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AstraZeneca COVID vaccine effectiveness

Question

According to STAT, early results from an AstraZeneca trial showed 70% efficacy. But that average hid a discrepancy:

Two full doses of the vaccine appeared to be only 62% effective at preventing disease, while a half dose, followed by a full dose, was about 90% effective. That latter analysis was conducted on a small subset of the study participants, only 2,741.

This half dose-full dose regimen was administered by accident, in a non-random fashion, therefore violating the randomization assumption of an RCT.

Because of this failure in study design, the CEO of AstraZeneca told Bloomberg News that the company wants to run "an additional global trial" to "confirm the 90% efficacy rate that the shot showed in a portion of an existing trial".

According to the WHO, vaccine efficacy is defined as:

% reduction in disease incidence in a vaccinated group compared to an unvaccinated group under optimal conditions (eg RCT)

How effective will a second RCT find the most effective regimen of the AstraZeneca SARS-CoV-2 vaccine to be?

This question resolves after a reliable media report of the final study results of a 2nd RCT of the AstraZeneca SARS-CoV-2 vaccine. It resolves based on the effectiveness reported for the most effective treatment arm.

This question closes retroactively 24hrs prior to the first interim results being reported. If no interim results are reported, this question closes 24hrs prior to when the final results are reported.

If no RCT is conducted or the final results of an RCT are not reported by the resolve date of this question, it resolves ambiguous.

If multiple RCTs are conducted, this question resolves based on the one for which the first reliable media report of the final study results.

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