Back in July, I argued that there might be strains of H1N1 flu with differing abilities to cause severe disease. I formed this hypothesis from a meta-analysis of several studies in ferrets. Briefly, ferrets that were infected with viral isolates from patients with severe symptoms exhibited more severe symptoms than ferrets that were infected with viral isolates from patients with less severe symptoms. This suggested to me that the severity of symptoms experienced by patients was at least in part dependent on which strain of virus they were infected with. [see Ferreting out the dangers of swine flu: A discussion of two animal model studies of the new H1N1 virus and More evidence that the new H1N1 virus is more dangerous than seasonal flu: the new Kawaoka paper for more details]
Today, there is more suggestive evidence that there is a strain of this virus that causes more severe symptoms:
From Reuters, August 28, 2009
Doctors are reporting a severe form of swine flu that goes straight to the lungs, causing severe illness in otherwise healthy young people and requiring expensive hospital treatment, the World Health Organization said on Friday.
There is still insufficient evidence to say for certain that there are strains of flu with differing abilities to cause severe disease. However, this possibility should be tested by collecting viral isolates from severe cases, sequencing them and comparing sequences from mild and severe cases. If there appear to be consistent differences in sequences from strains that cause severe and mild symptoms, further animal studies should be conducted to test this hypothesis.
If there are multiple strains of the new H1N1 with differing abilities to cause severe disease, it is possible that they are currently competing amongst themselves for dominance. If the more severe strain “wins”, the case fatality rate will increase, perhaps by a lot.
It is worth knowing if this is happening.