Arlington County Virginia, where I live, announced today that they were going to be spraying pesticides in my neighborhood because local mosquitoes have been testing positive for West Nile Virus, and a human case has emerged. (They're also keeping all the children at my kids' elementary school indoors during some of the most beautiful weather we've had in months.)
I wonder about this. I’m curious about the cost-benefit calculations, and suspicious that, as is so often the case these days, the calculus is all whacked because of media-driven hysteria over a novel phenomenon with an exotic name that has led to a perception of risk out of all proportion to reality.
The security expert Bruce Schneier likes to say (to paraphrase) ‘if you read about it in the newspaper, you shouldn’t be afraid of it.’ The fact that it’s in the media is a sure sign, he says, that the danger in question is unusual or freakish –- while its the things like slipping in bathtubs or falling down stairs that really get you. Or the flu.
Is West Nile Virus really dangerous enough to warrant the dumping of god knows how much pesticide into our community (grass, plants, soil, water tables, insects, animals’ bodies, insects again, rivers, etc)? (The authorities claim the pesticide is “safe” – another question is how much faith to put in that claim, but that’s a separate matter, even if for me it’s not a whole lot. It ain’t safe for mosquitoes anyway, I guess).
I turned to the CDC to see what I could find out. There I read that “approximately 80% of people who are infected with West Nile do not show any symptoms at all.” I read that about 1 in 150 of people who become infected will develop “severe illness,” and that of that 0.66% who get severe illness, “case-fatality rates range from 3% to 15%.” If my calculations are right that means between 1 in 1,000 and 1 in 5,000 of people who are infected will die.
How does that compare to the flu? The CDC says:
Each flu season is unique, but it is estimated that, on average, approximately 5% to 20% of U.S. residents get the flu, and more than 200,000 persons are hospitalized for flu-related complications each year. About 36,000 Americans die on average per year from the complications of flu.
5% of the US population of 300 million is 15 million; If 36,000 of them die, then that would be ~1/4 of 1%, or 0.23%, or 1 in 417 people.
20% of the US population would be 60 million; if 36,000 of that number die, that would be ~1/20 of 1%, or 0.06%, or about 1 in about 1667 people.
So according to the CDC’s figures, the death rates for each range from
- West Nile: 1 in 1000 to 1 in 5000
- The Flu: 1 in 417 to 1 in 1667
Maybe I’ve made a mistake in here somewhere or am missing some other key factor, but it looks to me like West Nile Virus is considerably less dangerous than the flu.
One thing that would be interesting to know in the case of both types of virus is how many of these fatalities are cases where a sick person who is on their last legs anyway gets dealt their final death blow by the virus. I suppose if the flu was mostly knocking out the vulnerable, while West Nile was striking down the healthy, that would make West Nile more dangerous if viewed in terms of “expected years of life cut short,” which would strike me as a legitimate way of evaluating a risk. But, the CDC does say that West Nile fatality rates “are highest among the elderly.”
I wonder if our public health authorities have sat down and seriously thought this through (hopefully with some ecologists at the table) or if they are just responding to hysterical constituent calls.
I wonder if they would dump pesticides and shut my children up in school to prevent a slight risk that someone would get the flu.
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