There is conjecture that the NYC virus came in from Europe, since NYC is a travel gateway,
whereas California may have gotten cultivars from Asia since they are the Western travel gateway.
Other possible explanations involve the rate at which the virus may have mutated.
Viri mutate readily due to their RNA simplicity, generally a single strand of genetic code.
Generally speaking, subsequent mutant cultivars of viri become less fatal as they mutate.
First case in NYC was a health care worker newly returned from Iran.