As others have posted in reply to Gray, a vaccine isn't a cure, you can't give it to already sick people. If I recall correctly, the flu vaccine needs more or less 2 weeks since injection to make your immune system resistant to a possible virus attack.
As such, apart from volunteers, who would you test it on? People who are more at risk, aka elders or seriously ill? Testing a completely experimental substance on the most frail of people doesn't look like such a bright idea to me.
That's why the first human trials that have begun in the US are on healthy 45 year-olds.