In a new analysis of serologic studies from 19 countries, researchers estimate that about 24% of the population were infected with the 2009 H1N1 influenza (pH1N1) virus during the first year of the pandemic, a finding modestly higher than what US health officials estimated in the immediate aftermath of the pandemic.
When the estimate is combined with recent estimates of the pandemic's death toll, it appears that about 1 in 5,000 (0.02%) of those infected with the virus died, says the report by a large team of researchers from the World Health Organization (WHO) and many other institutions. It was led by Maria D. Van Kerkhove, PhD, of the WHO and Imperial College London.
In accord with previous findings, the analysis showed that pH1N1 incidence varied widely by age-group, with the highest rates in children and the lowest in people 65 and older, who enjoyed a degree of protection by virtue of exposure to related H1N1 viruses much earlier in life. The study was published Jan 21 in Influenza and Other Respiratory Viruses.
The popular view of the 2009 pandemic is that it was fairly mild, even though it defied the typical seasonal flu age pattern by hitting younger adults more often than the elderly. But the authors note that its global impact is not well understood, largely because with the high volume of cases, the WHO recommended early on that testing focus on severe and fatal illnesses only. As a result, the numbers of cases and deaths reported to the WHO—fewer than 1 million and more than 18,449, respectively—are believed to be small fractions of the true numbers, they write.