Making the Case

PUBHLTH 405
Social History of Infectious Disease
University of Michigan School of Public Health

Jon Zelner
[email protected]
epibayes.io

Today’s Theme

Who will win???

Agenda

  • Finishing up discussion of mental models from (Greenhalgh 2021)

  • Chronicling the end of the Golden Square outbreak

  • Assessing the long-term impact of removing the pump handle

  • Prepping for Configuration Project workday next Monday

Mental Models

How have more recent, faulty mental models led us astray?

  1. What is evidence-based medicine, and what is Greenhalgh’s critique of its role in the COVID-19 pandemic?

  2. What weaknesses does she highlight in the Danamask RCT?

  3. What alternative sources of evidence does Greenhalgh argue should have been considered sooner in the SARS-CoV-2 pandemic?

  4. What historical experiences might have accounted for the different response to SARS-CoV-2 in China as compared to the U.S.? Does this connect to John Snow’s experiences in any way?

  • Are there similar fallacies you can identify from having worked on your contamination dossiers?

Small scale data provided compelling evidence for the droplet theory of SARS-CoV-2 transmission

Does this bias against certain types of evidence echo anything from The Ghost Map?

Why didn’t mechanistic evidence dislodge the droplet-only perspective on SARS-CoV-2 transmission?

Relationship between voice volume and aerosol production (From Asadi et al. 2019)

EBM’s hierarchy of evidence

  • What is meant by ‘risk of bias’?

  • Quality of evidence?

  • Where would Snow’s explorations fit in this hierarchy?

  • How could you re-draw this hierarchy to allow for more consilient explanations?

A version of the ‘evidence pyramid’ central to the philosophy of evidence-based medicine

Why is this kind of diagram inevitably controversial?

Two ways of aggregating evidence

Brick Wall: Collecting similar forms of evidence to test the strength of a hypothesis

Stone Wall: Collecting heterogeneous evidence to develop a robust and comprehensive explanation

Final Days of the Outbreak

Where are we?

By September 7th, the outbreak had begun to subside prior to the removal of the pump handle on the 8th

John Snow’s configuration-based argument

On your own:

  • How did Snow use exceptions to the predictions of his mental model to show the strength of his ideas?

  • Why do you think it is important to understand both the instances that align and those that disagree with a mental model?

  • What were the conclusions of the Vestry Committee on the cause of the outbreak? What may have kept them from adopting Snow’s theory?

  • What led to the removal of the pump handle?

Snow searched for exceptions that prove the rule in order to convince skeptics

Snow knew that the case would be made in the exceptions to the rule. What he needed now were aberrations, deviations from the norm. Pockets of life where you would expect death, pockets of death where you would expect life. (Johnson 2007, 140)

Snow’s experience with anesthesia contributed to his miasma debunking

[Snow’s] experience with choloform and ether had also endowed [him] with an intuitive grasp of the way that gases disperse in the environment. Ether could be deadly in a concentrated form, delivered directly to the patient’s lungs. But a doctor delivering it, standing a foot away from the patient, wouldn’t feel its effects in the slightest, because the density of ether molecules in the air dropped at a precipitous rate the farther removed one was from the inhaler itself. (Johnson 2007, 145)

Scale, scale, scale (again!)

“If the symptoms of the cholera concentrated around the small intestine, then there must be some telltale characteristic in the eating and drinking habits of cholera victims. If cholera was waterborne, then the patterns of infection must correlate with the patterns of water distribution in London’s neighborhoods.” (p. 148)

Snow lined up mechanisms with the spatial configuration of the neighborhood

“In his mind snow was already drawing maps. he’d imagined an overview of the Golden Square neighborhood, with a boundary line running an erratic circle around the Broad Street pump. Every person inside that border lived closer to the poisoned well; everyone outside would have had reason to draw water from a different source.” (p. 141)
  • 10 cases outside the boundary.
  • 2 were tailor & son on Cross st.
  • 3 were children who went to school near the pump.
  • 3 more regularly drank from the pump.
  • 2 more could be chalked up to the typical toll of Cholera in a given weekend.

What happened, exactly, in Golden Square?

In your pathogen groups, I’m going to ask you to draw a diagram illustrating the connections between the components of this system, including the steps leading up to and the aftermath of removing the pump handle:

  • What was the most likely route by which cholera was introduced to the Broad Street Pump?

  • Why did the outbreak gradually die out rather than spreading exponentially throught the neighborhood?

  • Why did the outbreak end when it did?

  • Related: What was the likely impact of removing the pump handle on risk in the neighborhood? What might have happened if it wasn’t removed?

  • You can be expansive in your definition of mechanisms and interactions. Think across levels and include biological, infrastructural, and social dynamics.

Strength of Snow’s explanation rested on nuts and bolts analysis.

  • Cracked cesspool in the basement of 40 Broad appears to have been the source of the outbreak.

  • Others got sick in 40 Broad over the course of the week, so why did they not add additional fuel to the 🔥?

  • Only the Lewis family had access to the basement cesspool, while others threw their waste out the…🪟.

How did the Cholera Commission report deal with the evidence that Snow brought to bear?

What is missing from this (long) list of section headings?

The first map of the outbreak was not made by Snow

Edmund Cooper’s map of the locations of Cholera deaths relative to the location of a supposed ‘Plague Pit’ suspected of being the source of the outbreak

Snow’s first map made two crucial modifications to the layout used by Cooper

Snow’s bar map indicated the number of deaths at each location, the location of the pumps, and stripped away extraneous environental detail

A later edition of the map included another piece of information to push back on miasma

Snow traced a boundary indicating the locations for which the Broad Street Pump was the closest by walking time, showing that many deaths were clustered within this area.

Next Time

Configuration Project Workday

References

Greenhalgh, Trisha. 2021. “Miasmas, Mental Models and Preventive Public Health: Some Philosophical Reflections on Science in the COVID-19 Pandemic.” Interface Focus 11 (6): 20210017. https://doi.org/10.1098/rsfs.2021.0017.
Johnson, Steven. 2007. The Ghost Map: The Story of London’s Most Terrifying Epidemic–and How It Changed Science, Cities, and the Modern World. Reprint edition. London: Riverhead Books.