The first death caused by swine flu virus outside the Americas occurred in Scotland on Sunday. The announcement generated more media interest than the declaration three days before by the World Health Organisation that the spread of the virus had moved into pandemic mode. But the declaration was expected and generated less fear than anticipated. The public can see that in Britain the virus is doing well – which is all that was needed to meet the pandemic criterion of sustained community spread in a region outside the Americas – and the message that the virus is mild is also well established, tempering the notion that the word ‘pandemic’ carries lethal overtones. But this means that a death requires explanation.
There is no such thing as a naturally avirulent influenza virus. Even the mildest ones that infect humans can kill. They do it routinely every winter. The people who die fall into two groups: the fit and healthy, and those with conditions that predispose them to develop complications. The only reason the fit and healthy die is bad luck; fortunately, with an averagely virulent virus like swine flu, their number is small. Known risk factors include being very young or very old, having lung or heart disease, diabetes, immunosuppression, and pregnancy.
The pregnant are especially at risk because of physiological and immunological changes which diminish their ability to fight the virus. Their hearts beat faster with an increased output; their oxygen consumption is raised and their lung capacity is reduced; and there is a shift away from cell-mediated immunity (particularly important in responding to the early stages of influenza) to antibody production (more important in preventing reinfection months and years later than stopping the growth of the virus during an acute attack).
The best estimate of the swine flu mortality rate comes from the US, where it stands at 0.25 per cent (1 in 400). So with more than 1700 cases recorded in the UK a death is not a surprise.
Virus gene sequencing is being done on a grand scale and there is no sign of mutational change. So far, the virus is the same as it was in the beginning. The place to watch for developments is Australia, just going into its winter flu season with swine flu well established in Victoria. Will it out-compete the other flu viruses? Will it swap genes with them and become hotter? Policymakers eagerly await the answers.