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”