Crossed question

I don't think so, because it would be too much of a coincident if all major characters were immune...

What if everyone is immune and everyone just goes along with it because they are too scared to admit anything different? Just a thought.

Leti Del Mar

Cassia isn't immune, she's wasn't aware of ever having used one before and it was stated by Ky that they had been used before(Xander being aware of this too), he wouldn't tell her why. If she had been immune she would have known that.
If the pills have any importance in Reached it will be with her brother or parents. I don't see any other reason they would be used by the main characters.

I do not think she is immune because then why would her grandfather tell her she doesn't need them (basically telling her not to ever take them) and I think it has something to do with her bigger purpose in life that she is not immune.