Overall, however, the under- and over-estimates probably even out, said Simonsen, making the global estimate - of 15 times more deaths than those confirmed at the time - about right.
The results paint a picture of a flu virus that did not treat all victims equally.
It killed two to three times as many of its victims in Africa as elsewhere. Overall, the virus infected children most (4 percent to 33 percent), adults moderately (0 to 22 percent of those 18 to 64) and the elderly hardly at all (0 to 4 percent).
Even though the elderly were more likely to die once infected, so few caught the virus that 80 percent of swine flu deaths were of people younger than 65.
In contrast, the elderly account for roughly 80 percent to 90 percent of deaths from seasonal influenza outbreaks. They were probably spared the worst of 2009 H1N1 because the virus resembled one that had circulated before 1957, meaning people alive then had developed some antibodies to it.
The relative youth of the victims meant that H1N1 stole more than three times as many years of life than typical seasonal flu: 9.7 million years of life lost compared to 2.8 million if it had targeted the elderly as seasonal flu does.
H1N1 had begun petering out by November 2009, and the WHO declared the epidemic at an end the following August.