Health: The Meaning of the “Phases” of Vaccine Testing

The Story: There has been a good deal of discussion recently of the development of vaccines intended to immunize people against a Covid-19 infection. Unfortunately, this discussion includes news reports that use, without explanation, a terminology with which those who do not work in a bio-pharm concern are seldom familiar.


In early October, CNN reported that Johnson & Johnson’s coronavirus vaccine was ready to enter its “phase 3 testing.” That and many other such statements in many other stories have left many listeners or readers wondering, “what are the phases?”, and “Is this third phase the last?”

The answer is that the vaccine development process under the supervision of the US FDA involves four distinct stages. The first requires the injection of the vaccine into healthy adult volunteers. The point is to see if they remain healthy. If they do, scientists may move to the second phase. Phase 2 still concerns safety, but involves a larger pool of volunteers, drawn from a wider range of age groups. If no safety issues have arisen from these two phases, the focus turns to efficacy.

In phase 3, then, some volunteers receive a placebo, others receive the drug under study, and all volunteers are monitored, usually over a period of months, to see how many people of each group get infected with the disease the vaccine is intended to protect them from. Yet that is not the end of the process.

After a successful completion of phase 3, a vaccine may be approved by the FDA. Nonetheless, testing continues into a phase 4, which broadens the tracking and can last for years, looking for efficacy over a long period of time, and both the short-lived and the long-lasting side effects and safety.

In Pill Form:

The development of a safe and effective vaccine has taken and may continue to take a frustratingly long time. Skipping steps, though, and rushing a product to market, threatens results that could be far worse than a sense of frustration.



Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.