It killed an estimated 284,500 people. That is some 15-times the number confirmed by laboratory tests at the time.
That's according to a new study by an international group of scientists published in the Lancet medical journal.
The original count by the world health organization put the number at 18,500 people.
The same study showed 51 percent of swine flu deaths occurred in Africa and southwest Asia which account for only 38 percent of the world's populations.