Epidemiology in the Wake of a Pandemic

As we progress through the COVID-19 pandemic, its effects on the study, containment and prevention of disease continue to unfold. Historically, public health emergencies have necessitated and inspired defining changes to epidemiology. Read on for a look back and a look ahead at epidemiology in the wake of a pandemic.

Recurrent outbreaks of plague between the 14th and the 17th centuries disrupted urban life and civic power structures. In London, the pandemics of plague progressively led to the systematic collection of death counts.¹

• Introduced in 1518, quantitative plague reports expanded in 1554-5 to include other individual causes of death
• Serious episodes of plague in 1592 and 1625 led to the creation of regular, weekly handbills listing parish-by-parish counts of deaths by plague

When statistician John Graunt analyzed this continuous weekly series for several decades of mortality data, he discovered a phenomenon never reported before: deaths from some causes other than the plague occurred with great consistency²—so the number of tuberculosis deaths in London in any year, for example, could be predicted from the number of deaths in the previous year. The observation protocols implemented to help contain the plague enabled Graunt to see that the occurrence of events in populations is predictable, potentially comparable and a possible source of scientific knowledge. This discovery of population thinking was the most decisive methodological advance in the history of social and human sciences.

In addition to population thinking, group comparison is the second defining trait of epidemiology, and we owe its first use to the cholera pandemics of the 19^th century.¹

John Snow is now known for his investigation of the cholera outbreak related to London’s Broad Street water pump. He believed there was an unobserved living thing that could multiply in the water supply, causing cholera upon ingestion from water or food or from sleeping in infected sheets. He conducted a survey, comparing Londoners living in the same parishes. They bore equal exposure to stench and air pollution but drank water of varying degrees of cleanliness. Lead pipe belonging to private companies carried water from different sections of the Thames, one contaminated and one not, to tanks and then to pumps in individual houses.³ Snow’s survey was the first use, at a population scale, of a comparative study design to explore a causal association.

In 1900, many in public health believed that susceptibility to tuberculosis was hereditary. People with tuberculosis could have children, which led eugenicists to fear that every person would eventually be susceptible to tuberculosis, which could wipe out the human species. With a long latency between tuberculosis infection and diagnosis, no methods existed to study whether those with tuberculosis had a survival advantage. To remedy this, German physician Wilhelm Weinberg invented a new design, referred to today as a cohort study: Using population registries in Stuttgart, he followed up 25,786 children from birth until they turned 20, comparing those from families in which a parent had, and had not, died from tuberculosis, respectively.⁴

He reported his findings in a detailed description of the new design and analytic methods of his study. Having reviewed hundreds of thousands of records and conducted individual follow-up with dozens of children, he found that children from tuberculous parents lived shorter lives. Tuberculosis conferred no selection advantage and did not threaten humanity with extinction.

Until 1918, public health in the United States was overseen by health officers and civil servants. In April of that year, while working for the U.S. Public Health Service, Wade Hampton Frost was made responsible for Surgeon General Rupert Blue’s Influenza Task Force. By conducting a months-long house-to-house survey in 18 communities, Frost established the prevalence of infection in the population. All of his published writings for the next two years address influenza.¹

In October 1918, the Johns Hopkins University School of Hygiene and Public Health (SHPH) admitted its first students. Founder William Henry Welch—also active in the response to the flu pandemic—approached Frost in 1919 and made him Chair of the newly formed, first-ever epidemiology department. Frost later became the first professor of epidemiology in the United States.

Between Frost’s and Welch’s commitment to the 1918 flu pandemic response, it is reasonable to believe that the pandemic spurred an important development: a new, academic parallel to the public service dedicated to public health and epidemiology.

We’re not through the pandemic yet, so it’s too soon to consider its impacts in hindsight. A recent article in the International Journal of Epidemiology, however, notes that, “it has become apparent that some issues have longer-term implications for field epidemiology and warrant discussion now. Moreover, the need to recognize and incorporate these lessons similarly applies to the broad field of epidemiology.”⁵ At a minimum, the article says, we’ve learned that we need to do the following:

Ensure timely availability of diagnostic tests.
Delays in the deployment of testing in this country led to uncertainty regarding the disease and the breadth of community spread, which contributed to rapid acceleration of the pandemic.⁶ Containment depends on rapid availability of results to enable three vital interventions:

• Timely contact tracing
• Implementation of isolation for the infected
• Implementation of quarantine of exposed persons⁷

Assess the effects of social determinants of health on disease burden.
As the pandemic progressed, the finding that certain groups are at higher risk for SARS-CoV-2 infection clarified the need for early analysis of social determinants, so that we can fast-track development of population-level interventions. We need to collect data on race: not only ethnicity, but also living conditions, work settings and other health circumstances that may affect individual and collective risk.⁸ In future public health emergencies, early consideration could lead to more timely identification of, and response to, these disparities in risk levels.

Strengthen existing and rebuild damaged global partnerships.
The COVID-19 pandemic has emphasized the crucial need for strong global partnerships in detection and effective response. While the 2005 International Health Regulations agreement (binding for all World Health Organization member states) addresses the importance of international collaboration and coordination, we must maintain trust-based relationships with other countries. By continuing to participate in multilateral public health enterprises, such as the World Health Organization, and strengthening collaborations on epidemiologic surveillance of health threats, we will enable more efficient detection and more effective implementation of interventions to control global public health emergencies.⁹

Ensure that primary communications are delivered by trusted, credible messengers.
During the COVID-19 response, unlike in previous public health emergencies, information has most often been communicated by politicians instead of public health experts.¹⁰ Politics have never complicated the response to public health emergencies as much as they have with COVID-19.⁵ An event history analysis of social distancing policies across the 50 states showed that the most important predictors were political, with Republican governors being slower to implement social distancing policies.¹¹

Our response communications, and the selection of those doing the communicating, should be guided by awareness that public health emergencies are easily politicized. Messaging, delivered by those demonstrating empathy, honesty, dedication and expertise,¹² should focus on educating people on basic public health literacy issues. This will improve public understanding of why we take certain preventive measures (wearing face coverings, maintaining physical distance and washing hands, for example) in order to increase acceptance and use of them.

Implement targeted, evidence-based interventions as early as possible.
The COVID-19 pandemic has shown us the critical importance of timing when implementing interventions. Data from the European Centre for Disease Prevention and Control on policies introduced in 149 countries showed that physical distancing interventions, such as school and workplace closures and restrictions on mass gatherings and population movement (that is, lockdown) were associated with significant decreases in incidence of COVID-19; those decreases were even greater, however, in countries employing earlier implementation of lockdown along with school and workplace closures.

Emphasize the importance of preparation training and exercises.
We need to intensify the focus on continual preparation for public health emergencies. In the last 20 years, American federal, state and local public health entities have faced a punishing succession of emergencies—9/11 and the anthrax attacks (2001), Hurricane Katrina (2005), H1N1 influenza (2009), Ebola (2014), Zika (2016) and COVID-19 (2020)—which underscores the need for responders at all levels to train for the ones to come. While each of these emergencies raised different challenges, all have required disciplined, strategic responses that would benefit from improved preparation.

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