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What do Vaccine Phase 1-3 Clinical Trials Really Mean?

By Aparna Kumar, Editors: Sami Morse & Kathleen Navas

As more and more SARS-CoV-2 vaccines progress into the third phase of clinical trials, many are hopeful a successful vaccine will soon emerge, but what does that entail?

After passing pre-clinical trials, vaccines enter phase I of clinical trials, the first stage of human testing. According to the U.S. Food and Drug Administration (FDA), vaccine studies in this phase recruit a small number of closely monitored subjects, screening for safety and immunogenicity. Phase I trials for vaccines tend to last 6 to 12 months.

If deemed safe and effective, the vaccine advances to the next stage: phase II. Expanding the study to hundreds of participants, phase II clinical trials determine optimal dose and assess demographic impacts on efficacy, such as differing ages, ethnicities, and genders. This stage can last up to a year.

Next, the vaccine faces phase III clinical testing. In this stage, the study grows to thousands of participants. Lasting from 1 to 4 years, the data collected from this stage will provide researchers with the evidence of efficacy and safety of the proposed vaccine, which is essential for FDA approval for widespread use. Efficacy, as defined by the NIH, is the ability for a vaccine to prevent a clinical disease. The FDA recommends that labs aim for 50% efficacy for a COVID-19 vaccine to pass phase III trials.

Because of the large number of participants in this phase, any variations in the testing population that might affect the results of the trials are normalized. Large scale testing also allows researchers to identify cases in which a vaccine does not work or is harmful to a patient with specific demographics. Testing is also fully randomized, double-blind, and placebo controlled so that there are no biases in administration of vaccines that may impact the data.

If a vaccine passes phase III trials and becomes available to the general public, there are many CDC programs that monitor the safety of the vaccine for years to come, such as the The Vaccine Adverse Event Reporting System, The Vaccine Safety Datalink, Clinical Immunization Safety Assessment Project, as well as the Immunization Safety Office which monitor vaccine safety to detect any unexpected irregularities. Researchers may also conduct an optional phase IV of trials, which monitor the safety and efficacy of the vaccines after they have been made available to the public. Phase IV trials allow for identification of potential long-term adverse effects that were not detected in the initial clinical trials.

The fastest vaccine that has ever been made is the mumps vaccine, which took 4 years to go from viral sample to widely distributed vaccine. In an attempt to fast track the vaccine for SARS-CoV-2, the Department of Health and Human Services has announced Operation Warp Speed in an attempt to quickly provide a vaccine for the public. When experts cited a 12-18 month expected time frame for the coronavirus vaccine, it didn’t seem believable, but between March and October, 11 vaccines have gone from Phase 1 to Phase 3 clinical trials, and there are already distribution plans in place. Scientists are fitting the same rigorous testing criteria into a shorter time frame in an attempt to put an end to this pandemic as soon as possible.


Results from ongoing research and the current understanding of COVID-19 are constantly evolving. This post contains information that was last updated on October 20, 2020.

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