How are vaccines tested?

How are vaccines tested?

(Plain Press, November 2020)        Vaccines developed to combat diseases such as COVID-19 go through a clinical trial process to test whether they are safe and whether they will work. The vaccines being developed to prevent infection with coronavirus are going through basically the same process, though some steps are being run concurrently or skipped in order to speed the process. 

   Before vaccines are tested in people, scientists first test them on cells in a laboratory and then give them to animals to see if there’s a reaction called an immune response, which is how the body fights an infection. 

Phase 1

   A vaccine is tested on between 20 and 100 healthy volunteers to test whether the vaccine is safe and what dose produces an immune response in people.  

   This step usually lasts a few months.

Phase 2

   The vaccine is then tested on up to a few hundred people, including those who are at high risk of contracting the virus. For coronavirus, this could include older adults and people with health conditions such as heart or lung disease. Researchers again check for safety issues and vaccine-related side effects. 

   This phase usually lasts a year or more. 

Phase 3

   A vaccine that reaches this phase will involve testing on thousands of people to confirm that it works, and is usually compared to a placebo, which in vaccine trials is often a saline injection. This is how researchers and regulatory bodies like The U.S Food and Drug Administration establish that the benefits of the vaccine outweigh any risks.

   This step can last from one to four years and continues to test the effectiveness in a large population and monitor for bad reactions.


   After a Phase 3 clinical trial, a developer can apply for approval to use a vaccine, which includes a review of all information collected during the clinical trials. If approved, scientists continue to collect information on the vaccine and any side effects and test how well it works when used in the wider population. 

How do different types of vaccines work?

   Genetic vaccines deliver small pieces of a virus into our cells to provoke our body to build an immune response. 

   Protein-based vaccines contain whole proteins or bits of protein from a virus that can stimulate an immune response. Some flu vaccines are made with proteins. 

   Viral vector vaccines use modified bits of a virus into our cells or our bodies. As they replicate, either quickly or slowly, our body responds with cells that help us build immunity. 

   Inactive or attenuated vaccines are created from a weakened virus or a “killed” virus. Some examples of vaccines made from weakened virus include measles, chickenpox and smallpox vaccines. Inactive vaccines include flu vaccines, polio and rabies.

Source: U.S. Food and Drug Administration, U.S. Centers for Disease Control and Prevention,

Editor’s Note: This information about vaccines accompanies articles from Ideastream’s Coping with COVID-19 series. The series is a partnership between the Northeast Ohio Solutions Journalism Collaborative and The Cleveland Observer. It is presented as part of ideastream’s Coping With COVID-19 project, which is funded by the Third Federal Foundation and University Settlement.

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