New Flu Review

04.10.2013 |


Photo by Marc Hall for NC State

The new strain of avian flu – known as H7N9 – has only been on scientists’ radar for a couple of weeks, but that’s been long enough to raise some questions.  NC State epidemiology expert Barrett Slenning, who spends a lot of time looking at the ways in which pathogens transfer from animals to people, was kind enough to answer a few of our most pressing questions about this new strain of avian influenza. For background on how flu strains are classified and named, go here.

Abstract: So how much danger are we in from this flu?

Slenning:  We have pretty good evidence, at the moment, that this H7N9 agent is poorly transmissible from person to person.

The first bit of evidence that this H7N9 does not transmit well from person to person: Of the 530 close contacts between confirmed H7N9 patients and others, none have resulted in transmission and creation of new cases, with only one such close contact still under investigation as of a day or so ago. In regular everyday seasonal flu you’d expect 20 to30 percent transmission rates to close contacts. So, in normal flu we’d expect 530 cases to result in over 100 new cases, and they’ve identified none so far with this H7N9. No doubt we will eventually find some, but it argues that this thing has a really tough time going from person to person.

The other bit of evidence relates to the demographic profile of who is currently getting sick in China. Of the first 14 cases, 13 were adults.  Only one was a child. Children are near ideal flu transmitting vessels: they congregate together, they have little immunity, they touch themselves and each other constantly,   and in most outbreaks they are a big part of the case numbers. Here, it does not appear to be so, which suggests that the virus is coming from an exposure other than human to human.

Abstract:  How is this strain related (or not) to H5N1?

Slenning: As the agent is researched more, this conclusion may change, but to my knowledge, none of the genetic components of this Type A H7N9 have been directly tied to H5N1.  Of course, multiple generations ago, there could be some common ancestors — go back far enough in time and you are just about guaranteed to find them — but as far as I know there are no recent connections.

Remember that Type A influenzae have  eight segments to their genome. If you are unlucky enough to be infected with two strains of Type A flu, the viruses can swap those genetic segments and create a brand new virus. That’s what we call reassortment. So this H7N9 could well have had its origin in a bird somewhere that was co-infected with an H7N? virus and an H?N9 virus. Talk about a sucky day.

Abstract: Is H7N9 made up of recombined avian viruses?  Do bird flu viruses pass to humans as readily as swine versions?  

Slenning:  It appears that this virus has only avian-sourced genetic components. That makes it different from the others with which we’ve been concerned the last decade or so (H5N1, H1N1, H3N2), which were mixes of avian, swine and human components.   However, the general scientific consensus is that the 1918 so-called ‘Spanish flu’ was an avian-only virus that jumped to people.

This is a good news/bad news kind of thing. The good news part of this story is that — as you suggested – avian-adapted influenzae tend to have more difficulty directly infecting humans than do swine- adapted pathogens. Humans have a different mix of potential target cells in our respiratory tracts than do birds, so the avian- adapted strains have difficulty infecting us. This is why the CDC and WHO are ‘concerned’ but are not waving their arms about in panic. Additionally, the Chinese CDC (and soon our own CDC) are making the basic components for a vaccine against this specific strain, but they are unlikely to manufacture it in quantities unless it shows signs of making more common jumps to people.

The first bad news part of it, though, is that for avian-only sourced influenzae it is much more likely that humans have not been previously exposed; therefore, we are unlikely to have much immunity to them. These viruses — if they get into people — can be tougher on us.

The second bad news part of this story is that this specific strain of H7N9 appears to be not so pathogenic in birds. That is, it does not cause severe disease in birds. This means that birds don’t show many clinical signs, making it harder to detect, and that infected birds will likely be fairly capable of going about their regular business, spreading the agent amongst themselves. H5N1 kills domestic birds quickly, so we are pretty quick at finding it.  This one has a much better chance of infiltrating into multiple types of birds without giving us many clinical signs as warning.

2 Responses to “New Flu Review”

  1. [...] Editor’s Note: You may hear about fatality rates or percentages when media report on new and dangerous flu strains, and often times the reports are conflicting. In this post, Barrett Slenning, an epidemiologist at NC State, explains how these fatality rates are calculated, and why the numbers may fluctuate. A previous post on H7N9 flu can be found here. [...]

  2. [...] version of this interview was prepared for the NC State Abstract by Tracey [...]

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