Vaccines help prevent illness from viruses and bacteria. They train your body to fight harmful invaders by causing an immune response. They work by using live or dead pathogens, part of a pathogen or mRNA. Vaccines go through an extensive series of safety testing before being released to the public. Versions of vaccines have been used for centuries.
Vaccines train your body to fight off harmful invaders that make you sick. These harmful invaders are called pathogens (or germs) and include viruses and bacteria. Vaccines can prevent you from getting sick with a serious illness and help stop the spread of disease.
Vaccine comes from the word “vacca,” which means “cow.” Edward Jenner created the term from his use of cowpox to prevent smallpox. Today, a vaccine is anything that causes the immune system to recognize a harmful substance in the body and attack it.
Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy
Vaccines are mostly used to prevent you from getting sick from viruses and bacteria. Some immunotherapies used to treat cancer are also called vaccines.
Vaccination and immunization are sometimes used to mean the same thing, but they’re slightly different. Vaccination is when you actually get a vaccine. Immunization is the process that happens in your body that protects you from getting the illness you were vaccinated for.
Vaccines work by training your immune system to recognize and destroy harmful invaders (pathogens) quickly, before they can make you sick. In some cases, they teach your immune system to recognize something already in your body that it didn’t know was harmful (like cancer cells) and destroy it.
Vaccines trigger your primary immune response. When bacteria, a virus or other pathogen gets into your body for the first time, your immune system has to recognize the threat and find the right tools to fight it off.
Your body first looks in its toolbox for a B-cell. Each B-cell is unique and fits a pathogen like a lock and key. The B-cell with the right “key” then makes a bunch of antibodies (immune system chemicals) that also have the key to that specific pathogen. Antibodies can use that key to grab onto harmful invaders so your immune system can destroy them. This is your primary immune response. While your immune system is mounting that response, you can get very sick.
Once your body has seen a pathogen, your body adds special immune cells (memory cells) to its toolbox that keep a lookout for the pathogen. The memory cells keep a kind of “wanted poster” of a harmful invader and know which tools (antibodies) to call in to fight off a future infection. Memory cells are very specific to the virus or bacteria they identify and destroy, which is why vaccines only give you protection against one specific illness.
The secondary immune response, or the second time your body sees the same invader, is much quicker. Your memory cells patrol your blood and are able to quickly respond to a repeat offender with a flood of antibodies. The invader is destroyed before it can make more copies of itself and make you sick.
Vaccines aim to trigger your primary immune response and generate memory cells without making you sick.
There are several ways that vaccines train your immune system to fight off harmful invaders. These include using a weakened pathogen, an inactive pathogen or parts of the virus or bacteria.
Live-attenuated vaccines use a weakened form of the entire bacterium or virus (whole agent). As it’s the closest to an actual infection, it causes the strongest response of all types of vaccines. You should only need one or two doses to have full protection for a lifetime.
Most people shouldn’t get sick from a weakened version of a bacterium or virus. But if you’re living with a condition that weakens your immune system, you should ask your healthcare provider before getting vaccinated with a live virus.
Examples of live-attenuated vaccines include:
Inactivated vaccines contain dead bacteria or viruses. You can’t get sick from an inactivated pathogen. They’re safe for people with weakened immune systems, but don’t provide protection that’s as strong as a live vaccine does. You may need to get additional vaccinations over time to remain protected.
Examples of inactivated vaccines include:
Subunit vaccines use only part of a virus or bacterium. Similar to inactivated vaccines, subunit vaccines are safe for people with weakened immune systems but might require additional doses to keep you protected.
Subunit vaccines can take longer for scientists to create because they need to find the part of the pathogen that makes the most effective vaccine.
You might hear some subunit vaccines called recombinant vaccines. This describes how scientists made more copies of the part of the pathogen used in the vaccine.
Types of subunit vaccines include:
Protein subunit vaccines
The immune response to protein subunit vaccines is caused by specific proteins taken from a pathogen. Examples include:
The immune response to polysaccharide vaccines is caused by chains of sugar molecules (polysaccharides) from a bacterium. Some pneumococcal vaccines are polysaccharide vaccines.
The immune response to conjugate vaccines is caused by a combination of a polysaccharide with a protein, like a toxoid (makes a stronger immune response). The Haemophilus influenzae type B (Hib) vaccine is a conjugate vaccine.
