Went to the grocery store shortly after reading it for the first time way, way back when in the 1970s. I remember thinking, "Hmmm... maybe I should stock up." Powerful book.
I'm currently re-reading another old favorite of mine, The Earth Abides, another great apocalyptic read.
Seems when things go bad I go back to these books. In each one, EVERYONE in Washington DC dies.
I love a happy ending!