Correlation without causation: The lesson of the Lazaretto
Correlation, of course, does not equal causation. But public health breakthroughs from the smallpox vaccine to the first smoking-and-cancer links came without it. So did the power of quarantine.
Correlation without causation: The lesson of the Lazaretto
The pandemic of the moment is MERS: Middle East Respiratory Syndrome. This mysterious, untreatable illness—which kills nearly a third of those who catch it—entered the U.S. for the first time in late April. A second imported case was confirmed this week in Florida.
Last month it was Ebola hemorrhagic fever in the West African nations of Guinea and Liberia. In 2009 it was the H1N1 or “swine” flu. A decade ago it was SARS, and a decade before that a whole host of so-called “emerging diseases” were identified as serious new health threats, including Hantavirus, “Mad Cow” disease, “flesh-eating” streptococcus, and an earlier incarnation of Ebola. Globalization, we’re told, has shrunk the world and made us all vulnerable to exotic infections that may be brewing on the other side of the world. After all, as we were reminded yet again in the 2010 blockbuster film Contagion, every potentially deadly disease outbreak is only a plane ride away from us.
The front line of protection against the entry of foreign germs is our quarantine system. The Centers for Disease Control and Prevention operate a network of 20 quarantine stations across the country, including one in Terminal A of Philadelphia International Airport. There, in conjunction with Customs and Border Patrol officials, the CDC keeps watch over arriving international passengers and cargo, responds to reports of illness on flights and ships coming from abroad, and conducts preparedness exercises against possible pandemics.
Just a mile west of the airport, a grand and stately but slowly decaying Georgian building looks out on the broad expanse of the Delaware River beneath the roaring airplane traffic. If walls could talk, this place could tell a story or two about pandemics and quarantine. It is the Lazaretto quarantine station and hospital, and it stood guard against the introduction of epidemic diseases into the Port of Philadelphia from 1801 to 1895. You could call it the original Ellis Island. After a series of devastating yellow fever outbreaks in the 1790s, Philadelphia’s Board of Health purchased the site in Tinicum Township and built a state-of-the-art quarantine facility there—far enough away from the city to prevent contact between residents and the ships and passengers detained there. Globalization is nothing new; Philadelphians in 1800 dealt with ships, goods, and people from distant countries every day, and they knew from hard experience that overseas trade made them vulnerable to disease.
And yet . . . after the new Lazaretto opened, the frequency and severity of yellow fever epidemics declined significantly. Quarantine shouldn’t have worked—it was based on an erroneous understanding of the cause and transmission of yellow fever—but it did. Health officials in Philadelphia, New York, and other large American seaports saw cases of yellow fever every year at their quarantine stations, but only rarely saw significant outbreaks in their cities after about 1805. Every year without an epidemic vindicated the controversial practice of quarantine.
This does not mean that quarantine is necessarily the key to pandemic preparedness today—whether for MERS or for any other disease. The Lazaretto’s lesson is not in the policy but in the methods by which threats were evaluated in the 19th century. Physicians squabbled vehemently over the nature and causes of yellow fever and other diseases, but these disagreements did not prevent a workaday consensus on prevention. Every known outbreak in history was studied in detail, and every observable variable was correlated with the spread and severity of the disease. Yellow fever outbreaks generally followed the arrival of ships from West Indian ports in times of hot, wet weather, and targeted crowded, filthy urban neighborhoods. Philadelphia’s Board of Health designed its quarantine and sanitation policies accordingly.
We are often reminded today—and nowhere more often than in public health—that correlation does not equal causation. But look at the long history of pubic health breakthroughs that we owe entirely to rigorous empirical observation and correlation: Edward Jenner’s smallpox vaccine, John Snow’s discovery that contaminated water was spreading cholera in London, the first studies linking tobacco smoking to lung cancer, the Framingham Heart Study’s implication of high blood pressure and high cholesterol in heart disease—and there are many more. Every one resulted in effective preventive measures, and every one happened in the absence of any knowledge of the specific cause and mechanisms of the disease in question. These milestones represent the power of correlation without causation.
Today we have the power to recognize MERS as caused by a hitherto unfamiliar coronavirus related to the one that causes SARS, and this fact alone scares us. We know about the mutating habits of the influenza virus, and we know about the laboratory behavior of thousands of other potentially disease-causing parasites, bacteria, and viruses. An abundance of caution is always wise in public health. But let us not lose sight of the means by which public health knowledge was generated for centuries before the germ theory of disease: empirical observation and correlation—not in the laboratory, but in the uncontrolled environment of the real world. That is one of the lessons we can learn from a stately 200-year-old building on the Delaware River next to the airport.
The Lazaretto Preservation Association of Tinicum Township is working to preserve the Lazaretto site for historical interpretation and public use. Contributions may be sent to LPATT, 629 N. Governor Printz Blvd., Essington Pa. 19029. The Friends of the Lazaretto invite one and all to a celebration of the Lazaretto’s history on June 14 from 10 a.m. to 4 p.m. in Governor Printz Park, which is next to the Lazaretto on Second Street at Taylor Avenue in Essington.
Or take a video tour through history with Dr. David Barnes.
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