Review of the Year 2009: Swine flu

The threat of a flu pandemic has hung over the world for the whole of the past decade. It started in 1997, with the outbreak of avian flu in Hong Kong that led to the death of a three-year-old boy and the slaughter of one million chickens. For a few days, panic gripped the city as virologists warned Armageddon was at hand.

Six years later, Professor Liu Jianlun, a specialist in respiratory medicine visiting Hong Kong from the Chinese city of Guangzhou, sneezed in a lift at the Metropole Hotel and sparked a global panic. The mystery virus he carried – severe acute respiratory syndrome (SARS) – spread to seven other guests staying on the ninth floor of the hotel. Within six weeks it had infected 2,300 people in 17 countries and claimed 80 lives.

Sars wasn't flu but it was awfully like it. Yet in months, as suddenly as it appeared, it disappeared. By June 2003, it was over – but not before the virus had infected 8,000 people and claimed 800 lives worldwide. The following year, 2004, avian flu re-emerged in poultry flocks in the Far East and began its march across the globe. With a 60 per cent death rate in humans – it has so far infected 442 people and claimed 262 lives – it posed a serious potential threat.

It has not, so far, mutated into a form readily transmissible between humans, as virologists feared. Instead, out of the left field, came swine flu from Mexico, to cause the first pandemic of the 21st century. Inititally swine flu was scary, causing the first UK outbreak of flu in summer in living memory. It also, unusually, targeted children and young adults rather than the elderly, who are the usual vicitms of seasonal flu. What would happen when winter arrived? We knew it was mild from the beginning but we thought that because it was a new virus – officially termed novel H1N1 influenza – it would spread widely and, even though mild, kill a lot of people by virtue of the sheer numbers infected. That has not happened. Estimates of the population potentially infected have been reduced from 30 per cent to 10 per cent – not much greater than for seasonal flu. It has emerged that people aged over 50 have some immunity because, although the virus itself is novel, proteins on its surface are similar in some respects to those on H1N1 viruses circulating up to the 1960s. In June, the World Health Organisation (WHO) announced a level six global alert – the highest – for the first time in 40 years. It was intended to help the world prepare for an imminent lethal viral attack not seen in decades and required the mobilisation of immense resources. Instead, what began with a thunderclap is ending – for this year at least – with a whimper.

True, if the virus does mutate it could cause mayhem on a scale not yet seen. But the risk is reducing as the vaccine is rolled out – almost two million have already had the jab in the UK and millions more will get it in the next couple of months. The existing vaccine should provide some cross protection even against a mutated virus.

But suppose it does not mutate? Will the WHO have to redefine the term "pandemic virus"? The only pandemic we have seen so far is the one at the end of the virologists' microscopes. It may indeed be a completely new flu virus of a kind never seen before, but there has been no "pandemic", in any clinically relevant sense of the term, anywhere in the world since swine flu broke out in Mexico last April.

Without doubt, fear of avian flu, shaped the world's response to swine flu. So far swine flu has – mercifully – proved to be a kitten, not a tiger, causing mild illness in most people with a low death rate and limited spread. Public health specialists are keeping their fingers crossed. But if swine flu remains as mild as it has proved so far, will it deserve the title "pandemic"?