The discovery of how strains of the flu virus emerge in Asia, circulate around the continent then migrate to start seasonal epidemics elsewhere may help scientists to improve immunisation programmes.

Research by an international team led by scientists from the University of Cambridge could help experts to select which strains to include in seasonal vaccines each year.

Asian countries such as China, South Korea, the Philippines, Thailand and Malaysia are responsible for “seeding” new outbreaks of influenza that spread around the world, because of the region’s unique climatic conditions, which mean that the virus circulates all year around.

Flu normally causes epidemics during the winter in temperate zones, and during the rainy season in the tropics. The reasons for this are poorly understood, though it is thought that it may reflect natural vitamin D levels, which are raised when people are exposed to sunlight, or greater indoor crowding during cold or wet spells.

The timing of the tropical rainy season varies in different parts of East and SouthEast Asia, and these regions also lie in close proximity to temperate parts of the continent where flu thrives during the northern hemisphere winter. This creates an overlap so that the virus is always active in at least part of the continent. This constant circulation means that outbreaks in one country can spread rapidly to its neighbours. New strains of the virus that evolve are likely to be carried around the world by international travellers.

Derek Smith, of the University of Cambridge, who led the study with his colleague Colin Russell, said: “Flu epidemics appear to be driven by seasonal factors such as winter, or rainy seasons. So there can be cities that are only 700 miles from each other, such as Bangkok and Kuala Lumpur, that have epidemics six months apart.

“There is a lot of variability like this in East and SouthEast Asia, so lots of opportunity for an epidemic in one country to seed an epidemic to another nearby country, like a baton passed by runners in a relay race.”

In the study, which is published in the journal Science, Dr Smith and Dr Russell analysed more than 13,000 samples of the commonest strain of the influenza virus, H3N2, collected worldwide by the World Health Organisation Global Influenza Surveillance Network in 2002-07.

This showed that new strains tend to emerge first in East and SouthEast Asia, before reaching Europe and North America about six to nine months later, and South America several months later still.

The viruses rarely circulate back to Asia: although it is possible for flu strains to return, by the time that they do so there is usually enough immunity among Asian populations to prevent fresh outbreaks.

A second study, by a team at Pennsylvania State University, published in the journal Nature, has reached similar conclusions.

The findings are important, as improved surveillance of the spread of flu will assist efforts to select the right strains for seasonal vaccination. Each year scientists must pick three strains for inclusion in the vaccine, about eight months before immunisation programmes are started and almost a year before flu will actually strike.

“The ultimate goal of our collaboration is to increase our ability to predict the evolution of influenza viruses,” Dr Smith said.

Annual influenza epidemics are thought to infect between 5 and 15 per cent of the world population each year, according to the WHO. This results in between three million and five million cases of severe illness, and between 250,000 and 500,000 deaths.

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