New to vaccine information? Start here.
You do not need to know which organization to search or which website to trust. Start with your question, and ACIC will point you toward credible, evidence-based information.
A few basics first
These short answers cover well-established vaccine science. Use the source links if you want to go deeper.
What does a vaccine actually do?
Vaccines prepare the immune system to recognize and respond to a disease threat. Depending on the vaccine, the immune system may be shown an antigen, a harmless form or part of a germ, or instructions that allow the body to make an antigen.
The immune response can then help the body respond more quickly if it encounters the disease in the future.
Why vaccinate against diseases we rarely see?
Vaccination has made several diseases that were once common in the United States much less common. That success does not mean the diseases no longer matter.
Vaccine-preventable diseases can still circulate or be introduced into communities, and lower vaccination coverage can increase the risk of outbreaks or resurgence.
How is vaccine safety evaluated?
Vaccines are studied before they are approved or authorized for use. Safety monitoring continues after vaccines are in use through multiple monitoring and surveillance systems.
Like other medical products, vaccines can cause side effects. Questions about known side effects, ingredients, safety signals, or how safety monitoring works deserve specific answers.
What’s in a vaccine?
Vaccine ingredients depend on the specific product. Ingredients can include the component that produces the immune response as well as substances used to improve the immune response, maintain stability, prevent contamination, or support manufacturing.
If you want to know about an ingredient in a specific vaccine, use the FDA’s current information and the product’s official labeling.
What are you trying to figure out?
You do not need to sort through every vaccine website. Choose the question closest to yours.
Which vaccines are recommended?
Find current recommendations for children, adults, and pregnancy.
Go to Vaccine Schedules →I have a safety question or concern.
Explore side effects, ingredients, safety monitoring, evidence, and common questions.
Go to Questions & Safety →Why am I seeing different recommendations?
Understand the difference between evidence, recommendations, requirements, and individual health decisions.
Understand Vaccine Guidance →I need a vaccine, records, or local help.
Find Allegheny County and Pennsylvania vaccine access, records, and program resources.
Find Local Resources →It’s okay to have questions.
People may want to know why a vaccine is recommended, what side effects are possible, what’s in a vaccine, how safety is monitored, or why two organizations seem to be giving different advice. Those questions deserve clear, evidence-based answers.
Start with your vaccine question →Want to understand the science more deeply?
These organizations maintain detailed vaccine education for people who want to explore beyond the basics.
CHOP Vaccine Education Center
Detailed information about the immune system, vaccine science, vaccine safety, ingredients, diseases, and vaccine development.
Explore CHOP Vaccine Education Center →American Academy of Pediatrics
Pediatric vaccine information and guidance for families and healthcare professionals.
Explore AAP immunization resources →World Health Organization
Plain-language explanations of vaccination, immunity, and vaccine science from a global public health perspective.
Explore WHO vaccine information →U.S. Food & Drug Administration
Information about vaccine regulation, approval, ingredients, product information, and safety monitoring in the United States.
Explore FDA vaccine information →How ACIC chooses vaccine information
ACIC prioritizes credible medical, scientific, professional, public health, and Pennsylvania sources. We use the source that best fits the question and link directly to the organization responsible for maintaining the information whenever possible.
Read ACIC’s Clinical Source Standard →
This page provides general educational information and does not replace individualized medical advice or clinical judgment. A qualified healthcare professional can help answer questions about your individual health or vaccination needs.