New H5N1 transmissibility study--does it show what the headlines claim?

I mentioned just last month the dearth of research into what really makes an influenza virus easily transmissible from host to host, noting:

If we had a better handle on the factors that caused an avian strain of influenza virus to be more efficiently transmitted among humans, then we could better focus our resources and know when to really sound the alarm--unlike now, when we're flying blind in many ways.

A new paper in PNAS has started to do just that, and the research findings have prompted such headlines as Bird Flu Pandemic may not develop (via Effect Measure). Revere has already provided a nice analysis of the paper in that post and today's follow-up, so I'll summarize and add some thoughts of my own after the jump.

There are two main ways that influenza viruses evolve: the accumulation of point mutations (single base-pair changes, "antigenic drift"), or by swapping out gene segments wholesale ("antigenic shift"). While the latter was long thought to be critical for the generation of a pandemic strain of virus (and was responsible for the 1957 and 1968 pandemics), we now know that the 1918 virus was not a reassortant, but instead, appears to have been a completely avian virus that adapted to a mammalian host via the accumulation of point mutations. Still, the focus of the current research was on reassortants between influenza viruses of serotypes H5N1 ("bird flu") and human H3N2. To do this, they first used a human H3N2 virus, and four different H5N1 viruses (isolated in 1997, 2003, and two isolated in 2005) to validate their ferret transmission model, by housing ferrets together in a cage separated by a perforated wall. This allowed the virus to spread via droplets, while preventing the infected and uninfected animals from having any physical contact. As expected, the human virus spread from the infected to uninfected ferrets, but the H5N1 viruses did not.

Then, they used reverse genetics to create reassortant influenza viruses, containing some human H3N2 genes and some avian H5N1 genes, which were then again used to infect a new batch of ferrets. However, to generate these reassortants, they used the 1997 H5N1 virus--which, as they note, is "genetically distinct from H5N1 viruses isolated since 1997." (You can find a phylogenetic tree using the 1997 virus--HK486--compared to more recent isolates in this paper by many of the same authors). I'm not sure why they chose this one--it seems that their findings would have been more relevant had they used a more recent H5N1 isolate, that was a better representative of the viruses circulating currently.

Anyhoo, they ultimately found that none of the reassortant viruses transmittted efficiently between ferrets, leading to the paper title and the media headlines. However, they only give data on 4 combinations that they tried, and they note that others (such as ones that would have mimicked the human/avian gene combinations that gave rise to the 1957 and 1968 pandemics) failed to generate a reassortant virus, so these couldn't be tested in the ferrets. And as Revere notes, left untested was the effect simple point mutations may have on transmission. Of course, these are much more difficult to examine experimentally since there are so many more permutations that would need to be examined, and that makes it completely understandable that this was left untested for the purposes of this new paper, but it becomes a critical black box that has been left out of the many of the media's write-ups of the research.

What also wasn't tested was reassortment with other human influenza viruses, such as an H1N1 serotype. What would be an interesting experiment--but one that would probably be deemed a "fishing expedition"--would be to generate a panel of reassortants to inoculate into animals, and let selection take over--use the animals as the filter in order to identify unique isolates with a higher propensity for transmission from a mixed input inoculum. A limitation to this, though, would be that differences in replication between the inoculated viruses may obscure transmission potential. The current paper showed that some of the reassortant viruses were able to replicate in the animals, and therefore reassortants that are able to replicate but not spread to other ferrets may out-compete potential transmissible viruses at the initial stage of infection. Still, it's something that could be experimentally tested, given the funds and viral isolates.

Finally, I'll emphasize that this is much more than just an academic exercise. msnbc is reporting today the presence of another potential cluster of cases in Indonesia, in the same area where the previous human cluster occurred. It's not been confirmed yet as H5N1, but even if it's not, it underscores the need to increase our understanding of just what makes influenza easily tranmissible--and to do so quickly.

References

Maines et al. 2006. Lack of transmission of H5N1 avian-human reassortant influenza viruses in a ferret model. PNAS (early edition). Link.

Maines et al. 2005. Avian Influenza (H5N1) Viruses Isolated from Humans in Asia in 2004 Exhibit Increased Virulence in Mammals. J. Virology. 79:11788-11800. Link.

Image from http://www.indcjournal.com/archives/ferret.jpg