Tuesday, September 20, 2005

H5N1 and lethality

The pandas thumb has posted an article on the lethality rates of H5N1 (based on a new study) and how the virus may be evolving.
The problem with the figure Garrett cites is that it kills ~55% of the cases we know about. This is a classic case of sample bias. Those who are most sick (and thus, most likely to die) are also most likely to go to a hospital or clinic to be examined—and therefore, are also the most likely to have a clinically-confirmed case of influenza due to the H5N1 strain. Hence, the mortality data we have for H5N1 only comes from this sickest segment of the population—artificially raising the mortality rate. Puzelli et al.’s study, then, is timely due to the fact that it shows that sub-clinical infection with avian-type influenza viruses does occur (in almost 4% of their cohort of poultry workers).
Now while this is true and it could well be that the virus is less lethal than has previously been described, there are several worrying aspects of this kind of revelation. The most important, as discussed at the article at PT, is that this is where pandemics can come from. It isn't the people who are infected with the virus at a 'lethal' dose and die from the infection. For the virus that kills its host it will end up at an evolutionary dead end and won't have the chance to get anywhere. Subclinical infections, where the host isn't adversely affected and the virus gets to persist and spread for a while are the dangerous kinds of infections.

Here the avian influenza virus has time on its side, because here it can build up mutations and run the standard evolutionary experiments that influenza is capable of, namely by swapping segments of its genome with existing human influenza viruses. As the main impediment so far to H5N1 being able to spread between humans is simply the lack of key genes to be able to functionally replicate in humans, this is an incredibly worrying study. More so than if the viruses lethality rate was found to be as high as originally thought, because at least a high lethality rate reduces the chance of the virus having time to start aquiring new genes.

This virus is becoming more and more of a threat every day as a result. It really should not be ignored or trivalised.