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After AstraZeneca's Big Vaccine Claim, Questions

90% efficacy was seen in participants given a half-dose by accident
By Arden Dier,  Newser Staff
Posted Nov 26, 2020 7:00 AM CST

(Newser) – Initial results regarding the AstraZeneca and University of Oxford COVID-19 vaccine had the industry buzzing Monday—but that enthusiasm is beginning to erode. The vaccine was said to be up to 90% effective, quite cheap at $3 to $5 per dose, and able to be stored for months in a fridge. But there's an issue with that first claim. Per the AP and Reuters, the vaccine was only 90% effective in a group of 2,741 volunteers who were mistakenly given a half-strength dose followed by a full dose—a slip-up AstraZeneca and Oxford didn't detail in their press releases and have since said was a contractor's mistake. Another 8,895 participants received two full-strength doses as planned, but the vaccine was only 62% effective for them. Researchers suspect the first scenario prompted a stronger immune response, but can't yet say for sure.

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Further, the results came from "two differently designed clinical trials in Britain and Brazil, a break from standard practice," the New York Times reports. To boot, the participants who received the half-strength dose were all 55 or younger, a fact Operation Warp Speed head Moncef Slaoui revealed on Tuesday, per the Wall Street Journal. So it's possible the higher effectiveness could be due not to dosage size but the participants' age. There were 131 symptomatic COVID-19 cases during the trial, but AstraZeneca and Oxford haven't said publicly how many cases were in each study group, or how they broke down by age. All of this raises doubts about whether the reported efficacy will hold up to further testing; a global trial comparing the dosing regimens is in the works. But "we still have efficacy that meets the thresholds for approval with a vaccine that’s over 60% effective," an AstraZeneca executive tells the Journal. (Read more AstraZeneca stories.)

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