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Epidemiologists Are the 'Disease Detectives' Protecting Public Health

epidemiology
Sandrine Belouzard, virologist and researcher, works in her epidemiology laboratory of the Infection and Imminence Center at the Pasteur Institute of Lille on F,eb. 17, 2020 in Lille, France. The research institute sequenced the genome of coronavirus using blood samples taken from the first confirmed French cases. Sylvain Lefevre/Getty Images

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Although they are often working behind the scenes studying diseases, when there is an outbreak of virus, epidemiologists quickly take center stage and serve on the front line to understand and control its spread. Their work is even more vital when the disease is a new one, like with the novel coronavirus (SARS-CoV-2).

These "disease detectives" practice epidemiology, a branch of medicine dealing with the incidence, distribution and control of diseases and other health-related issues, according to the Oxford Dictionary. The term was borrowed from the Greek word "epidēmia," which means prevalence of disease, with the addition of "logy" for study, and came into use during the 19th century.

They consider the person, place and time of a disease outbreak, and use quantitative tools to describe them and examine at-risk factors.

So, for example, with novel coronavirus — or COVID-19, the disease it causes — epidemiologists are working to understand how easily, quickly and by what mechanisms it spreads, as well as its mortality rate. They also are involved with recommending control tactics and interventions, and will be tasked with assessing effectiveness.

What Is an Epidemiologist?

There are two lines of study for epidemiologists: infectious diseases and chronic diseases. Chronic disease epidemiologists study conditions like heart disease, diabetes, cancer, arthritis and other diseases. They interpret data related to these diseases and also make public health recommendations based on findings, according to the U.S. Centers for Disease Control and Prevention (CDC).

But it's the job of the infectious disease epidemiologist to tackle disease outbreaks. They are mathematical modelers who "try to stay ahead of the wave in terms of an outbreak and model potential interventions," says Dr. Saad B. Omer, director of the Yale Institute for Global Health. They do this by examining relevant factors in ways similar to how journalists attack hard news stories, with the 5W's:

  • what: diagnosis or health event
  • who: person
  • where: place
  • when: time
  • why/how: causes, risk factors and modes of transmission

Infectious disease epidemiologists use quantitative methods to learn how a disease is being transmitted and how infectious it might be. The field includes a lot of math and statistics as these "detectives" use data and look at patterns.

It is a diverse crowd and filled with people from many backgrounds, like biology, physics and public health. Many hold Ph.Ds., but some are physicians who also see patients. Some epidemiologists work in offices, but they might also be in a lab or in the field developing and testing interventions. The bottom line is they are all interested in public health.

epidemiologist
While in the field, dressed in a full complement of personal protective equipment (PPE), a CDC epidemiologist was implementing a rapid diagnostic test, which was used during the Ebola response, to quickly test for the presence of the Ebola virus.
Centers for Disease Control

How Do Epidemiologists Track Infectious Diseases?

In order to perform quantitative analysis and mathematical modeling, epidemiologists need data.

"Surveillance is the cornerstone of epidemiology," says Dr. Benjamin Lopman, professor of epidemiology at Rollins School of Public Health at Atlanta's Emory University. Epidemiologists need to know how many people are getting a disease, where they are and who they are. Generally, organizations like the CDC or local, state or federal government public health agencies collect the data epidemiologists use.

"Whenever a new pathogen emerges, there is so much to learn," Lopman says. Understanding how transmissible a disease is crucial in controlling its spread (more on that in minute). To quantify and measure transmission, epidemiologists use the Basic Reproduction Number (R0) (pronounced "r-nought"), which is the number of individuals that will be directly infected by an already infectious person.

The R0 is used to estimate the severity of an outbreak. If the number proceeding R is greater than 1, it means that every person infected will infect that number of people. For instance, the R0 of measles is 10-15, while for seasonal flu it's 1.5, according to Lopman. The estimated R0 for COVID-19 is currently 3. So that means on average, every person positive for coronavirus will create three new cases.

There are several factors that affect transmission, such as how well a pathogen can survive on surfaces and how quickly people might get sick or pass it on.

"When infection leads to a debilitating illness, it can be easier to detect and track cases, thus leading to easier containment," says Dr. Matthew Freeman, associate professor of epidemiology at Rollins School of Public Health. But sometimes, people can be asymptomatic, meaning they're carriers without symptoms.

With the novel coronavirus outbreak in Washington state, the first case was determined to be someone who had known contact from travel. However, several weeks later, a similar strain was found in other people in the area who had not traveled, which meant that the virus had been begun circulating in the population, a term known as "community spread."

