Swine flu
The H1N1 pandemic began in 2009 (Wiki Commons)

Almost a quarter of people were infected with swine flu in the first year of the outbreak of the pandemic in 2009.

The rate of the H1N1 infection was highest in children and young people, with 47 percent of 5-19-year-olds showing signs of the virus.

Research from the World Health Organisation and Imperial College London examined data from 19 countries where the disease was present. This included the UK, US, China and India.

It found older people had lower rates of infection, with around 11 percent of over 65s infected.

Scientists believe the lower rates in older people could be a result of previous circulating influenza viruses giving them some protection from the swine flu strain.

Blood samples from this group showed 14 percent already had antibodies that reacted to the infection.

The research, published in the journal Influenza and Other Respiratory Viruses, found that between 20 to 27 percent of the people living in infected areas had swine flu.

Fatality ratio 0.02 percent

Scientists think this rate was probably similar in countries where there is no data available, meaning a quarter of the population could have been infected.

Senior author Anthony Mounts, from 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."

Another author, Maria Van Kerkhove, from Imperial College London, said: "This study is the result of a combined effort by more than 27 research groups worldwide, who all shared their data and experience with us to help improve our understanding of the impact the pandemic had globally."

The study tested 90,000 blood samples collected during and after the pandemic to test for antibodies produced to fight the infection.

It also estimated that the fatality ratio was less than 0.02 percent - around 200,000 people are thought to have died from the disease. WHO declared the pandemic over in August 2010.