Toxoid vaccines don’t use any part of the pathogen to make the vaccine, but instead, use a weakened form of the toxin (toxoid) that some bacteria produce. Your body has an immune response to the toxin rather than the bacterium itself. You need booster shots to stay protected over time. Diphtheria and tetanus vaccines are examples of toxoid vaccines.
Nucleic acid vaccines use your own cells to make part of a virus or bacteria. They can use DNA or messenger RNA (mRNA) to do this, but current vaccines use mRNA. The vaccine contains the instructions that your body uses to make antigens (the unique part of the pathogen that your body recognizes as an invader). Your body has an immune response to the antigen and will remember how to attack it if it tries to infect you in the future. Just like the mRNA your body produces for its own instructions, the mRNA from the vaccines breaks down in your body in a few days. We have mRNA vaccines for COVID-19.
Now that scientists have the technology to create mRNA vaccines, they can be made for new pathogens fairly quickly — this is how the COVID-19 vaccines were made so fast. This is a huge advantage over vaccines that contain all or part of a pathogen.
Vector vaccines use a harmless virus (vector) to deliver the pathogen you want to be vaccinated against. Ebola vaccines and some COVID-19 vaccines are vector-based.
Not all vaccines are used to prevent disease. Cancer vaccines, a form of immunotherapy, train your immune system to recognize and attack cancer that’s already in your body. As cancer cells often hide from your immune system, vaccines could be a way to get your immune system to notice them again and destroy them.
Vaccines are made of the active ingredient (which causes the immune response that gives you protection), ingredients that help the vaccine work well and ingredients that prevent contamination and make the vaccine last longer. Sometimes, they’ll also have small amounts of products that were used to make the vaccine.
Vaccines can include:
Today the most commonly used vaccines include those against:
Almost everyone should get vaccinated against vaccine-preventable illnesses, unless you have a weakened immune system or your healthcare provider recommends that you don’t. If you have a weakened immune system, live vaccines can be dangerous. If you’re pregnant, live vaccines could pose a risk to the fetus. Ask your healthcare provider if it’s safe for you to get vaccines.
You usually get vaccines:
You can find out what vaccines are recommended for you based on age and risk factors from the Centers for Disease Control and Prevention/CDC (in the U.S.), the National Health Service/NHS (in the U.K.) or your local public health authority.
Vaccines are often given as shots, but there are also oral and nasal vaccines.
Some vaccines require more than one dose to be effective. This can be because:
An immunization schedule is a recommendation on when you should get vaccinated for each vaccine-preventable illness, usually based on age. You can find immunization schedules through the CDC (in the U.S.), the NHS (in the U.K.) or your local public health authority. The schedule will tell you how many doses you need and how far apart to get the doses.
Most people get vaccinated so they won’t get sick with an infectious disease. Viruses and bacteria can not only make you feel terrible, but they can also cause serious complications, which can be life-threatening or long-lasting. But vaccines can also help protect the people around you — your loved ones, people in your community and around the world — by stopping the spread of an illness.
There are personal, community and global benefits of vaccination. Vaccines:
Infants and people with weakened immune systems are particularly vulnerable to getting seriously ill with infectious diseases. But they aren’t yet vaccinated or can’t get vaccinated against them. The more people close to them and in their community who are vaccinated, the less likely it is they’ll get infected.
If enough people are vaccinated, a disease will stop spreading and eventually no one will get sick with it anymore. When there aren’t any more cases of a specific illness in an area, it’s considered eliminated. But as long as there are some cases still in the world, if enough people stop getting vaccinated, the disease can come back.
When there aren’t any more cases of a specific illness in the entire world, it’s considered eradicated. Smallpox is an example of a human disease that’s been eradicated through vaccination.
For people without underlying health issues, the main risks of getting vaccinated are side effects or an allergic reaction.
If you have a weakened immune system or underlying health issues, live vaccines could make you sick. Ask your healthcare provider before getting vaccinated.
Like every medication, vaccines go through a series of safety tests. Thousands of volunteers receive a vaccine before it’s released to the public. Different doses are tested to find the right balance between how well it works and how serious the side effects are. Vaccines don’t get approved if they don’t work without causing serious reactions.
