As a rule, researchers don’t begin jabbing people with experimental vaccines until after rigorous safety checks.
They test the vaccine first on small batches of people — a few dozen during Phase 1, then a few hundred in Phase 2, then thousands in Phase 3. Months normally pass between phases so that researchers can review the findings and get approvals for subsequent phases.
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There are ways to slash time off this process by combining several phases and testing vaccines on more people without as much waiting.
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It’s here that talk of fast-tracking the timeline meets the messiness of real life: What if a promising vaccine actually makes it easier to catch the virus, or makes the disease worse after someone’s infected?
That’s been the case for a few H.I.V. drugs and vaccines for dengue fever, because of a process called vaccine-induced enhancement, in which the body reacts unexpectedly and makes the disease more dangerous.
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So researchers might produce a viable vaccine in just 12 to 18 months, but that doesn’t mean you’re going to get it. Millions of people could be in line before you. And that’s only if the United States finds a vaccine first. If another country, like China, beats us to it, we could wait even longer while it doses its citizens first.
You might be glad of that, though, if it turned out that the fast-tracked vaccine caused unexpected problems. Only after hundreds or thousands are vaccinated would researchers be able to see if a fast-tracked vaccine led to problems like vaccine-induced enhancement.
“It’s true that any new technology comes with a learning curve,” said Dr. Paul Offit, the director of the Vaccine Education Center at the Children’s Hospital of Philadelphia. “And sometimes that learning curve has a human price.”