"bless you!" alternative [duplicate]
Interestingly enough, in the US the German expression is often used and is well understood.
gesundheit interj
Used to wish good health to a person who has just sneezed.
TMD Online
There is no alternative. But the term no longer has any real religious significance, just as the exclamation “Christ!” no longer has that kind of significance. Saying it does not erode at your atheism. I say it, and I’m a committed atheist.