Vaccine Safety: How Vaccines Are Tested & Monitored

It is completely normal to have questions about vaccine safety. Here is how the rigorous testing and monitoring process works to keep your baby safe.

Content reviewed against published CDC, FDA, AAP, WHO guidelines

Last reviewed: July 2026 · Editorial policy

Last reviewed: July 2026

Key takeaways

Vaccines undergo 10-15 years of development and testing before FDA approval, including large clinical trials in children.
After approval, multiple independent surveillance systems (VAERS, VSD, CISA) monitor safety continuously.
Every ingredient in a vaccine has a specific purpose and has been studied extensively for safety at the amounts used.
The amount of aluminum in all childhood vaccines combined is less than what a baby receives from breast milk or formula in 6 months.
Serious vaccine reactions are extremely rare — about 1 in a million doses for anaphylaxis.

How Vaccines Are Developed and Tested

Pre-clinical Research (Laboratory)

Before human testing, vaccines are studied in laboratories and animal models to identify promising candidates and evaluate initial safety. This phase alone can take several years.

Phase I Trials (Small Groups)

20-100 healthy volunteers receive the vaccine. Researchers monitor for serious side effects and determine dosing. For pediatric vaccines, adult trials come first.

Phase II Trials (Hundreds of People)

Hundreds of participants, including people in the target age group, receive the vaccine. Safety, dosing, immunogenicity (immune response), and schedule are evaluated.

Phase III Trials (Thousands of People)

Thousands to tens of thousands of participants receive either the vaccine or a placebo in randomized controlled trials. This phase establishes efficacy and identifies less common side effects. These trials often run for 1-4 years.

FDA Review and Approval

The FDA reviews all clinical trial data, manufacturing processes, and labeling. An independent advisory committee (VRBPAC) of scientists, physicians, and public health experts votes on whether to recommend approval.

Post-approval (Phase IV) Monitoring

After approval, safety monitoring continues indefinitely through VAERS, the Vaccine Safety Datalink (VSD), and the Clinical Immunization Safety Assessment (CISA) Project. These systems can detect rare adverse events that even large clinical trials might miss.

What Is in Vaccines (and Why)

Ingredient Type	Example	Purpose
Antigen	Weakened or inactivated virus/bacteria	Trains the immune system to recognize the pathogen
Adjuvant	Aluminum salts	Strengthens immune response so less antigen is needed
Preservative	Thimerosal (multi-dose flu vials only)	Prevents bacterial/fungal contamination
Stabilizer	Gelatin, sugars	Keeps vaccine effective during storage and transport
Residual	Formaldehyde (trace)	Used during manufacturing to inactivate viruses; removed to trace amounts

Note: Thimerosal has been removed from all childhood vaccines (except some multi-dose flu vaccine vials) since 2001 as a precautionary measure. Multiple large studies have confirmed no link between thimerosal and autism or other health problems. Single-dose flu vaccines without thimerosal are available.

How Vaccine Safety Is Monitored After Approval

VAERS (Vaccine Adverse Event Reporting System)

A national early warning system co-managed by CDC and FDA. Anyone — patients, parents, healthcare providers — can submit reports of health events that occur after vaccination. Reports serve as signals for further investigation; they do not prove that a vaccine caused the event.

VSD (Vaccine Safety Datalink)

A collaboration between the CDC and 9 integrated healthcare organizations covering over 12 million people. VSD uses electronic health records to conduct rigorous studies comparing vaccinated and unvaccinated populations to identify real associations.

CISA (Clinical Immunization Safety Assessment)

A network of vaccine safety experts at medical research centers that provides clinical consultations for complex adverse events and conducts focused research studies on vaccine safety.

Frequently asked questions

How do we know vaccines are safe for babies?

Vaccines undergo 10-15 years of testing before FDA approval, including three phases of clinical trials involving thousands of participants. After approval, multiple surveillance systems (VAERS, VSD, CISA) continue monitoring for rare side effects. The CDC, FDA, and independent advisory committees review safety data on an ongoing basis.

What ingredients are in vaccines and why?

Vaccine ingredients include antigens (the active component that trains immunity), adjuvants like aluminum salts (to strengthen immune response), preservatives like thimerosal in multi-dose flu vials (to prevent contamination), and stabilizers like gelatin (to keep vaccines effective during storage). Each ingredient has been extensively studied for safety at the amounts used.

Is the aluminum in vaccines harmful to babies?

No. Babies receive more aluminum from breast milk and formula in the first 6 months than from all vaccines combined. The small amounts of aluminum salts in vaccines are quickly eliminated by the body. Studies show no link between vaccine aluminum and health problems.

What is VAERS and how does it work?

VAERS (Vaccine Adverse Event Reporting System) is a national early warning system co-managed by the CDC and FDA. Anyone can report a health event after vaccination. VAERS reports do not prove causation — they are signals that trigger further investigation by scientists using more rigorous methods.

Are vaccines tested on children before being approved?

Yes. Pediatric vaccines are specifically tested in children during clinical trials. Phase III trials typically include thousands of children and compare vaccinated groups to placebo groups, monitoring for safety and efficacy over months to years before FDA approval.

Bottom line

Vaccines are among the most rigorously tested medical products in the world. Before your baby receives any vaccine, it has been studied for over a decade and reviewed by independent experts. After approval, multiple surveillance systems continuously monitor safety. Serious reactions are extremely rare.

Having questions about what goes into your baby's body is a sign of being a thoughtful parent. The science is clear: vaccines are safe and effective.

Our commitment to accuracy

Every page is reviewed against published guidelines from authoritative medical organizations including the CDC, WHO, AAP, ACOG, ASHA, and NIH.

2,705 evidence-based guides6 authoritative medical sources

Editorial Policy Medical Review Process Sources & References