins to tag them for destruction.6

It takes time for your adaptive immune system to build up a response to a foreign invader. On average, it takes several days to a few weeks for your B cells to begin making antibodies. Here’s a simplified breakdown of how your immune system responds to an infection7:

  1. Immune cells travel around your body looking for foreign invaders
  2. When they find bacteria or viruses, they break them up into small pieces (antigens)
  3. The immune cells show the antigens to T cells, which are then activated and release chemicals to create inflammation
  4. Other immune cells are activated by inflammation, and they begin fighting and destroying the pathogen
  5. B cells are activated and begin making antibodies specifically targeted at the pathogen, which are then released into your bloodstream
  6. Some B cells travel to the bone marrow (spongy tissue inside your bones) and become memory cells, where they wait for another infection with the same pathogen

Your Immune System and Vaccines

Now that you have a basic understanding of how your immune system protects you, you can better understand how vaccines work. Vaccines introduce a dead version or pieces of the pathogen to your immune system. They give your body the opportunity to learn how to recognize the pathogen and create memory B cells that live in your bone marrow. If you come in contact with the same pathogen again, your immune system can quickly activate your memory B cells to create antibodies. Your body can begin fighting off the invader immediately, preventing you from becoming sick.7

You may need more than one vaccine over several weeks to months to become immune to a pathogen. For example, the HPV vaccine requires a series of three vaccinations to build up enough antibodies and memory B cells. It’s important to receive all vaccinations in a series for them to be effective — missing one or more of your doses may mean you’re not fully protected.

Why Should You Get Vaccinated?

Vaccines give your immune system a boost against foreign invaders, lowering your chances of getting sick. Even with modern medicine, some bacteria and viruses can cause serious illness and even death. Vaccinating yourself is one of the best ways to protect yourself from preventable infections and diseases and lower your risk of hospitalization.8

For example, hepatitis B can become a serious and even deadly liver infection. There’s currently no cure, so it’s important to get vaccinated against the hepatitis B virus to protect your liver and overall health. HPV can cause several types of cancer in both men and women — by getting vaccinated, you’re lowering your risk of infection and cancer and the chances of passing the virus on to others.8

Herd Immunity and Vaccines

Vaccines protect not only you but others around you as well. When you’re vaccinated, your body quickly destroys pathogens before you can spread them. If enough people in a population are vaccinated, pathogens have a hard time spreading and eventually are eliminated entirely. As a result, those with weakened immune systems who can’t be vaccinated are also better protected and less likely to get sick themselves. This concept is known as herd immunity.7

People with weak immune systems who need protection by herd immunity include:9-11

  • Immunocompromised individuals — People who are infected with human immunodeficiency virus (HIV), have acquired immunodeficiency syndrome (AIDS), or who are undergoing cancer treatment don’t have a properly functioning immune system
  • Infants — Infants aren’t able to make their own antibodies until they’re a few months old, and they can’t safely receive certain vaccinations until they’re older. Infants can receive antibodies in breast milk.
  • The elderly — As we age, our immune systems become weaker and have a harder time fighting infections

Side Effects of Vaccines

As with all therapies, there are some side effects and risks of getting vaccines. Vaccines aren’t 100 percent protective, and it’s still possible to become infected and sick after getting vaccinated. This is known as breakthrough infection, and rates depend on the vaccine.12

For example, the flu vaccine is developed every year by researchers predicting which influenza strains will appear the most. The influenza virus mutates (changes) rapidly, making it difficult to predict them. If you get the flu vaccine, your risk of infection is reduced, but you may still have a breakthrough infection with another strain that wasn’t included in the vaccine.13

Other side effects of vaccines can include:14

  • Redness, swelling, and pain at the injection site
  • Mild fever and chills
  • Achy muscles and joints
  • Feeling tired or fatigued
  • Headaches

It’s important to note that these side effects aren’t because the vaccine has infected you — they’re due to your immune system creating inflammation and responding to pieces of the pathogen. Even if you do come down with the flu, your case will probably be weaker than it would have been if you had not been vaccinated.

In extremely rare cases, you may have an allergic reaction after getting a vaccination. The U.S. Department of Health and Human Services (HHS) estimates that out of every 1 million vaccinations, only 1 or 2 people have severe reactions. If you begin experiencing any of the following symptoms, seek medical attention immediately:

  • Swelling of your face, tongue, lips, or throat
  • Difficulty breathing
  • Weakness or dizziness
  • Red, itchy rash (hives) covering your body
  • Rapid heartbeat

HHS provides a   for you or your doctor to report any allergic reactions or serious side effects of vaccinations to the government.

The Future of Vaccine Research

While there are dozens of vaccines currently available to protect us against bacteria and viruses, there’s still a need to address other infectious diseases. Doctors and researchers continue to develop new vaccines against shigella, HIV, and more through vaccine clinical trials.