Vaccine Safety Testing

One of the things I frequently hear from parents when discussing immunizations is a concern for the general safety of vaccines. Many parents are under the impression that there is a lapse in safety testing protocols when it comes to vaccines. So, let’s take a look at the process of vaccine approval in the U.S.


Just like any other medication, vaccines go through a rigorous approval process conducted by the Food and Drug Administration (FDA). Each new vaccine goes through the following steps:

  1. Prior to any human testing, vaccines are put through a variety of safety and efficacy tests in the laboratory, including animal testing.
  2. Phase 1: This is the first phase of human testing. A small study sample is tested for major adverse reactions.
  3. Phase 2: A larger group of people is given the vaccine and studied further for adverse reactions, as well as any common side effects that may arise as a result of the vaccination. This helps establish the overall safety of the vaccine.
  4. Phase 3: An even larger group of people is studied, which allows for detection of even less common adverse events and side effects. The new vaccine is also tested in conjunction with the other vaccines that would also be given at the proposed interval(s) in order to make sure they are compatible.


If any problem with the vaccine is identified at any of the testing stages, development may be stopped or delayed. After trials are complete, the information is presented to the FDA for formal approval.


But the testing doesn’t end there. Long after a vaccine is approved, surveillance continues using the Vaccine Adverse Event Reporting System (VAERS). This is a passive reporting system that allows anyone to report something that happened to them after receiving a vaccine. Reports range from fevers to seizures to turning into the Incredible Hulk (really!). It’s important to realize that literally anything can be reported, which means that the data must be constantly monitored to see if patterns develop. Further testing may be done at any point in order to support the development and use of the vaccine (also referred to as phase 4 studies).


These testing procedures ensure vaccine safety as well as efficacy. Understanding this process is important to being able to interpret and counter some arguments against immunization. Given that the process may take many, many years before a vaccine is approved and licensed, I feel quite confident saying that they are thoroughly investigated prior to being available to my patients.