Without good public health surveillance, epidemiologists wouldn't able to properly track the spread of disease. Early testing is a critical part of this equation.

"One of the tools epidemiologists have in our toolbox is surveillance," Omer says, but if the first view of a disease occurs when patients end up in the ICU, "the concern is that you are getting just the tip of the iceberg."

How Do Epidemiologists Control an Infectious Disease?

Implementing a containment strategy as soon as possible when a virus is discovered is one of the core jobs of an epidemiologist. The CDC uses a three-tiered response to help public health departments respond aggressively if a threat, like coronavirus, is confirmed.

  • Tier 1: Genes and germs for which no treatment options exist or genes and germs that have never or only rarely been detected in the U.S
  • Tier 2: Genes and germs not commonly detected in a geographic area
  • Tier 3: Genes and germs that are known threats in a geographic area but not endemic

The CDC's containment strategy includes:

  • rapid identification
  • infection control assessments
  • colonization screenings when needed
  • coordinated response between facilities
  • continue assessments and colonization screenings until spread is controlled

Contact tracing is another way epidemiologists control viruses from spreading. "People in close contact with someone who is infected with a virus are at higher risk of becoming infected themselves, and of potentially further infecting others," Lopman says in a follow-up email. "Closely watching these contacts after exposure can help then to quickly be diagnosed, get care and be isolated if need be, which can prevent further transmission of the virus."

Contact tracing, Lopman explains, is a tried-and-true method to control infectious diseases. "It's been used to control sexually transmitted infections, and to eradicate smallpox. However, it's only really useful when transmission is limited to a few chains of transmission and in the 'containment phase' of an outbreak response." Once transmission in the community is widespread, he explains, contact tracing becomes impossible.

ebola vaccine
Dr. Mike Montello, a former research director at the U.S. National Institutes of Health, is seen here preparing the first batch of Ebola vaccines to be given in the Ebola vaccine trials, which were launched at Redemption Hospital, formerly an Ebola holding center, on Feb. 2, 2015 in Monrovia, Liberia.
John Moore/Getty Images

How Do Epidemiologists Help Prevent the Spread of Disease?

The collection of data to learn about and understand diseases allows epidemiologists to better control the spread of a disease because they can make informed recommendations about interventions. The CDC website explains that epidemiologists propose "appropriate, practical, and acceptable public health interventions to control and prevent disease in the community."

Omer says early on in an outbreak, epidemiologists use mathematical models to consider how transmittable a disease is to estimate its rate and how likely it is to spread. Later, they can look back to see how successful the measures they made were.

For example, the novel coronavirus outbreak appears to be on the decline in China and South Korea. Comparing the effectiveness of containment strategies and interventions in those countries could help inform epidemiologists' input into policy decision-making at various levels in other countries where the outbreak is just starting to advance.

Those interventions, as we are seeing now across the globe with coronavirus, might include closing schools, social distancing, canceling large events, travel restrictions or the development of a potential vaccine. Drastic measures like large quarantines or even entire cities on lockdown might be necessary recommendations to prevent the spread of a fast-moving virus. While epidemiologists use scientific methods to help answer questions about the benefits of interventions and can make recommendations, it is policymakers who ultimately make the final decisions.

But it's not only policymakers who rely on epidemiologist recommendations for decision making. For example, many events, like The Coachella Valley Music and Arts Festival and the Architectural Digest Show, have been postponed due to the coronavirus outbreak and rescheduled for summer and others for fall. Though on March 12, the NBA, MLS and NHL all canceled, rather than postponed, the rest of their 2020 seasons. The NCAA Big Ten Tournament also was canceled. Emory's Lopman and Freeman explained in a follow-up email that some models are suggesting that with aggressive action now — coupled with lower transmission of the virus in warm weather — the coronavirus pandemic could subside by the summer.

"However, there are a lot of unknowns. So many people are susceptible since they've not been infected with this virus, it is quite possible that risk may remain throughout the summer and also that we could see reemergence in the fall," Freeman says.

By that time, epidemiologists involved with COVID-19 may be deep into the final phases of their detective work, which is to "assess whether an intervention — be it a vaccine, technology or behavior change intervention — leads to a reduction in infectious disease," Freeman explains.

"This is where science meets people," Omer says. "It informs our decision making in a more systematic way." In the field of epidemiology, quantitative tools are used to determine real-life interventions.

It's an awesome responsibility, and it comes with a huge burden to be as right as possible.

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