>>In a real catastrophe, most of them will die fairly close to where they live.<<
As I said, in a War of the Worlds scenario.
I am still unclear on the scenario being suggested by the OP. And how it would kill 50+ million people in a few days (well, really it would have to kill the nearly 200 million living in cities and suburbs to have the scenario make any sense).
Just in case, the OP should have people stock up on recordings of Slim Whitman’s “Indian Love Song...”
“I am still unclear on the scenario being suggested by the OP. And how it would kill 50+ million people in a few days”
Not a few days, I think, but a couple of months. Mainly they would kill each other.