The control group will not be given the vaccine, but a placebo.

Clinical trials are phased. Typically you start with a very small group of people to evaluate toxicity (e.g. below treatment levels of a drug), then a slightly larger group looking for side effect, then a significantly larger group looking for desired effects, and finally a broad group looking for safety and efficacy.

This is a combined phase 1/2 trial. It tests that the vaccine doesn't hurt or kill people too much, and that it has some immune system effect.

After that a phase 3 study is done on tens of thousands of people from all possible demographics.

That phase 3 study for the Oxford vaccine is already underway, and its initial findings are what the article is referring to. It was one of the first (possibly the first, I'm not entirely sure) to enter phase 3.