What does this mean?
A phase 6 influenza pandemic means that the virus is now spreading between people across countries in a sustained fashion and will have a global distribution (although not necessarily affecting every country).
But doesn’t seasonal influenza spread globally anyway?
The other part of the definition of an influenza pandemic is that the causative agent must be a new strain of virus. Thus seasonal influenza is not called a pandemic because it involves strains of influenza A that we have seen before, even though it spreads worldwide. Conversely, the H5N1 avian influenza A that we’ve been so worried about in recent years was a new strain, but thus far hasn’t been able to transmit from person to person efficiently enough to affect people globally.
Note that the definition of pandemic does not refer in any way to the severity of illness affecting the infected individuals.
In general, a pandemic might be expected to cause more severe illness because affected people will lack preexisting immunity to the new strain. For this reason reports of deaths in young (presumably) healthy people in Mexico from H1N1 are concerning, and such deaths were indeed a notable feature of the devastating 1918 Spanish influenza pandemic that followed World War 1 killing 50 million people. However, thus far, H1N1 influenza cases that have occurred outside of Mexico do not seem to be any more severe than seasonal influenza (which kills 500,000 people a year!). The very young, very old, and immuno-compromised are, as always, at greatest risk. Nevertheless, the virulence of H1N1 influenza remains poorly defined and has the potential to increase so we must remain vigilent.
‘Life in the Fast Lane’ 2009 H1N1 Influenza A (‘swine flu’) resource list.