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A global flu pandemic cannot be avoided solely by containing an outbreak at its source, because the H5N1 bird flu virus is now so widespread that a form able to spark a pandemic could emerge more than once, say researchers.

They say that instead of containment, plans should focus on how to limit the chance of pandemic-forming viruses emerging in the first place.

In a paper published yesterday (20 February) in PLoS Medicine, Marc Lipsitch of the Harvard School of Public Health and colleagues say that, at best, containment would only delay a full pandemic.

Experts believe a pandemic strain of flu could emerge if H5N1 gained the ability to spread between people, either by mutating or by swapping genes with a human flu virus.

Lipsitch's team says that if this can happen once, it is just as likely to happen again "the next day or week, nearby or in another country".

He explains that if an initial outbreak of a pandemic virus were to be contained, most people would not be exposed to it, and so would not have protective immunity if a second pandemic form emerged.

Containing a pandemic virus at its source would buy some time to prepare for future outbreaks, but not much, so efforts should focus on how to limit the chances of a pandemic-forming virus emerging in the first place, say the researchers.

Lipsitch accepts that drastic measures such as changing how chickens are raised or treated when ill would cost money and disrupt lives, so might only be acceptable to people after a pandemic virus emerges. 

Last year, researchers including Neil Ferguson of Imperial College, United Kingdom, said a pandemic might be prevented if its initial outbreak is detected early and anti-flu drugs are distributed rapidly (see Bird flu pandemic 'could be avoided if —').

Ferguson believes that Lipsitch's team's conclusions are based on a common misconception that, with the emergence of H5N1 in Asia, a pandemic is more likely today than it was before.

He argues that although a pandemic is inevitable in the long run, nothing indicates it will happen in the next few years, nor that it will be triggered by H5N1.

Ferguson told SciDev.Net that if a pandemic does occur this year or next, the chances of another occurring in close succession are slight.

"Put another way, if we succeed in containing a pandemic which started this year, we are likely to buy ourselves at least ten years until the next one."

Ferguson believes that containing an outbreak is critical, particularly for Africa and South Asia, which are unlikely to have access to potential vaccines.

"It is the only policy that will protect them, even temporarily," he says, adding that the paper by Lipsitch's team may distract from this goal.

Lipsitch told SciDev.Net that he feels the risk of a flu pandemic has increased recently, largely because more people in Asia can now afford to raise chickens for food.

This, he argues, means there is more contact between humans and birds, making the risk of a bird flu virus infecting people and exchanging genes with a human flu virus higher than ever before.

Link to full paper in PLoS Medicine

Reference: PLoS Medicine doi: 10/1371/journal.pmed.0030135.sd001 (2006)


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