[MUSIC] Welcome, I'm Justin Lessler, faculty at the Department of Epidemiology at the Johns Hopkins' Bloomberg School of Public Health. In this module, I'll be discussing the basics of identifying and investigating outbreaks using epidemiological techniques. At the end of this module, you should be able to describe what an outbreak is and how they are identified. Identify the major steps in an outbreak investigation. Outline the elements of a good case definition. Describe the the major types of outbreaks and their epidemic curves. Describe the key elements in a risk factor study, and discuss the basics of outbreak reporting and response. There are numerous resources available for those who wish to better understand this material or explore these issues further. Epidemiology by Leon Gordis is a classic test on epidemiology that will help students to gain deeper understanding of the principles and techniques covered in this module. The US Centers for Disease Control and Prevention has a huge number of resources for those who are interested in learning how to investigate and respond to outbreaks. These include a collection of epidemiological case studies that walks students through real outbreak investigations conducted by the CDC. The solve outbreak app allows users to try their hand at investigating an outbreak in an interactive format. The morbidity and mortality weekly reports are a great resource for those who want to read about past and recent outbreaks. On the international stage, TEPHINET a network of global field epidemiology and training programs also has a huge number of resources. Step 0, what is an outbreak? First, let's define what an outbreak is. Both epidemic and outbreak refer to a period of time when more cases of a disease occur than is typical in a particular place. If a disease is rare, this might be only a few cases. While if it is more common, we would need to see a very large number of cases before calling something an outbreak. For instance, if we saw even two or three cases of Ebola in New York City, we would call that an outbreak. But we'd have to see many thousands of influenza cases in the same area before we said there was an outbreak of the flu. The term outbreak and epidemic are often used interchangeably. But generally, outbreak is used to refer to something smaller or more contained in space and time. For example, we often refer to the HIV epidemic when discussing the entire course of the disease across the whole world. However, we would only call something and HIV outbreak if we were talking about a particular population over a small period of time. For instance, there was a outbreak of HIV in Indiana from 2011 to 2015. Here's an example about an outbreak of meningitis. In the United States, even though a small number of linked cases of meningococcal diseases is considered an outbreak. Here the county Public Health Department declared a meningococcal disease outbreak at San Diego State University based on only three cases of the disease among undergraduates in a month. Ebola is a rare disease and doesn't require very many cases to be considered an outbreak. Here we are showing information about an outbreak of Ebola in the Democratic Republic of Congo, where they declared the outbreak based on only two cases of laboratory confirmed Ebola viral disease. The outbreak has continued and as of the 30th of October of 2018 there were 257 cases inside Congo. In contrast, for influenza, we have to see a very large number of cases in order to consider something an outbreak. So, for instance, a outbreak of influenza was declared in the 2017, 2018 influenza season because hospitalizations were at their highest rate in nearly ten years. And over 50 people out of every 100,000 had been hospitalized for the flu, far above regular numbers. Influenza is a good example for considering how we define an outbreak and it’s not just a number of cases we see. So for influenza, we have a couple of ways of looking at an outbreak. One is to compare the amount of deaths that we see from pneumonia and influenza compared to a seasonally adjusted average. So in this figure, we're comparing a seasonal baseline of influenza cases shown in black, it goes up and down each year. And a defined epidemic threshold is slightly above that seasonal baseline that also goes up and down, here shown in black. And then we consider when the number of observed cases, shown here in red, go over that epidemic threshold to be an outbreak. So here we have four periods where we would consider there to be an influenza outbreak because the red lines have gone above the black lines. That is, the number of observed cases has gone above the seasonally adjusted average. The CDC also has an alternate definition of an outbreak based on the percentage of doctor's visits for influenza-like illness, sometimes called ILI. When this gets above a predefined threshold of around 2.5%, they declare an epidemic. To summarize defining an outbreak, an outbreak is in a period where are more cases than expected for a disease, also called an epidemic. If the disease is rare, even a few cases may be an outbreak. If the disease is common, we need a way to say what is normal and when we are seeing an important increase over that. In practice, multiple metrics, not just number of cases, are used.
