The U.S. already had a crisis of chronic disease, especially in working-age people, which is one reason why the coronavirus wreaked havoc, Stokes adds.
. . . is there any way to ascertain what the piece is seeking to attribute to Stokes?
Hi, Andrew—From a grammatical standpoint, I agree with Gustavo that the clause is ambiguous. Syntax permits the clause of attribution ("Stokes adds") to be adjoined either to the sentential relative clause ("which is one reason why the coronavirus wreaked havoc") or to the matrix clause containing it.
HOWEVER, if the clause of attribution adjoins only to the sentential relative, then what was added by Stokes is that the referent of "which"—namely, the U.S.'s already having had a crisis of chronic diseases, especially in working-age people—is one reason why the coronavirus wreaked havoc.
THEREFORE, we can be certain Stokes acknowledges that the U.S. already had a crises of chronic disease, whether or not "Stokes adds" adjoins syntactically to the matrix clause, since the sentential relative clause points backward to the content of that clause by means of the relative pronoun "which."