Vaccine Information
Browse individual vaccine pages and plain-language summaries.
HOW VACCINES ARE TESTED, APPROVED, AND MONITORED
Vaccines are thoroughly tested for safety before approval and regularly monitored afterward.
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TESTING
This includes small groups of 20-100 people, larger groups of 100-300, and finally thousands of people (usually between 1,000-3,000).
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APPROVAL
A vaccine is approved by the appropriate regulatory agencies only after testing shows how effective the vaccine is at preventing illness and how safe it is.
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ONGOING MONITORING
After approval, vaccines continue to be monitored for quality, effectiveness and safety. This includes three monitoring systems ensuring vaccine safety.
- Vaccine Adverse Event Reporting System (VAERS): A U.S. system where anyone can report health problems after vaccination to help spot possible safety concerns. These reports aren’t verified and don’t prove if a vaccine caused a problem but help scientists decide what to study further.
- Vaccine Safety Datalink: A system that uses real medical records from health care organizations to study vaccine safety more closely. Researchers use it to check whether a vaccine actually caused a problem and to measure how often side effects happen.
- Clinical Immunization Safety Assessment Project: A network of doctors and scientists who study rare or complex vaccine reactions and help health care providers understand and manage them.
SIDE EFFECTS
Some side effects may happen after vaccination. These can vary by vaccine and are temporary. The most common side effects are arm soreness, redness at the injection site, tiredness, and mild fever. A health care provider can help you understand what to watch for based on the vaccine you receive.
Minor side effects from vaccines can be uncomfortable. But getting sick with these diseases can cause more severe and long-lasting health issues than temporary side effects from vaccines.
HOW VACCINES WORK
Understand the step-by-step process of how a vaccine works in the body.
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EXPOSURE
Vaccines introduce the body to a small part of a germ, a killed or weakened version of a germ, or instructions to the body on how to produce part of a germ.
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RECOGNITION
Specific cells in the immune system recognize the harmless germ, and signal other cells to get ready to fight it.
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RESPONSE
The immune system responds by producing tiny defenders (or antibodies) that help find and fight viruses. Each of these antibodies matches a specific germ, helping the body recognize and respond in the future.
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PROTECTION
If the real germ enters the body, the immune system uses that memory to fight it off far more quickly and effectively. This quick response keeps you from becoming really sick and can often keep you from getting sick at all.
DIFFERENT VACCINE TYPES
There are many types of vaccines, but they all work with your body to protect you from getting sick. Learn more about some common types of vaccines below.
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Live Attenuated
These vaccines use a live but weakened form of a germ that can’t make you sick.
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Messenger RNA (mRNA)
These vaccines don’t use an actual virus. Instead, they give the body instructions to make a harmless piece of the virus (a protein), so the immune system can learn to recognize and fight it.
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Inactivated
This type of of vaccine uses an inactivated, or killed germ, which can't make you sick. It is safe and effective, but the protection may not last as long as with live vaccines. That’s why you sometimes need booster shots to stay protected.
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Protein Subunit
This type of vaccine uses just one small part of a germ (usually a protein) to teach the immune system how to recognize it and fight it.