Questions deserve answers, not judgment.
If something about a vaccine has made you stop and wonder, start with the question you have. ACIC can help you understand what credible evidence shows, what it cannot tell us, and where to go for more information.
What are you wondering about?
Choose the question closest to yours. You do not need to sort through a long list of vaccine websites first.
What reactions can happen after vaccination?
Learn the difference between an expected side effect, an adverse event, and a reason to seek medical care.
What’s in a vaccine, and why is it there?
Find reliable information about ingredients and where to check the contents of a specific vaccine.
How are vaccines evaluated before use?
Understand clinical testing, FDA review, and why safety evaluation continues after approval.
How are possible safety problems detected?
See how different safety systems identify and investigate possible concerns.
I saw something reported in VAERS. What does it mean?
Understand what a VAERS report can tell us — and what it cannot establish by itself.
I want to check a vaccine claim or concern.
Use a few basic questions to judge the evidence before deciding what a claim means.
How vaccine safety is evaluated
Safety evaluation does not end when a vaccine becomes available. Different parts of the process answer different questions.
How are vaccines evaluated before use?
Vaccine development includes laboratory research and clinical studies in people. FDA reviews scientific information about safety, effectiveness, manufacturing, purity, and potency before deciding whether to approve a vaccine.
Pre-approval studies are important, but they cannot answer every safety question. Once a vaccine is used in much larger populations, ongoing monitoring can help identify possible problems that may be uncommon or difficult to detect in clinical trials.
How are possible safety problems detected after a vaccine is in use?
The United States uses more than one vaccine-safety system. Some systems are designed to detect possible warning signals. Others use health records, research studies, and clinical expertise to investigate questions in more detail.
A possible signal is a reason to look more closely. It is not, by itself, proof that a vaccine caused a health problem.
I saw something reported in VAERS. What does that mean?
VAERS, the Vaccine Adverse Event Reporting System, is an early-warning system co-managed by CDC and FDA. It accepts reports of health events that happen after vaccination.
A VAERS report does not, by itself, show that a vaccine caused the reported event. Reports can describe events that are related to vaccination, unrelated events that happened afterward, or events that need more investigation.
VAERS is useful for identifying unusual patterns or possible safety signals. Those signals can then be examined using other data and study methods.
Common safety questions
Start with the category that matches your concern. The details depend on the specific vaccine, so these answers link to sources that maintain more complete information.
What reactions can happen after vaccination?
Vaccines can cause side effects. Common reactions to many vaccines include soreness where the vaccine was given or a temporary fever, but the possible reactions differ by vaccine.
A health problem that happens after vaccination is called an adverse event. That timing alone does not establish that the vaccine caused the event.
What’s in a vaccine, and why is it there?
Ingredients vary by vaccine. Some ingredients produce the immune response. Others may be used to strengthen that response, keep the vaccine stable, prevent contamination, or support the manufacturing process.
If you are asking about a particular ingredient, the most useful next step is to look at information for that ingredient and the labeling for the specific vaccine.
I heard something about a vaccine. How do I check it?
A claim can sound convincing without showing whether the evidence actually supports it. These questions can help you slow the process down and look at what is known.
What exactly is being claimed?
Is the claim about one vaccine, all vaccines, one health event, or a particular group of people? Broad claims often require broader evidence.
What kind of evidence is being used?
A personal story, a VAERS report, a clinical trial, and a large population study provide different kinds of information. They should not be treated as interchangeable.
Does the evidence show timing or causation?
Something happening after vaccination does not automatically mean vaccination caused it. Researchers look for patterns, comparison groups, biological evidence, and other information when evaluating causation.
Is the information current and specific?
Recommendations, product labeling, and safety information can change. Check the date and whether the source is discussing the same vaccine, population, and question you are asking.
I have a question about a specific ingredient.
Start with the FDA ingredient information and the labeling for the specific vaccine. CHOP’s Vaccine Education Center also provides plain-language explanations of commonly asked-about ingredients.
I have a question about how a particular vaccine was tested.
Trial design can differ by vaccine, product, population, and stage of development. For a specific claim about a vaccine’s testing, look at the evidence for that product rather than assuming every vaccine was studied in exactly the same way.
I found a study or report and I’m not sure what it means.
Look at what question the research was designed to answer, who was studied, what comparison was used, how large the study was, and whether the result has been replicated or reviewed alongside other evidence. One study rarely answers every question about a vaccine.
ACIC’s source standard explains how we choose sources for vaccine and public-health information.
Read ACIC’s Clinical Source Standard →Something happened after vaccination. What should I do?
Start with the person’s health, not the reporting system.
If you are concerned about symptoms after vaccination, contact a qualified healthcare professional for medical advice. If there may be a medical emergency or severe reaction, seek emergency medical care.
Patients, parents, caregivers, and healthcare professionals can report an adverse event after vaccination to VAERS even when it is not clear whether the vaccine caused the event.
Reporting helps vaccine-safety systems look for possible patterns, but VAERS does not provide individual medical diagnosis or treatment.
Want to go deeper?
These organizations provide more detailed information about vaccine safety, ingredients, regulation, monitoring, and research.
FDA
U.S. vaccine regulation, product information, approval, ingredients, and safety information.
Explore FDA vaccine information →CHOP Vaccine Education Center
Detailed, plain-language information about vaccine safety, ingredients, immune science, and common concerns.
Explore CHOP Vaccine Education Center →CDC Vaccine Safety Systems
Information about the systems used to detect, monitor, and investigate possible vaccine safety concerns.
Explore vaccine safety systems →How ACIC chooses vaccine safety information
ACIC uses the source that best fits the question. Vaccine safety sources should be grounded in credible evidence and explain both what the evidence shows and its limitations.
Read ACIC’s Clinical Source Standard →
This page provides general educational information and does not replace individualized medical advice, diagnosis, or clinical judgment. Safety considerations can differ by vaccine and by person.