PARIS, August 2, 2009 (AFP) - Take two neighboring countries, of equal population and prosperity, each with advanced health systems.

Swine flu is bound to affect them equally, wouldn't you say?

After all, we live in a globalised world. Borders are open or porous. People -- and therefore microbes -- are highly mobile.

Which means the H1N1 virus is bound to hit both countries at roughly the same time and spread at the same rate, surely.

Yet a remarkable disparity between Britain and France shows how false such thinking can be.

And understanding why this has happened could help save lives if H1N1 genetically reshuffles into something more lethal.

"Influenza in its distribution is always heterogeneous. It doesn't go evenly everywhere," says Angus Nicoll, head of the flu programme at the European Center for Disease Prevention and Control (ECDC) in Stockholm.

In the past two weeks, there have been nearly a quarter of a million cases of A(H1N1) in England alone. Twenty-seven have died in England since the outbreak began, and another four in Scotland.

In France, there have been 1,022 cases and a single death, announced on Thursday. And that is the official tally over three months, since the disease first emerged.

Experts say the data are reliable and even though different counting methods are used, the gulf in cases is clear. They conclude that Britain got whacked first -- and badly.

Why?

Marc Gentilini, a professor of infectious disease and former head of the French Red Cross, suggests British health surveillance may not have been as rigorous as its French counterpart in spotting and tackling early cases, especially in inner-city areas with mixed, mobile communities and big households.

"But this is just an idea. Frankly, it's impossible to know at this stage," says Gentilini.

But Alyson Warhurst, a professor at Warwick Business School in central England, says she is unsurprised by what has happened.

On June 12, a day after the World Health Organization (WHO) declared the pandemic, her risk-assessment company, Maplecroft, ranked 213 countries according to their vulnerability to viral spread.

It placed Britain squarely at the top -- unexpected given that the country has one of the best health services in the world. France, whose health system persistently ranks at or near the top, ranked 33rd in risk.

But Warhurst explains Britain was susceptible because it has a very dense population, a very high proportion of urban dwellers and a large number of people arriving from abroad.

"While the risk factors for France are high in terms of tourist arrivals, population density (there) is not so high as in the UK," she explained to AFP.

Canadian researchers have found strong evidence for the role of jet travel in propagating the virus.

Kamran Khan of St. Michael's Hospital, Toronto, analysed the flight itineraries of more than 2.3 million passengers who left Mexico, where the new flu erupted, in March and April.

Their destinations mirrored places where swine flu swiftly started up, notably cities in the United States, Canada, Spain, Argentina and Brazil.

France was the third biggest destination while Britain ranked 11th, figures that confirm Warhurst's belief that foreign arrivals are a factor in spreading flu but not the only one.

There are several other hypotheses to explain swine flu's faster spread in Britain, says Thomas House, an epidemiologist at the University of Warwick.

They include damper weather compared with France -- the virus is more vulnerable to dry conditions -- and even different school systems.

France's schoolchildren began their summer vacation at the end of June, while Britain's began theirs two weeks later, and during this period, its flu cases took off.

Yet within the puzzle is another puzzle: swine flu prevalence in Britain is not homogenous. It varies greatly among regions and even within cities.

That raises intriguing questions about "micro-networks" of transmission at work, in transport or socially.

All this offers priceless knowledge if pandemic flu -- right now, relatively mild -- becomes more virulent, said House.

For instance, closing an airport may be impossible, but screening incoming travellers may be useful. School closures, or extending school holidays, could in some cases also be helpful. Targeting districts or communities that seem more at risk would also be beneficial. Antiviral drugs, if stockpiled and used in time, can also brake the spread.

"There are some things we can do in public hygiene, in keeping people away from certain locuses where there is strong mixing, giving antivirals, all these things make a difference and are very sensible things to do," said House.

"It's certain that by doing those we will reduce human suffering compared to previous pandemics."