The swine flu pandemic of 2009 killed an estimated 284,500 people, some 15 times the number confirmed by laboratory tests at the time, according to a new study.
The study, published on Tuesday in the London-based journal Lancet Infectious Diseases, said the toll might have been even higher - as many as 579,000 people.
The original count, compiled by the World Health Organization, put the number at 18,500.
Those were only the deaths confirmed by lab testing, which the WHO itself warned was a gross underestimate because the deaths of people without access to the health system go uncounted, and because the virus is not always detectable after a victim dies.
The new study also shows the pandemic's impact varied widely by region, with 51 percent of swine flu deaths occurring in Africa and southeast Asia, which account for only 38 percent of the world's population.
"This pandemic really did take an enormous toll," said Dr. Fatimah Dawood of the U.S. Centers for Disease Control and Prevention, who led the study. "Our results also suggest how best to deploy resources. If a vaccine were to become available, we need to make sure it reached the areas where the death toll is likely to be highest."
Swine flu, caused by the H1N1 influenza virus, infected its first known victim in central Mexico in March 2009. By April it had reached California, infecting a 10-year-old, and then quickly spread around the world, triggering fears and even panic.
The CDC warned Americans not to travel to Mexico if they could avoid it. Egypt ordered the slaughter of all the country's pigs in a misguided attempt to contain the virus, which was in fact spread from person to person.
The fears reflected the unusual nature of the virus, which contained bits and pieces of bird, swine and human flu viruses, a combination never before detected.
Scientists were unsure how transmissible or deadly this mongrel flu would be, but early signs were ominous: the World Health Organization declared swine flu a pandemic in June 2009, when labs had identified cases in 74 countries.
Such lab-based identification is the gold standard, but every expert acknowledges that it misses more cases than it catches.
One reason is that "some people who contract flu do not have access to health care," said CDC's Dawood, so their illness and even death goes unnoticed by authorities. Another reason is that the virus is not always detectable by the time a victim dies.