Swine Flu remains non-fatal outside Mexico
It has been 5 days since the first cases of Swine Flu (now more often called H1N1 influenza-A) have appeared outside Mexico. There has not been a death from anyone who contracted the H1N1 infection without going to Mexico. All the non-Mexican H1N1 victims, even those who contracted the disease while in Mexico, have suffered from “mild” disease, meaning that their lives were not threatened by the illness.
The whole scare around H1N1 centres around the fact that a significant number of victims in Mexico died from H1N1 infection. In particular, the people who died were young adults, rather than the old and the very young.
In outbreaks of “seasonal influenza” - the normal colds and flu that we see in winters - it is the very young and the very old that suffer the most, due to their immune systems being weakener at the extremes of age. With “pandemic influenza”, the virus is one that is genetically different in some way from the viruses that cause seasonal influenza. This is called “antigenic shift”. People therefore don’t have a natural immunity to the new virus, and are susceptible to getting severe infections.
The H1N1 deaths in Mexico raised the possibility that this was a new strain of influenza virus against which humans had little immunity. Subsequent testing of the virus indeed confirmed that H1N1 is a swine-human-bird hybrid influenza virus that has not been seen before.
Why is H1N1 not killing people outside Mexico?
The WHO still does not have a satisfactory answer to this question.
A likely answer is that there still have not been enough people infected with H1N1 outside Mexico for there to be any deaths from it. The disease is thought to have a 0.1-1.0% mortality in Mexico, meaning that between 100-1,000 people will need to be infected before we see the first deaths.
An alternate explanation is that the virus has mutated and evolved into one that is less dangerous than the one which was originally seen in Mexico. While this is an attractive answer as it suggests the virus will fade away, there is no scientific evidence that the strains of virus isolated from non-Mexican sufferers is significantly different from the virus that has caused deaths in Mexico.
Thirdly, it may simply be that Mexican and WHO officials over-estimated the number of people who died from the disease - people may have died of other causes, wrongly attributing the deaths to H1N1.
It is still too soon to say which of these possible explanations is the right answer. The world is still nervously watching to see how this H1N1 story pans out.
