I just started the game yesterday so I’m not sure if there maybe is a rating of S+ but it looked to me like a simple typo in the tutorial:
“GOAL 1: Unlock 1 research upgrade
[…] The Cure Ratings are broken down into bands ranging from S+ all the way down to F.”
Shouldn’t that be A+?
PS: There is also a typo in the forum registration form: “Confirmation of registration - what is the name opf the company that runs this website” 
No, I actually use the style of rating common in Japanese games where the ratings go C, B, A, S.
It’s a little quirk that I’m rather fond of. In case you are interested, the maximum rating you can get without using a Syringe Injector or Regulatory Connections research (two late game technologies) is A. So the S is kind of like, even better than we thought we could do. The ‘S’ stands for ‘Super’.
Good to know. Thanks for the explanation 