H1N1 + H5N1 = ?

Until the surprise outbreak of what the CDC is now calling "novel influenza A (H1N1)" a couple of weeks ago, if you had asked which strain of flu virus was most likely to cause the next pandemic, most experts (and even news-consuming lay people) would have told you the same thing: H5N1

The H5N1 flu virus is either a panzootic in birds (a panzootic is just like a pandemic, but for animals), and has shown the ability to infect a number of other species in a more limited way, including the friendly neighborhood tom cat... and yes, the occasional human.

And we've been quite lucky that it's only been the occasional human, because of the 421 confirmed cases we've seen over the years, 257 have been fatal.  That's a case fatality rate (CFR) of just over 61%.  It's a mean and nasty bug.

So, at this moment when much of the world is learning about recombinant flu viruses for the first time, it's not surprising that a lot of people (or at least a lot of the people who post comments in blogs and bulletin boards) are wondering whether the H1N1 virus and the H5N1 virus could combine to create a virus far more lethal than H1N1 and far more capable of human-to-human transmission than the H5N1 virus.

The bad news is that, yes, it's possible.

The good news is that it's not terribly probable, because -while the H5N1 virus is capable of infecting humans and a broad range of animals- for the most part it continues to be a disease of birds.  It typically doesn't reproduce very well in pigs at all, and humans seem to be a dead-end host for the virus so far.

The Novel Influenza A/H1N1 virus, of course, is mostly a disease of humans (though it is apparently willing to move back in with the pigs too).

In an interview with Science Insider, virologist Yi Guan of SARS fame shared his thoughts on the possibility:

Discussing the our inability to predict whether the H1N1 virus will become more virulent....

Q: It depends on further mutations?

Y.G.: It depends on mutations and whether the virus further reassorts with other viruses—like H5N1. That could be a super nightmare for the whole world.

Q: You’re talking about the Armageddon virus?

Y.G.: The chance is very, very low that these two viruses will mix together, but we cannot rule out the possibility. Now, H5N1 is in more than 60 countries. It’s a panzootic, present everywhere except North America.

Q: If the nightmare comes true?

Y.G.: If that happens, I will retire immediately and lock myself in the P3 lab. H5N1 kills half the people it infects. Even if you inject yourself with a vaccine, it may be too late. Maybe in just a couple hours it takes your life.

Sounds pretty scary, and it could be... but you'll recall that he said the chance of it happening is very, very low.  

This is a good illustration of the difference between a hazard (something that can cause harm) and a risk (the likelihood of that harm actually occurring).  

An H5N1/H1N1 recombinant virus represents a serious hazard, but the risk of it actually happening is thankfully rather low.

That doesn't mean we can afford to take our eyes off the ball, however.  The H5N1 virus is no less of a threat today than it was before the H1N1 outbreak began, and we ignore it at our peril.  President Obama says that the President of the United States needs to be able to walk and chew gum at the same time, and he's right.  Here's to hoping the global public health preparedness community can do the same.