There are three phases of a clinical trial for a new vaccine, all of which come after a pre-clinical trial which determines that the vaccine is safe.

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Before any new vaccine becomes available, including the new COVID-19 vaccines, it must go through the steps and stages that the FDA requires to assure the vaccine is safe and effective. Vaccines work by triggering your body’s defense system – called your immune system – to fight off an infection from invading germs like viruses or bacteria. Once you have immunity, your immune system will quickly recognize the invader and fight it off.

Jump to: How Do COVID Vaccines Work?

The part of the vaccine that triggers the immune response is called an antigen. The antigen could be a weakened or dead germ, a part of a germ, or the genetic material of the germ. Exposing your immune system to the antigen causes your immune system to make antibodies that will recognize the antigen in the future. These are the stages and phases of vaccine development:

In the research and development stage, vaccine researchers do testing in laboratories to find out what type of vaccine will work. Testing may start in animals before it is tested in people. Testing before done before human testing is called the pre-clinical stage.

The FDA will review the pre-clinical testing to find out if it is safe to start human testing, called clinical development. Clinical development occurs in three phases of clinical studies:

• In phase 1, the focus is safety. The vaccine is usually tested on a small number of people, called research subjects, that volunteer to be tested. The number may be between 20 and 30 subjects. The subjects are healthy and the trial will see if giving the vaccine has any significant side effects, called adverse effects.
• In phase 2, a vaccine that did not have significant adverse effects moves in to a larger trial. There be hundreds of people. Subjects in these studies will be tested with different doses and they will be randomly assigned to a group that receives the actual vaccine, or an already approved vaccine, or an inactive (placebo) vaccine. This type of trial is called a randomized controlled trial.
• In a phase 3 trial, a vaccine that was effective and safe in a phase 2 is given to thousands of subjects. These trials are also randomized and controlled. The immune response to the vaccine, how well it protects people from infections and severe disease, and any adverse reactions will be studied.

If a new vaccine makes it through all these trials, and the FDA finds that the results meet their requirements for safety and effectiveness, the vaccine will be approved. In some cases, as with the COVID 19 vaccines, there may be an emergency use approval before full approval.

COVID 19 Vaccines: How Do COVID Vaccines Work?

As of October 2021, there are three FDA-approved COVID-19 vaccines in the United States. These are the three options:

• The Pfizer vaccine (also called the Pfizer-BioNTech vaccine) was the first COVID 19 vaccine approved by the FDA for emergency use in December 2020. It now has full approval and is recommended for people ages 12 or older. This vaccine is given as two injections 21 days apart. It takes full effect about two weeks after the second injection. Common side effects are chills, headache, arm pain, and tiredness. These side effects usually go away in one to two days. This vaccine causes immunity by injecting a piece of genetic material from the COVID 19 virus that triggers the immune response. This is called an mRNA vaccine. Studies show that this vaccine is very effective at preventing COVID19 infections and even more effective at preventing severe infection and death.
• The Moderna vaccine was approved for emergency use by the FDA shortly after the Pfizer vaccine. It is now fully approved for people age 18 and older. This vaccine is given as two shots 28 days apart and reaches full effect two weeks after the second shot. Side effects are similar to Pfizer. Its effectiveness at preventing infections and severe disease is also similar. The Moderna vaccine is also an mRNA vaccine.
• The Johnson & Johnson vaccine was the last vaccine approved by the FDA. It is now recommended for people ages 18 and older. This vaccine is given as single shot and reaches full effect within two weeks. Side effects are similar to Pfizer and Moderna, but may be milder. This vaccine is not an mRNA vaccine. It uses a harmless cold virus as a shell to carry COVID 19 protein genes that trigger the immune response. The effectiveness of this vaccine is very good but not quite as good as the Moderna and Pfizer vaccines. Giving a booster shot increase the effectiveness up to the level of the other vaccines.

After a vaccine is approved, the FDA will continue to check on its safety and effectiveness over time. Vaccines have been given to many millions of babies, children, and adults over many years. They have a long history of being safe and effective. In fact, according to the CDC, the risk of dying from COVID 19 if you are fully vaccinated is about one in 10,000.