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The Journey of a Vaccine

Learn about the four phases of clinical research, what questions researchers try to answer in each, and how a vaccine is developed, approved, and manufactured.

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    NIH Lays Groundwork

    Earlier NIH research on coronaviruses helped scientists quickly identify vaccines to test for SARS-CoV-2.

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    Phase 1: 20 to 100 Healthy Volunteers

    Researchers try to answer these questions:

    • Is this vaccine safe?
    • Are there any serious side effects?
    • Is the vaccine causing an immune response?

    First Studies in Humans

    Just over two months after researchers identified the new virus's genetic sequence, the NIH/Moderna vaccine candidate entered Phase 1 clinical trials.

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    Phase 2: Several Hundred Volunteers

    Researchers try to answer these questions:

    • What are the most common short-term side effects?
    • What’s the body’s immune response?
    • Are there signs that the vaccine protects against infection?

    Safely Speeding Up the Process

    When early Phase 1 data showed safety, researchers began Phase 2 testing. Overlapping the phases helped speed the process.

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    Phase 3: 1,000+ Volunteers

    Researchers try to answer these questions:

    • How do disease rates compare between people who get the vaccine and those who do not?
    • How well can the vaccine protect people from disease?

    FDA approves a vaccine only if:

    • It is safe and effective
    • Its benefits outweigh the risks

    NIH Builds a Network

    NIH created the Coronavirus Prevention Network (CoVPN) to conduct Phase 3 efficacy trials for COVID-19 vaccines and treatments.

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    Phase 4: Treatment is approved by the FDA and made available to the general public.

    FDA closely monitors the safety of the vaccine after the public begins using it. Researchers continue to collect data on the vaccine’s long-term benefits and side effects.

    Vaccines Arrive

    In December 2020, the U.S. FDA granted emergency authorization for two COVID-19 vaccines based on safety and efficacy data from clinical trials.

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