Oxford/AustraZeneca vaccine's initial half dose was an accident. That this mistake took the rating towards 90% was a miscalculation. And if all these sounds weird, that's because it is.
There's no Oxford vaccine yet. They will rerun the 3rd phase test.
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https://timesofindia.indiatimes.com/india/astrazeneca-manufacturing-error-clouds-vaccine-study-results/articleshow/79419695.cms