I've played the final mission twice now, and on both playthroughs rescue never came to the stadium, despite there being a message that it had arrived. I've seen from a playthrough online that there was supposed to be a helicopter to come in and evacuate the survivors, but that wasn't the case for either of the times I've played the mission. I don't know the cause of this problem, other than the fact that I had picked up all of the vaccine samples before the rescue helicopter arrived on both playthroughs.