It mentions he's not showing any symptoms. From what I've read, they aren't 100% (more like 90%, I think?) in prevention. But it's kind of like the flu shot, where if you get the shot and get the flu, you have a much milder case of the flu.

ETA: Also, the article doesn't say when he got the second shot. I think it takes 2 weeks after the second shot to reach full immunity.