Most vaccines have been in use for a long time and contribute every year to keeping millions of people healthy worldwide. Before being integrated into a vaccination program, each vaccine undergoes thorough testing to ensure that it is safe and effective. In this article, we explain in easy-to-understand terms the five (5) stages of vaccine development. You can also find them on the World Health Organisation (WHO) website.
1. First stage: laboratory research on the microbe
Researchers study how to activate the immune system using the specific microbe of the disease they want to prevent.
The immune system is the body’s natural defense force, protecting against diseases by recognizing and fighting the microbes that cause them (such as viruses, bacteria, and parasites). During this stage, researchers identify a part of the microbe (called an antigen) that can trigger the alarm to mobilize the immune system for combat. This mobilization of the immune system is called the “immune response.”
The antigen is chosen based on certain criteria, including two main ones:
- Stability: The antigen must be stable and able to maintain its integrity during production, storage, and administration of the vaccine.
- Ease of production: The antigen must be able to be produced on a large scale at low cost.
Once this part of the microbe (antigen) is identified, researchers develop an experimental vaccine that will be tested in the next stages below.
2. Second stage: laboratory testing on animals
During this stage, researchers administer the vaccine to laboratory animals whose bodies function somewhat like the human body. This initial test aims to see if the vaccine is not harmful to the body and if it can mobilize the body’s immune defenses. In other words, it verifies that the vaccine is safe (vaccine safety) and that it can trigger an immune response in animals genetically similar to humans. When researchers are sure that the vaccine is not harmful and can trigger the immune response, they move on to the third stage, which is subdivided into 3 phases called “clinical trials”, which are conducted on humans.
3. Third Stage: Clinical Trials
Phase 1: Testing on a small group of volunteers
The vaccine is given to a small group of people to ensure that, like in the animal tests of the 2nd stage, it is safe (not harmful to humans), and that it effectively triggers an immune response. This phase also helps determine the correct vaccine dosage to administer to humans for the best possible effectiveness with the fewest side effects. This is usually done on young and healthy adults.
Phase 2: Testing on several hundred volunteers
The vaccine is given to a larger number of people to further assess its safety and efficacy (effectiveness). Participants in this phase have the same characteristics (such as age, sex) as the people for whom the vaccine is intended in real life. A group not given the vaccine is usually included in this phase as a comparison group to determine if the changes observed in the vaccinated group are caused by the vaccine or if they occurred by chance.
This phase usually involves several trials to evaluate different age groups and different vaccine formulations. That is why you will hear about “phase 2a, phase 2b,” etc.
Phase 3: Testing on thousands of volunteer people
Here, the vaccine is given to thousands of people. At the same time, another group of people is given a product similar to the vaccine but does not contain the vaccine. The two groups will be compared after a while to check again the effectiveness and safety of the vaccine, and to determine if the changes observed in the vaccinated group are caused by the vaccine or if they occurred by chance.
The purpose of this phase is to determine if the vaccine is effective against the disease it is designed for and to study its safety in a larger number of people. Most of the time, phase 3 trials are conducted in several countries and at several sites within the same country to ensure that the vaccine’s performance results apply to several different populations.
NB: During phases 2 and 3, precautions are taken to ensure that the results cannot be falsified in favor of or against the vaccine: during these tests, neither the voluntary participants themselves nor the scientists/researchers know who among them received the real vaccine or the similar product.
4. Fourth stage: regulatory approval
The safety and efficacy of a vaccine must be proven in a large population before it is approved and introduced into a national vaccination program. When the results of all these clinical trials are available, international health authorities and those of each country carefully review the study data and decide whether to authorize the use of the vaccine or not.
5. Fifth stage: continuous monitoring
After introduction, the vaccine is continuously monitored to ensure that over time, it remains safe for humans and still effective in preventing the disease for which it was manufactured. Based on new information obtained during this monitoring, adjustments may be made regarding vaccine use.
Key Takeaways
In general, vaccine development follows a fairly rigorous process, which ensures that only vaccines with the best possible effectiveness and the fewest side effects are approved and introduced into national vaccination programs. Also, it is important to emphasize that the decision to introduce or not a vaccine is made taking into account the benefits of this vaccine compared to the dangers associated with the targeted disease. Certainly, there are special cases like the COVID-19 vaccine, which has raised many legitimate concerns given the global context and the effects observed during the follow-up stage.
Our article explaining in detail the role of vaccination will give you more details on “the Science behind Vaccines.” You can also better understand the specifics by reading our articles on the measles vaccine and its multiple doses. Given the current news about malaria vaccination, I invite you to read our article specifically about the RTS,S/AS01 or Mosquirix vaccine, newly introduced into the Cameroon Expanded Vaccination Program on January 22nd (2024).
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Dr Christelle Mpoulet, Field Epidemiologist Physician and Freelance Writer (production of medical content combining scientific rigour and editorial clarity for an effective and impactful health communication)
