Wednesday, February 14, 2007

The end of SARS and the use of synthetic pathogens to combat epidemics

You know - SARS - the plague that was going to destroy civilization. It went away. Puzzled the heck out of me. It was supposed to recur every year or two, but it hasn't. Nobody seemed to be curious about this. Was a giant conspiracy at work?

Well, no. A quick pubmed search showed that lots of epidemiologists have been quietly puzzling about what happened, especially in China. I liked this one:
A double epidemic model for the SARS propagation (9/2003).

... We find that, in order to reconcile the existing data and the spread of the disease, it is convenient to suggest that a first milder outbreak protected against the SARS...
I particularly like it because I wrote:
I still don't understand why all hell didn't break loose then. My best guess is that there were multiple strains of SARS circulating simultaneously, and an innocuous one spread faster -- immunizing the susceptibles in advance of the killer strain.
Ok, so I put my theory out more than a year after Ng et al published a full model supporting it! I gotta work on my timing. They must have started work on their model very shortly after the epidemic had started to fizzle. I didn't blog on this thought, but my theory back then (2004, not 2003!) was the Canadian nurses got so sick because they were so good at isolation -- they prevented exposure to the benign, immunizing, coronavirus and thus suffered the full impact of the malign virus.

All of which lead to some random observations and questions:
1. This is a fascinating story that ought to appear in a popular magazine, or at least in The Economist or Scientific American. I don't recall seeing anything. There's a curious "chaotic" aspect to what gets written when. I wonder if blogs will change any of that, or if they simply amplify the current fads.

2. As a non-practicing physician who works on clinical knowledge representation I often think about the limits of the mental models I once used to care for patients. Back in the day we were taught to think of 'one infection, one disease'.

Are medical students still taught to think that way, or are physicians now taught that illness (or its absence) may be the result of a number of interacting simultaneous infections (and of course susceptibilities, treatments, phases of the moon, etc, etc)? Of course I'm not sure what one would do with such knowledge! Still, it does help make one's predictions more modest.

3. This suggests a radical, but not entirely novel, approach to a future serious epidemic. Create a contagious synthetic pathogen that's relatively benign, but induces immunity to the major pathogen -- and spread it actively. I say not entirely novel, because this is how Polio was suppressed. The oral vaccine was an active contagious pathogen that was excreted in stool. It immunized a vast number of persons -- but some became sick, disabled, or dead. When Polio was less of a threat we switched to a non-pathogenic inoculation. The difference is the successful Polio strategy was probably unintentional (I suspect some people understood even in the 1950s), but in the future we'd be deliberately exposing an entire population to an immunogenic pathogen that would almost certainly harm many people.
Oh, about that conspiracy. Somewhere in central China in 2003, a brilliant scientists realizes that she can save the world by unleashing a synthetic coronavirus she's been developing in a top-secret bioweapon facility ... The novel almost writes itself ...

2/15/07: Emily points out that this is rather like fighting a fire by setting fires -- backburning I think it's called. My son has a book on it called 'Hotshots'. A useful analogy.

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