No matter which type of vaccine is used, the active ingredients are broken down in your body or destroyed by your immune system within a few days. This means vaccines can’t cause long-lasting health effects.
Vaccine ingredients are tested to make sure that the amount of the ingredient you receive is safe. Most vaccines contain ingredients already found in your body or things you eat. For instance, aluminum salts, used as an adjuvant, are in drinking water and some medicines like antacids. Formaldehyde, used in making some vaccines, exists in small amounts in your body naturally (and even smaller amounts in any vaccine). Gelatin, used as a stabilizer, is in many foods.
Thimerosal, the ingredient most people worry about, has been studied extensively for safety. Studies show no evidence that thimerosal is harmful to humans. Thimerosal doesn’t contain the same kind of mercury that causes poisoning and it’s cleared from your body quickly. In the U.S., it’s currently only used in multidose vials of flu vaccine.
No, vaccines don’t cause autism spectrum disorder (ASD). A small study done in the late 1990s that linked the MMR vaccine to ASD has since been discredited. Numerous studies done around the world since then have found that vaccines don’t cause autism. These studies have compared hundreds of thousands of people based on whether or not they were vaccinated and whether or not they had a diagnosis of ASD. Even those at the highest risk of ASD are no more likely to be diagnosed after getting an MMR vaccine than those who aren’t vaccinated.
No, vaccines can’t alter your DNA. Viruses and bacteria used in vaccines are destroyed by your immune system, which gives you protection against future infections. DNA and mRNA in vaccines don’t interact with your DNA and can’t do anything to change it.
If you have a healthy immune system, you can’t get a disease from a vaccine. If you have a compromised immune system, there’s a risk that you could get sick from a live vaccine. Inactivated vaccines, subunit vaccines and mRNA vaccines don’t have anything in them that could actually infect you. Live vaccines use a weakened form of a virus or bacteria that can’t make healthy people sick. The side effects you experience from vaccines are reactions from your immune system making antibodies.
Talk to your healthcare provider about what vaccines they recommend for you. If you’re pregnant, are being treated for an ongoing health condition or have a weakened immune system, ask your provider which vaccines are safe for you.
Contact your provider if you have any serious side effects after getting vaccinated.
If you’ve never known anyone who’s had a particular disease, you might wonder why you should bother getting vaccinated for it. But think of it this way: The disease is uncommon because so many people have gotten vaccinated against it.
When enough people stop getting vaccinated or stop getting their kids vaccinated because they think they can’t get sick, the disease can begin spreading again. For instance, if someone who’s not vaccinated travels to a place where the disease still exists, they can bring it back to their community and cause an outbreak. Large outbreaks can overwhelm hospitals and put infants, older adults and people with weakened immune systems at risk for life-threatening illnesses. Instances of polio and measles in the U.S. in recent years are examples of how outbreaks of disease can still happen if not enough people are vaccinated.
Vaccines don’t always fully prevent you from getting sick, but that doesn’t mean they didn’t work. Some reasons you might still get sick with a disease you were vaccinated for include:
Vaccines can take decades to go from a theory in a lab to testing in humans. Once they get to human testing, it can take months or years to get approved. Vaccines go through the same testing that other drugs do. Several trials are required to find the safest dose of the vaccine, how effective it is and whether there are serious side effects.
There’s evidence that people have been inoculating against diseases since as early as the 10th century, but the first modern version of a vaccine was for smallpox. Back in the 1700s, people started noticing that people who had cowpox didn’t get smallpox. They found ways to infect people with the pus from cowpox to prevent them from getting smallpox, a much more serious illness. From there, Edward Jenner created the first vaccines for smallpox. Smallpox no longer exists (it’s eradicated) because of vaccinations.
A note from Cleveland Clinic
Vaccines have rid the world of smallpox and greatly reduced the threat of polio, measles and other diseases. Vaccines have saved millions of lives — from your friends and neighbors to people around the world.
The science of today’s vaccines may seem complex and even a little scary. But it’s based on knowledge we’ve had for centuries about how our bodies fight disease. Vaccines show your body the tools it needs to stop the battle before it starts.
Last reviewed by a Cleveland Clinic medical professional on 09/07/2022.
Learn more about our editorial process.