By Rebecca Robbins and Benjamin Mueller
...The regimen (click here) that appeared to be 90 percent effective was based on participants receiving a half dose of the vaccine followed a month later by a full dose; the less effective version involved a pair of full doses. AstraZeneca disclosed in its initial announcement that fewer than 2,800 participants received the smaller dosing regimen, compared with nearly 8,900 participants who received two full doses.
The biggest questions were, why was there such a large variation in the effectiveness of the vaccine at different doses, and why did a smaller dose appear to produce much better results? AstraZeneca and Oxford researchers said they did not know.
Crucial information was also missing. The company said that the early analysis was based on 131 symptomatic Covid-19 cases that had turned up in study participants. But it did not break down how many cases were found in each group of participants — those who received the half-strength initial dose, the regular-strength initial dose and the placebo....