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What are vaccines?
Vaccines are injections (shots), liquids, pills, or nasal sprays that you take to teach your body's immune system to recognize and defend against harmful germs. For example, there are vaccines to protect against diseases caused by:
- Viruses, like the ones that cause the flu and COVID-19
- Bacteria, including tetanus, diphtheria, and pertussis
What are the types of vaccines?
There are several types of vaccines:
- Live-attenuated vaccines use a weakened form of the germ.
- Inactivated vaccines use a killed version of the germ.
- Subunit, recombinant, polysaccharide, and conjugate vaccines use only specific pieces of the germ, such as its protein, sugar, or casing.
- Toxoid vaccines that use a toxin (harmful product) made by the germ.
- mRNA vaccines use messenger RNA, which gives your cells instructions for how to make a protein (or piece of a protein) of the germ.
- Viral vector vaccines use genetic material, which gives your cells instructions for making a protein of the germ. These vaccines also contain a different, harmless virus that helps get the genetic material into your cells.
Vaccines work in different ways, but they all spark an immune response. The immune response is the way your body defends itself against substances it sees as foreign or harmful. These substances include germs that can cause disease.
What happens in an immune response?
There are different steps in the immune response:
- When a germ invades, your body sees it as foreign.
- Your immune system helps your body fight off the germ.
- Your immune system also remembers the germ. It will attack the germ if it ever invades again. This "memory" protects you against the disease that the germ causes. This type of protection is called immunity.
What are immunization and vaccination?
Immunization is the process of becoming protected against a disease. But it can also mean the same thing as vaccination, which is getting a vaccine to become protected against a disease.
Why are vaccines important?
Vaccines are important because they protect you against many diseases. These diseases can be very serious. So getting immunity from a vaccine is safer than getting immunity by being sick with the disease. And for a few vaccines, getting vaccinated can actually give you a better immune response than getting the disease would.
But vaccines don't just protect you. They also protect the people around you through community immunity.
What is community immunity?
Community immunity, or herd immunity, is the idea that vaccines can help keep communities healthy.
Normally, germs can travel quickly through a community and make a lot of people sick. If enough people get sick, it can lead to an outbreak. But when enough people are vaccinated against a certain disease, it's harder for that disease to spread to others. This type of protection means that the entire community is less likely to get the disease.
Community immunity is especially important for people who can't get certain vaccines. For example, they may not be able to get a vaccine because they have weakened immune systems. Others may be allergic to certain vaccine ingredients. And newborn babies are too young to get some vaccines. Community immunity can help to protect them all.
Are vaccines safe?
Vaccines are safe. They must go through extensive safety testing and evaluation before they are approved in the United States.
What is a vaccine schedule?
A vaccine, or immunization, schedule lists which vaccines are recommended for different groups of people. It includes who should get the vaccines, how many doses they need, and when they should get them. In the United States, the Centers for Disease Control and Prevention (CDC) publishes the vaccine schedule.
It's important for both children and adults to get their vaccines according to the schedule. Following the schedule allows them to get protection from the diseases at exactly the right time.
- Adult Immunization Schedule: Recommendations for Ages 19 Years or Older, United States, 2023 (Centers for Disease Control and Prevention)
- Get Vaccines to Protect Your Health (Adults Ages 19 to 49) (Office of Disease Prevention and Health Promotion) Also in Spanish
- Understanding How Vaccines Work (Centers for Disease Control and Prevention)
- Vaccine Basics (Department of Health and Human Services)
- Vaccines for Adults (Mayo Foundation for Medical Education and Research) Also in Spanish
Diagnosis and Tests
- Screening Checklist for Contraindications to Vaccines for Adults (Immunization Action Coalition) - PDF Also in Spanish
- Heart Disease, Stroke, or Other Cardiovascular Disease and Adult Vaccination (Centers for Disease Control and Prevention) Also in Spanish
- HIV Infection and Adult Vaccination (Centers for Disease Control and Prevention)
- Keeping Your Vaccine Records up to Date (Centers for Disease Control and Prevention) Also in Spanish
- Liver Disease and Adult Vaccination (Centers for Disease Control and Prevention)
- Vaccinations and Flu Shots for People with Cancer (American Cancer Society) Also in Spanish
- Vaccinations for Adults with Diabetes (Immunization Action Coalition) - PDF
- Vaccine Safety: MedlinePlus Health Topic (National Library of Medicine) Also in Spanish
- Vaccines Protect Your Community (Department of Health and Human Services) Also in Spanish
- Weakened Immune System and Adult Vaccination (Centers for Disease Control and Prevention)
- What's in Vaccines? (Centers for Disease Control and Prevention) Also in Spanish
- Current Vaccine Shortages and Delays (Centers for Disease Control and Prevention)
- There Are Vaccines You Need as an Adult (Centers for Disease Control and Prevention) Also in Spanish
- Travelers' Health: Destinations (Centers for Disease Control and Prevention)
- Vaccines.gov (Department of Health and Human Services) Also in Spanish
- Who Should Not Get Vaccinated with These Vaccines? (Centers for Disease Control and Prevention)
- Vaccine-Preventable Disease Photos (Immunization Action Coalition)
Health Check Tools
- Adult Vaccine Assessment Tool (Centers for Disease Control and Prevention) Also in Spanish
Statistics and Research
- Research: Vaccines (National Institute of Allergy and Infectious Diseases)
- Safeguarding Our Health: Vaccines Protect Us All (National Institutes of Health) Also in Spanish
- VaxView: Vaccination Coverage in the U.S. (Centers for Disease Control and Prevention)
- ClinicalTrials.gov: Vaccines (National Institutes of Health)
Journal Articles References and abstracts from MEDLINE/PubMed (National Library of Medicine)
- Article: Efficacy and Safety of a Protein-Based SARS-CoV-2 Vaccine: A Randomized Clinical...
- Article: NVX-CoV2373 vaccine efficacy against hospitalization: A post hoc analysis of the...
- Article: A Pilot Single-Blinded, Randomized, Controlled Trial Comparing BNT162b2 vs. JNJ-78436735 Vaccine...
- Vaccines -- see more articles
- Glossary of Vaccine Terms (Centers for Disease Control and Prevention)
- Overview of the Immune System (National Institute of Allergy and Infectious Diseases)
Find an Expert
- Centers for Disease Control and Prevention Also in Spanish
- National Institute of Allergy and Infectious Diseases
- National Vaccine Program (Department of Health and Human Services, Office of Infectious Disease and HIV/AIDS Policy)
- Childhood Immunization: MedlinePlus Health Topic (National Library of Medicine) Also in Spanish
- Summary of Maternal Immunization Recommendations (Centers for Disease Control and Prevention) - PDF
- Vaccines and Pregnancy (Organization of Teratology Information Specialists) Also in Spanish
- Vaccines and Pregnancy: 8 Things You Need to Know (Centers for Disease Control and Prevention) Also in Spanish
- Vaccinations and Older Adults (National Institute on Aging)
- Immunizations for people with diabetes (Medical Encyclopedia) Also in Spanish