I heard there was a Phase 1 study to find a vaccine for COVID-19. What are the phases of a clinical trial?

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June 22, 2020

The Food and Drug Administration (FDA) is working with the U.S. Government and international partners to learn about the coronavirus disease (COVID-19), find treatments, and create a vaccine. What we know about COVID-19 is changing rapidly.

Normally, new medical products go through four phases of testing. Each phase (or stage) has a different purpose. Research was already being done on other viruses before the COVID-19 outbreak. This means COVID-19 research will likely have different starting points.

What is the usual starting point for research with real people?

Phase 1 is called the first stage of research involving people, which is also known as human subjects research. Phase 1 is the first time a new treatment is tried in humans. The goal of this research is to find out if a new drug or treatment is safe. Volunteers are watched closely to see if there are any unwanted effects.

Phase 1 studies usually only include small numbers of people. If the treatment is found to be safe, it can move to the next phase.

What happens in Phase 2 research?

Phase 2 is the first step to find out if a new treatment really works. Phase 2 trials include more people. The larger number of participants allows researchers to see if the treatment shows ways that it is effective. Phase 2 studies are also used to determine the most effective doses or frequencies of treatments, and to test the treatment in selected groups of subjects. For example, a person’s age or gender might affect their response to treatment. With more people, it’s possible to see if the treatment works the same for people of different ages or genders.

Phase 2 research also continues to watch for side effects. With a larger number of people, it’s possible to see different types of side effects.

If a new treatment works in Phase 2, why do we need Phase 3 studies?

Some people might have good results from treatments in Phase 1 or 2 trials. However, these good results could be due to a lot of factors other than the treatment. Even though Phase 2 trials have a larger number of subjects than Phase 1, even larger studies are needed to make sure the treatment effect is real and is safe.

Phase 3 trials can have as many as 3,000 people. With a larger group, researchers can set stricter guideline for saying when and how a treatment is effective. For example, they might control for a lot more factors to be sure that it’s really the treatment that makes the difference.

In some cases, a treatment may already exist. Phase 3 studies can compare two or more treatments to see which one works best.

Unexpected effects are tracked even more closely in Phase 3. As more people try the treatment, it becomes clearer which unexpected effects are likely, whether they are harmful, and who is more likely to have them.

Phase 3 studies are needed to get reliable answers about safety and effectiveness of the treatment being studied. If results are good, the new treatment can be approved for doctors to prescribe to patients.

Why keep studying a treatment after it has been approved?

Phase 4 trials happen after a drug is approved by the FDA. Phase 4 trials continue to track safety. More information is obtained about who is helped by a treatment. This research also finds out more about risks of combining different medical treatments and products.


This step-by-step process helps make sure new medical products can have the most benefits and least risks for people and communities. At each phase of research, it’s important to have diverse groups of patient volunteers. Diverse groups may include differences such as race, ethnicity, age, gender, and income, and overall health status. A new treatment can only move forward if it successfully passes each phase.

Because COVID-19 is a worldwide emergency, the process of getting new treatments to people who need them may be different, but the necessary steps for ensuring effectiveness and safety of patients will not be compromised.

Any progress on clinical trials to prevent and treat COVID-19 will surely make the news headlines. To learn more about the research terms you may hear in the news, check out Clinical Trials and You. To learn why research volunteers are so important for improving health, including ending the novel coronavirus pandemic, visit WePartner4Research. Follow us on Facebook for information about how clinical trials can help us find ways to prevent and treat COVID-19. Let us know what questions you have about how clinical trials work.

The official name of the virus that started a worldwide outbreak in late 2019 is “Severe Acute Respiratory Syndrome Coronavirus 2.”  The abbreviation is SARS-CoV-2. When the virus infects someone, the diagnosis is called “Coronavirus Disease 2019.” The abbreviation is COVID-19. In COVID-19, ‘CO’ stands for ‘corona,’ ‘VI’ for ‘virus,’ and ‘D’ for disease.

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