At least one in five people worldwide were infected with swine flu during the first year of the 2009-2010 H1N1 pandemic, an international research group said on Friday, but the death rate was just 0.02 percent.
The results echo other studies that found children were hit harder by the H1N1 strain, which swept around the world, than they are by regular seasonal flu outbreaks and that people over 65 were less vulnerable.
More accurate early surveillance is needed to plan for and respond to future pandemics, scientists said, in the wake of the international research led by the World Health Organization (WHO) and Imperial College London.
"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 ... pandemics," said the WHO's Anthony Mounts who helped lead the study.
The information could be used to assess severity and develop mathematical models to predict how flu outbreaks spread and what effect different interventions, such as school closures, vaccination or preventative treatment, might have, he said.
The study, which used data from 19 countries, collated results from more than 24 studies involving some 90,000 blood samples collected before, during and after the pandemic.
The results, published in the journal Influenza and Other Respiratory Viruses, showed 20 to 27 percent of people studied were infected during the first year of the pandemic.
Because infection rates were likely to have been similar in countries where data were not available, this means as many as a quarter of the world's people may have been infected, the researchers said.
The WHO declared H1N1 swine flu a pandemic in June 2009 when laboratories had identified cases in 74 countries. By November 2009 it had started to peter out but the WHO did not declare the epidemic at an end until August 2010.
While this study did not set out to look at death rates, the researchers said they had used previously published and still-in-progress death rate estimates to calculate the proportion of people infected who died from the pandemic virus.
Based on an estimate of around 200,000 deaths, they said the case fatality ratio was probably less than 0.02 percent.
The WHO's official data show 18,500 people were reported killed by the H1N1 flu. But a study published in The Lancet last year said the actual death toll may have been up to 15 times higher at more than 280,000.
Maria Van Kerkhove of Imperial College London, who worked on the study, said it would improve understanding of the H1N1 pandemic's impact and fuel efforts to improve prediction of future pandemics.
"It puts it into context and gives us a fuller picture," she said in a telephone interview. "That's incredibly important because we know this will happen again and there is a lot of effort being put into trying to prepare now...for the next one."