The Oxford Vaccine’s True Efficacy Remains Uncertain

The vaccine candidate sponsored by the University of Oxford and AstraZeneca has shown promise in preventing COVID-19 in Phase III clinical trials, but the evidence is not yet convincing enough to put it in the same category as competitors from Pfizer-BioNTech and Moderna. Consequently, it is likely that the Food and Drug Administration (FDA) will insist on more and better data before granting regulatory approval.

The preliminary results from the Oxford-AstraZeneca Phase III trial provided cause for optimism, but left many questions unanswered. While reporting an overall vaccine efficacy rate of 70 percent—a rate that exceeds the minimum 50 percent threshold set by the World Health Organization (WHO) and FDA for approving COVID-19 vaccines—the trial sponsors further announced that the efficacy of their vaccine was 90 percent for a limited subset of participants who received a half dose and then a full dose one month apart. 

Just under 1 in 4 participants in the vaccine arm of the trial received the half-dose/full-dose regimen, which was the result of a manufacturing error and not a part of the original trial protocol. The rest, 3 out 4 trial volunteers in the vaccine arm, received two full doses one month apart. 

For participants who received the full-dose/full-dose regimen, the observed vaccine efficacy rate was 62 percent, or 28 percentage points below the half-dose/full-dose regimen. Because so many more participants received two full doses rather than a half dose followed by a full dose, the average efficacy rate when combined across the two dosing regimens was closer to the efficacy rate found among those receiving the full-dose/full-dose regimen. 

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