Monday, January 23, 2006

Flu Kinetics - Single Number Disinformation

The most recent WHO fact sheet contains this alarming but misleading statement [the alarming part isn't the misleading part, it's very alarming]:

"Highly pathogenic viruses can survive for long periods in the environment, especially when temperatures are low. For example, the highly pathogenic H5N1 virus can survive in bird faeces for at least 35 days at low temperature (4oC). At a much higher temperature (37oC), H5N1 viruses have been shown to survive, in faecal samples, for six days."

Brooklyndodger(s) comments: BrooklynDodger suggests this statement would be a lot more informative, and also true, if stated in terms of half lives or decay times. It's misleading to say that room temperature chicken turds are dangerous for 6 days, but then become safe at midnight.

The potential for infection might also be better understood if there were estimates of an ID50, infectious dose of virions for 50% of the population, with some notion of the population distribution. Actually, you'd want to know the virions per gram of turd, so as to calculate an ID50 in terms of turd grams.

Those two data sets would yield a probability of infection at a given time. That in turn might distinguish human to human transmission from transmission from chicken-free but chicken contaminated articles and materials. It would also yield a science based approach to flock culling, and a re entry time for new chickens into a formerly infected coop.

The Dodger guesses that these infective flu particles are virions, and not a virus infected cell of some type. [For example, legionella live and reproduce inside other microorganisms, and animal macrophage. Anthrax spoors live "forever" in dirt. The Dodger guesses that virions have to be wet.]

Theoretically, it's possible to say the infectious material has a true zero risk when the last virion decays. But practically, the Dodger guesses there's a first order decay process with a huge number of virions which never get to zero even at infinity.

[But, what about autoclaving, wouldn't that kill them all? That's a first order process too? The Dodger guesses you could guess the number of virions in the sample, 10 to the whatever power, and figure out how long it would take to get down to 1 and then 0.]

Yes, the Dodger says, there aren't infinite virions in the chicken turd, so there is a true zero. But it's a risk assessment number to be calculated. With a distribution.

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