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What are the phases of a vaccine trial?

Posted on 23rd September 2020 by


In the midst of the COVID-19 pandemic, everyone has a vested interest in the development of a SARS-CoV2 vaccine that can successfully protect the population and prevent further deaths from occurring. The herd immunity of a vaccinated population would ideally allow our society to somewhat return to our pre-pandemic lifestyles, even with the pathogen existing in our environment. Although we are eagerly anticipating its creation, and many companies are racing to be the first to produce and distribute it globally, a vaccine has to first display a safe pharmacological profile, show efficacy in humans, and importantly, have benefits that outweigh the risks before it can receive approval from drug regulatory bodies. 

Initially, preclinical research and development is carried out in the laboratory using in vitro techniques or, if necessary, in vivo techniques in animals. Animal models can be given the vaccine to see if an immune response is produced.  If the data from preclinical research shows promise, it is then used to justify subsequent clinical studies in humans. 

Phase I Clinical Trial

Phase I clinical trials occur in a small group of healthy adult individuals, approximately 20-100 to evaluate safety. They answer the following questions; 

  • Is this vaccine safe?
  • Does the vaccine seem to work? Does it produce an immune response?
  • Are there any serious side effects?
  • How is the size of the dose related to the side effects?

Phase II Clinical Trial

Phase II clinical trials study the intervention in a larger group of people (several hundred), stratified by age, to determine efficacy and to further evaluate its safety. They answer the following questions; 

  • What are the most common short-term side effects?
  • How are the volunteers’ immune system responding to the vaccine?

Phase III Clinical Trial

Phase III studies study the safety and efficacy of the intervention in large groups of trial participants (from several hundred to several thousand) over time. They answer the following questions; 

  • How do people who get the vaccine and people who do not get the vaccine compare?
  • Is the vaccine safe?
  • Is the vaccine effective?
  • What are the most common side effects?

The regulators will then review the trial results and determine whether to approve the vaccine or not. 

Phase IV Clinical Trial

Drug companies may decide to conduct Phase IV trials after a vaccine is released. The manufacturer can continue to test the vaccine for safety, efficacy, and other potential uses. In the case of COVID-19, it may be necessary given the variability in the widespread population taking the vaccine.

Therefore, it is clear that each stage of the trial provides important information regarding whether the vaccine works in individuals, and whether there are side effects. Usually it takes 8 to 10 years to conduct a robust set of clinical trials before approving a vaccine. This does beg the question of how COVID-19 vaccine developers will combine the phases of the trials to deliver a vaccine to the population in record time. We will have to watch this space!

For more information you can view a: Coronavirus Vaccine Tracker

References (pdf)


Cathey Saha

I am a student studying a Bachelor of Science and Biomedical Science double degree at Monash University. I'm very passionate about pharmacology, pharmaceutical sciences and medical writing. With a strong understanding of study designs, research methods and reading systematic reviews, I hope to be involved in creating effective therapeutics-based robust evidence in the near future. For more information or discussion, follow me on twitter - @CatheySaha - or message via LinkedIn - https://www.linkedin.com/in/cathey-saha-56a7b3140/ . View more posts from Cathey

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