User blog comment:B214/Question and Answers 6/@comment-99.235.66.221-20161130072057/@comment-201.239.133.207-20161214021447

don't forget that while Vali did manage to resist Samael's poison, he could not change into his armor while doing so (which is obvious, if you want to fend off a poison that is killing you, you wouldn't waste power in something as meaningless as armor), also Issei in some way did also resist the effects of the poison, because even if it killed him in the end, it did not prevent him to fight and finish Shalba, nor it seemed that it weakened him drastically like it did Vali and Cao Cao (granted Cao Cao is human so it's undestandible that the poison completely paralyzed him, and Vali most likely seemed drastically weakened because he didn't want to waste power, but i stand for what i said)