Skip to content

Vaccine Development

Developing vaccines is a complex and rigorous process that involves a number of steps before it is available to the public. Here’s a primer to better understand the process.

Clinical Trials

Vaccines are some of the most tested medical interventions in the world. They go through a rigorous testing process that can take years with several phases of clinical trials and licensure. The process includes:

Exploratory Phase: This phase happens in research labs and works to identify potential antigens (a foreign substance that helps the body create an immune response that might prevent a disease).

Pre-clinical Phase: In this phase, researchers take the antigen(s) they identified in the Exploratory Phase and test them in the lab (not on humans) to see if they produce an immune response.

Clinical Development

  • Phase I is a very small study (usually no more than 100 people) to determine if the vaccine is safe and to learn about what type of response the vaccine produces.
  • Phase II is a larger study with hundreds of participants that learns more about:
    • Safety
    • Effectiveness (known as immunogenicity or how strong an immune response is produced by the vaccine)
    • When the optimum time is to get the vaccine (the immunization schedule)
    • How much of the vaccine is needed (the dose size)
  • Phase III is the largest of the phases, usually with thousands of participants that continue to measure the safety (because rare side effects may not show up in small groups) and effectiveness of the vaccine.


Even after the rigorous research process, a vaccine still has a long road before it reaches your doctor’s office.

If a vaccine is found to be safe and effective during the clinical development phase, it is reviewed by the Food and Drug Administration (FDA) for approval. The FDA will look at all the data compiled through the clinical trial phases to decide whether or not it will approve the vaccine for use in the U.S. The FDA will only approve a vaccine if it is safe, effective, and the benefits outweigh any risks.

Almost every country in the world has an equivalent to the United State’s FDA which means vaccine data are reviewed by dozens of regulatory bodies around the world for safety and effectiveness.

In the United States, once a vaccine is licensed and approved for use, the ­Advisory Committee on Immunization Practices (ACIP) reviews all the data and provides guidance on how a vaccine should be used. The members of ACIP are vaccines experts including members of several physician academies. After the ACIP makes its recommendation, the Director of the Center for Disease Control and Prevention (CDC) reviews the data and ACIP’s recommendation. If the director approves the recommendation, the vaccine is added to the U.S. vaccine schedule.

Ongoing Safety Monitoring

Once approved, a vaccine continues to be monitored for potential side effects (sometimes called “adverse events”) through a series of safety systems:

  • Vaccine Adverse Event Reporting System (VAERS): VAERS Analyzes reports of adverse events that happen after vaccination. Anyone can submit a report to VAERS, and submissions do not mean that a vaccine definitely caused the event. See the FAQs below for more information on VAERS.
  • Vaccine Safety Datalink (VSD): Analyzes healthcare information from more than 24 million people. It groups health information and does not contain personal information so that data is useful but still protects people’s confidentiality.
  • Post-Licensure Rapid Immunization Safety Monitoring (PRISM): Analyzes healthcare information from more than 190 million people. Like VSD, it groups health information and does not contain personal information so that data is useful but still protects people’s confidentiality.
  • Clinical Immunization Safety Assessment Project (CISA): CISA is a collaboration between the CDC and 7 medical research centers that provides expert technical advice and research focused on vaccine safety and decreasing side effects.

Emergency Use Authorizations

In the United States, the FDA can grant temporary “emergency use” of a vaccine when there is a vaccine-preventable epidemic but the vaccine has not yet gone through the approval process. In order for a vaccine to be given an EUA, all of the phase I and II safety clinical trials must be complete and the vaccine must be found to be safe. However, instead of having final data that proves a vaccine is effective, the EUA allows a vaccine to be made available if preliminary data indicates “reasonable to believe” that the vaccine “may be effective.” An EUA is only given in emergency situations where a vaccine could save lives and help end an epidemic.

Several vaccines were approved under EUAs. The Ebola vaccine for West Africa was used prior to its final approval and was key in ending that epidemic. Three different vaccines for COVID-19 were also approved for emergency use prior to full FDA approval.

An EUA is temporary and the vaccine will still need to go through the full approval process. Any vaccine used under an EUA is still subject to all the same monitoring as other vaccines.

Want more information?

How do I know this information is credible?

We work for parents so we make sure that parent concerns are addressed using facts and science and our content is reviewed by experts who have spent their careers studying vaccines. Learn more about how we ensure we are bringing you the best information to help you make healthy choices for your family.

Understanding VAERS

VAERS is an important safety monitoring system. One of the strengths of VAERS is that anyone can report to it. That means that anyone who believes that something happened because of a vaccine can submit that to VAERS. This is great because it involves everyone in ensuring vaccines are safe. But not all of us are vaccine experts so it also means that many VAERS reports are not vaccine related. It requires some expertise to determine if a VAERS report is really related to the vaccine. And sometimes it is clear a VAERS report is not at all related to vaccines.

Some examples include:

  • Fight in school (VAERS ID 0403086-1)
  • Frostbite (VAERS ID 0284528-1)
  • Infection of mites (VAERS ID 0156190-1)

So, when you hear that a vaccine has a number of VAERS reports remember that most of them are probably not related to the vaccine at all. And those that are, are usually mild. Serious adverse events are about 1 in 1 million vaccinations. It’s important to understand the full picture in order to make safe and informed decisions for your family.

Back To Top