It might have been officially declared today, but it has been clear for several weeks that the world is experiencing the first flu pandemic for 41 years.

The surge of infections in Australia that prompted the announcement was not the first sign that swine flu is spreading freely around the globe. It has presented the World Health Organisation with evidence so overwhelming that it could no longer ignore facts that have been obvious to most scientists and health officials for some time.

A pandemic is declared, according to WHO definitions, when at least two continents report “community-level outbreaks” in which some new infections cannot be traced to known cases. Several European countries started to experience such sporadic cases last month which would have given the WHO justification to confirm that the pandemic had started.

Why, then, has it waited until now to make its move?

The first answer is that swine flu has become political. The United Nations’ member states, including Britain, have been urging caution for fear that declaring a pandemic too early might provoke panic.

Though a pandemic disease need not be especially severe or lethal — the term describes the ease with which it spreads — this remains poorly understood. Most people associate pandemic flu with the body count of 1918-19, when 50 million people died.

The second ground for delay was that the WHO’s pandemic alert system has just six phases, with level six signifying a full pandemic. Move to level six too early and there is nowhere to go to alert the public to an escalating danger.

The organisation held back, but the confirmation of more than 1,000 cases in Australia and Chile, as well as in the US and Mexico, has forced its hand.

There was also a danger that moving to level six too early could lead some countries to activate pandemic plans drawn up around the more lethal H5N1 virus, which would have been a costly overreaction to the more moderate H1N1 strain.

What really matters, however, is not whether the H1N1 pandemic is officially declared, but how public health authorities around the world are responding to it.

In Britain the move to level six will have little if any practical significance. The Health Protection Agency (HPA) is using measures such as school closures to slow the spread of swine flu, and it had ordered enhanced testing for the virus in hospitals before the pandemic was declared.

The WHO decision will not immediately trigger the manufacture of a pandemic flu vaccine in place of the seasonal variety. The production run for the northern hemisphere flu season is almost complete and a seed virus for making a new vaccine is not quite ready, so a switch next month remains likely.

That means that while Britain has ordered vaccine to cover the entire population, the first batches will not arrive until October at the earliest, by which time the autumn infection spike might have begun. The HPA will have to prioritise, vaccinating first NHS and essential workers and then vulnerable groups.

As the southern hemisphere is entering the flu season now, the experience in countries such as Australia, Chile and Argentina will be watched closely by British health officials and scientists.

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