Paris -- Britain is most at risk to the spread of an influenza pandemic, closely followed by The Netherlands, Germany, Italy and South Korea, according to a ranking of 213 countries released Friday.

Russia, Canada, Israel, Singapore, Hong Kong and Japan are also classified as being at "extreme risk" of a flu virus spreading within their borders due to some mix of dense populations, busy airports, and high levels of tourism and urbanisation.

The World Health Organisation (WHO) declared Thursday that the A(H1N1) swine flu that has swept across 74 countries since April -- infecting tens of thousands and claiming 145 lives -- had become a pandemic, the first in four decades.

But even if most rich countries are vulnerable the rapid transmission of the disease, they are far better equipped to cope with its impact, said Alyson Warhurst, a professor at Warwick Business School in Britain and main architect of the global ranking.

"Capacity to contain the spread is going to be much weaker in poor countries with very poor infrastructure and lack of education. That would be much of sub-Saharan Africa," she told AFP.

In a separate "capacity" index, all but seven of the 40 nations least able to contain a pandemic are on the African continent.

At the other end of the spectrum, the European Union -- along with the US, Japan and other wealthy states with strong health care systems and communications networks -- comprise the 40 countries most able to thwart or slow down a full-on pandemic.

A third index, also compiled in cooperation with British-based Maplecroft, a firm specialising in global risk analysis, identified nations most likely to see the emergence of a new pandemic flu strain on their territory.

"In these countries, people are not so poor as to not have assets," said Warhurst.

"They own swine and poultry, but they lack clean water and sanitation, as well as health education outreach. These tend to be emerging economies."

It is no coincidence that five of the 11 countries topping this list are those where the H5N1 "bird flu" has hit hardest, the research showed.

Since 2003, the H5N1 strain has killed some 250 people -- 60 percent of those known to be infected -- in a dozen countries, including 115 in Indonesia, 56 in Vietnam, 25 in China, 17 in Thailand and 23 in Egypt, according to the WHO.

Many of these nations -- including Mexico, the epicentre of the current outbreak -- share attributes favourable to the emergence of flu strains with pandemic potential: large rural population, underdeveloped sanitation systems, and especially large swine and domestic fowl populations.

All the major pandemics over the last century have combined genetic material from viruses found in pigs, birds and humans, with swine often being the final "mixing vessel" before the strains mutated into a form easily spread among people.

Fortunately, up to now, virtually all human H5N1 infections have come directly from birds.

What scientists most fear is a new strain combining the potential to spread of A(H1N1) and the virulence of H5N1.

Under current WHO rules, lethality is not one of the criteria used to decide whether a flu outbreak has reached the pandemic threshold, and is thus not taken into account in any of the new risk assessments, said Warhurst.

Many virologists and epidemiologists have said the global alert system should be adjusted to add a measure of virulence, such as a case/fatality ratio.

AFP/Expatica