So now its official. The World Health Organisation has warned that a global influenza outbreak is imminent. How serious will it be? History and modern virology offer a rough guide.
There were three flu pandemics in the last century. By the far the worst was the Spanish flu outbreak in 1918-19. In the turbulent conditions at the end of World War I, an estimated 25 per cent to 30 per cent of the world population fell ill and at least 40 million people died. The toll was many times higher than the 8.3million who died in the fighting. In the United States, the flu mortality rate was about 3 per cent of those infected.
The first pandemic of the 21st century appears to be coming from North America. At least nine countries, including New Zealand, have reported confirmed cases of the flu infection. The US Centers for Disease Control in the US have identified the pathogen responsible for the human-to-human spread of respiratory illness in both Mexico and the US as a unique version of the H1N1 strain of the influenza A virus, the only type of flu virus that can cause a pandemic.
Because it is new, people have little or no immunity. It is a version of the H1N1 strain that flared up in 1918. H1N1 is also the same strain that causes seasonal outbreaks of flu in humans regularly.
But this latest subtype is different from the 1918 and seasonal flu.
According to preliminary analysis, it is a hybrid that contains genetic material from flu viruses previously found in pigs in North America, Europe and Asia. It also contains genetic segments from North American human flu and bird flu.
The predominance of pig-origin material in the gene segments of the new virus prompted the WHO to call it a form of ''swine'' flu, even though it has been shown to pass readily from person to person as seasonal flu does when people in close proximity cough and sneeze.
Although the bug may have evolved from or in pigs, it has not been shown to cause disease in them. The World Organisation for Animal Health pointed out on Monday that the new virus had not so far been found in pigs or any other animals. It called for urgent scientific research to determine the susceptibility of animals to the pathogen, adding that if it was proven to cause disease in animals this ''could worsen the regional and global situation for public health''. It would also have an adverse impact on food production and international trade. Already a number of countries are culling pigs and banning pork imports without any scientific basis.
We do not yet know how quickly or how far the new virus will spread among humans or whether it will trigger severe respiratory illness in large numbers of people, causing many to die. The answer to these questions will determine the impact of an H1N1 pandemic on economic activity and essential services around the world. The virus may also change during the pandemic, becoming more or less virulent.
The 1918-19 outbreak occurred in two waves. The first began in March 1918 in Europe and in different states in the US. The infection then travelled back and forth between Europe and the US on troop ships, before moving, by land and sea, to Australia, Asia and Africa.
This first wave was highly contagious but not especially deadly. So when the second wave began in late August 1918, no country was prepared for the explosive outbreaks characterised by a tenfold rise in the death rate.
A study published by the WHO in 2005 said that the viral disease of 1918-19 had characteristics that were not seen before or subsequently.
Deaths from flu, whether during seasonal epidemics or pandemics, usually occur in the very young or very old. With Spanish flu, most deaths were among those aged from 15 to 35, a prime-of-life group normally resistant to illness.
It is not yet clear on which age groups H1N1 will have most impact, and on which countries and regions. It may turn out to be relatively mild.
Latest research on the 1918-19 pandemic indicates that most of the deaths were not from primary viral pneumonia. They were the result of bacterial infections in weakened respiratory systems, at a time when anti-microbial drugs were not available. Nor were mechanical respirators and supplemental oxygen in use. In a similar situation today, antibiotics and modern medical resources would save many of the infected.
Science and health care have certainly made major advances since 1918. But the world's population is also much bigger and vast numbers of people, especially in developing parts of Asia and Africa, lack the resources and health services that could help protect them from pandemic flu.
With the increase in global transport, travel and trade, as well as urbanisation, poverty and overcrowding, an infectious new virus is likely to spread quickly.
Still, concerns in the past few years about another flu pandemic based on the H5N1 bird flu virus have strengthened international defences, putting surveillance and reaction mechanisms in place that can be used to minimise the impact of the virus from North America.
Many countries and the WHO have also stockpiled anti-viral drugs which doctors say have been effective in reducing the severity of illness from the new virus when taken soon after symptoms start. Governments must ensure that such drugs are used for treating the sick, not bought up in advance and hoarded by people who fear they may fall ill.
The WHO says that early detection and treatment of cases, and infection control in all health facilities, are essential to minimise the impact of a pandemic. With luck, the virulence of this one will be relatively low, on the scale of the Asian flu (H2N2) pandemic that caused about two million deaths in 1957-58 and the Hong Kong flu (H3N2) pandemic that killed about half that number in 1968-69. In contrast, seasonal flu is estimated to kill between 250,000 and 500,000 people a year. Road traffic accidents cause a far higher annual death toll. A sober sense of proportion may be needed as the new pandemic spreads.
The writer is a visiting senior research fellow at the Institute of South-East Asian Studies in Singapore.
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