A New Game: Find the Point of Vaccination

I've just come up with a new game. The following is a graph of diarrhea-caused deaths of children in Mexico month by month for eight years. At one point on the graph, a vaccine was introduced that was designed to stop diarrhea-caused deaths of children. The game is, can you pinpoint when the vaccine was introduced?

(If I could do Encyclopedia-Brown-style upside down type I'd do it here!)

ANSWER: May 2007. Any point from April to September in 2007, after the seasonal spikes of death stop, will be counted correct for the purposes of your grade.

Just for reference, most of my test questions are not this easy.

For more details, you can find the original paper here. And thanks to Matthew Herper from Forbes magazine for writing the article that brought my attention to this graph.