# 5974
The announcement yesterday out of Iowa that 3 more children have contracted a rare emerging swine flu strain (see Iowa DPH Reports 3 Novel trH3N2 Cases) has produced a number of headlines over the past 18 hours.
The most significant piece of new information since then is that all three children had contact with one another, and that it appears that one child transmitted the virus to the other two.
No additional transmission has thus far been detected, and the the mystery of how and where the first child acquired the virus remains.
As might be expected, Helen Branswell of the Canadian Press has some of the best and most complete coverage of this story.
A link to her story (which I encourage you to read in its entirety), then I’ll be back with a little more.
U.S. sees 3 new infections with new flu virus
The Canadian Press
Date: Tuesday Nov. 22, 2011 8:59 PM ET
U.S. health officials have spotted three more children infected with a new swine-origin flu virus, this time in Iowa.
And while earlier cases of infection with this virus are believed to have been cause by exposure to pigs, this time the evidence points to person-to-person spread.
Occasionally we’ll see a novel swine influenza virus (usually an H1N1 or H3N2 variant) jump to humans. Although the 2009 swine flu pandemic was an exception, most of the time this turns out to be a one-off dead-end transmission, and goes no further.
The key phrase being `most of the time’.
While it is a rare event, as we saw in 2009 it is possible for one of these viruses to adapt well enough to human physiology to spark an extended outbreak, or even a pandemic.
Since only a tiny fraction of influenza flu viruses are ever sequenced, we really don’t know how often these types of novel infections occur.
It’s probably more often than we think.
But surveillance systems must be incredibly lucky, or wait until a novel virus has infected a sufficient percentage of the population, before they are likely to start picking up cases.
In these ten most recently reported cases, the virus has been a swine H3N2 virus with the M segment gene borrowed from the 2009 H1N1 virus. Essentially a hybrid – a new reassortant virus - that until this summer had not been seen before.
Reassorted viruses can result when two different flu strains inhabit the same host (human or otherwise) at the same time. Under the right conditions, they can swap one or more gene segments and produce a hybrid virus.
Where this virus goes from here is anyone’s guess.
Its appearance across four states, and in some humans without direct animal contact, certainly suggests low-level human transmission.
But the ability to transmit from one human to another isn’t enough to ensure its survival. In order to thrive, it must have an R0 number (basic reproductive number) sufficient to sustain an outbreak.
The R0 number signifies the average number of secondary infections caused by one infectious person entering a totally susceptible population.
If less than 1.0, outbreaks are likely to sputter and die out. If greater than 1.0, the outbreak is more capable of spreading.
And as we’ve seen with roughly two dozen other reported novel swine infections since 2005, and with a significant number of H5, H7, and H9 avian flu infections, it is possible to have limited outbreaks of a novel flu virus without it sparking an epidemic.
For now, this virus doesn’t appear to be sufficiently well adapted to human physiology to spread in an efficient and sustained manner.
This virus may simply simmer at low levels in the human population for a spell, then die out. Or it may further adapt to humans, and produce a larger public health threat down the road.
We’ll simply have to keep our eyes on this story, to see where it goes.
In the meantime, maintaining good flu hygiene this winter, and getting your flu shot, remain the best strategies to avoid getting sick during this flu season.
