Which is the best vaccine? The simple answer is the strongest of course. But it never is that simple. It is too early to know this for certain but the data that exist so far suggest that all of the vaccines being used have been equaly effective in reducing hospital admissions. So while they vary in strength with the prevention of infection at all, they do not vary in their ability to do the thing we need most urgently, which is to take pressure off our overloaded doctors and nurses before they burn out completely.
The Astra Zeneca drug may be being weaker than the rest, but it has the enormous advantage of being so much the cheapest. If a drug is ten times cheaper, a poor country can afford to buy ten times more of it, and so save far more lives.