Well to be honest I have been applying mostly for topics related to cancer, you can add a line that is specyfic but most of it is similar. At first I honestly wrote an individual cover letter for everything, later, when I was desperate I started copying and pasting and guess what? I started getting replies.
I applied for 26 (including some I would never be considered for because I am not qualified for them), got 3 interviews which I attended and three which I rejected once offered a place. One interview invitation was in January and all the five remaining from end June to mid August. I was surprised coz on this site it states that the later the more competitive, but quite a lot of institutes urgently searches for students at the beginning of summer.