AstraZeneca has produced confusing results for its COVID vaccine. The trial had two groups with two very different results. The first group got two full doses. In that group, the vaccine was 62% effective. The second group got a half dose and then a full dose, which proved 90% effective.
The first half of this excellent podcast by STAT News explains the situation in detail. The difference in effectiveness may be due to the smaller size of the second group. It could also be due to the lower average age in that group. A third possibility is that since the first dose was smaller, it may have been easier for the body to express the spike protein needed to prevent COVID.
The different dosing in the second group was due to a manufacturing error, which makes me question exactly how careful AstraZeneca is being. However, even a 62% percent effective vaccine has very real value at a time when daily COVID deaths worldwide are often exceeding 10,000.