Almost half of all children and teenagers in the UK and other countries hit by swine flu in 2009 succumbed to the virus, a study has shown.
Overall, at least a fifth of the general population was infected. But death rates were low at less than 0.02%.
The new figures are from an international study looking at the 2009 H1N1 swine flu pandemic in 19 countries, including the UK, US, India and China.
Children and young people showed the highest rates of infection, according to the findings, with 47% of those aged five to 19 developing symptoms.
In contrast, only 11% of people aged 65 and older caught the flu.
Multiple exposures to previously circulating flu viruses may have given older people some immune protection, said researchers.
Blood samples taken before the pandemic showed that 14% of the over-65s had antibodies that reacted to the 2009 strain.
The study, led by experts from the World Health Organisation (WHO) and Imperial College London, is published in the journal Influenza and Other Respiratory Viruses.
Scientists pooled the results of around two dozen research papers looking at more than 90,000 blood tests carried out before and after the pandemic.
Senior author Dr Anthony Mounts, from the WHO, said: "Knowing the proportion of the population infected in different age groups and the proportion of those infected who died will help public health decision-makers plan for and respond to pandemics.
"This information will be used to quantify severity and develop mathematical models to predict how flu outbreaks spread and what effect different interventions may have."