AstraZeneca’s best COVID vaccine result was a fluke. Experts have questions

Enlarge / Vials in front of the AstraZeneca British biopharmaceutical company logo are seen in this creative photo taken on 18 November 2020. (credit: Getty| NurPhoto)

Pharmaceutical giant AstraZeneca and the University of Oxford made an exciting announcement Monday: the COVID-19 vaccine they developed together appeared up to 90 percent effective at preventing disease. But in the days since, that exciting news melted into a pool of confusion after it became clear that the 90 percent figure came about from a complete accident. Now, experts are scratching their heads over what actually happened in the trial and what it means for the vaccine's future.

The questions all swirl around the vaccine's dosage regimen. In initial press releases, AstraZeneca and Oxford explained that researchers had used two different dosage regimens to test their experimental vaccine, AZD1222. In one regimen, trial participants received two full" vaccine doses, 28 days apart. In the other, participants received a half dose of vaccine followed by a full dose 28 days later.

Pooling results from trials in the United Kingdom and another in Brazil, the researchers found the two-full-dose regimen was 62 percent effective at preventing COVID-19-a good, but not great result. The half-dose/full-dose regimen, on the other hand, appeared 90 percent effective-a rather impressive result.

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