The AZ press release -
https://www.astrazeneca.com/media-ce...zd1222hlr.html gives a little more detail on the statistical significance;
Quote:
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One dosing regimen (n=2,741) showed vaccine efficacy of 90% when AZD1222 was given as a half dose, followed by a full dose at least one month apart, and another dosing regimen (n=8,895) showed 62% efficacy when given as two full doses at least one month apart. The combined analysis from both dosing regimens (n=11,636) resulted in an average efficacy of 70%. All results were statistically significant (p<=0.0001). More data will continue to accumulate and additional analysis will be conducted, refining the efficacy reading and establishing the duration of protection.
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The 'p' value of <0.0001 is pretty compelling, roughly saying that there is a 0.01% chance of these results happening by chance. The release correctly says that this is still preliminary data with more to come. Without challenge testing (deliberately infecting trial subjects with SARS-COV2 to see what happens) there needs to be a lot of people in a Phase III trial.