美国CDC说是从美国始发

In the spring of 2009, a novel influenza A (H1N1) virus emerged. It was detected first in the United States and spread quickly across the United States and the world. This new H1N1 virus contained a unique combination of influenza genes not previously identified in animals or people.

https://www.cdc.gov/flu/pandemic-resources/2009-h1n1-pandemic.html