7.3 Epidemic curves and outbreak investigation

Epidemic curves and outbreak investigations are crucial tools in infectious disease epidemiology. They help track the spread of diseases and guide public health responses. By analyzing these curves, experts can identify patterns, predict future cases, and implement targeted interventions to control outbreaks.

Outbreak investigations follow a systematic approach, from confirming an outbreak's existence to implementing control measures. This process involves descriptive and analytical epidemiology, hypothesis testing, and effective communication with stakeholders. Understanding these steps is essential for managing public health crises.

Epidemic Curve Analysis

Interpreting Epidemic Curves

Top images from around the web for Interpreting Epidemic Curves

• 

  Tim Churches Health Data Science Blog: Analysing COVID-19 (2019-nCoV) outbreak data with R - part 2 View original

  Is this image relevant?

• 

  Tim Churches Health Data Science Blog: Analysing COVID-19 (2019-nCoV) outbreak data with R - part 2 View original

  Is this image relevant?

• 

  Frontiers | Real-Time Standard Analysis of Disease Investigation (SADI)—A Toolbox Approach to ... View original

  Is this image relevant?

• 

  Tim Churches Health Data Science Blog: Analysing COVID-19 (2019-nCoV) outbreak data with R - part 2 View original

  Is this image relevant?

• 

  Tim Churches Health Data Science Blog: Analysing COVID-19 (2019-nCoV) outbreak data with R - part 2 View original

  Is this image relevant?

1 of 3

Top images from around the web for Interpreting Epidemic Curves

• 

  Tim Churches Health Data Science Blog: Analysing COVID-19 (2019-nCoV) outbreak data with R - part 2 View original

  Is this image relevant?

• 

  Tim Churches Health Data Science Blog: Analysing COVID-19 (2019-nCoV) outbreak data with R - part 2 View original

  Is this image relevant?

• 

  Frontiers | Real-Time Standard Analysis of Disease Investigation (SADI)—A Toolbox Approach to ... View original

  Is this image relevant?

• 

  Tim Churches Health Data Science Blog: Analysing COVID-19 (2019-nCoV) outbreak data with R - part 2 View original

  Is this image relevant?

• 

  Tim Churches Health Data Science Blog: Analysing COVID-19 (2019-nCoV) outbreak data with R - part 2 View original

  Is this image relevant?

1 of 3

• Epidemic curves are graphical displays of the number of incident cases of a disease or health condition plotted over time, typically during an outbreak or epidemic
• The shape and characteristics of an epidemic curve provide insights into the pattern, mode of transmission, and timeline of a disease outbreak
  • A steep upslope in cases suggests a rapid increase in disease incidence and a highly contagious or virulent pathogen (influenza)
  • A more gradual upslope indicates a slower spread or less contagious pathogen (tuberculosis)
  • A plateau or peak in the curve represents the point at which the outbreak reaches its maximum number of cases before declining
• The time scale used on the x-axis of an epidemic curve can vary depending on the duration of the outbreak and the incubation period of the disease, ranging from days to weeks or months
• Interpreting epidemic curves requires consideration of the population at risk, the incubation period of the disease, and potential reporting delays or biases in case detection

Common Patterns of Epidemic Curves

• Common patterns of epidemic curves include point source, continuous common source, and propagated outbreaks
  • Point source outbreaks have a single exposure event and typically show a sharp increase in cases followed by a rapid decline (foodborne illness from a contaminated meal)
  • Continuous common source outbreaks have a persistent exposure over time and show a more gradual increase and decrease in cases (waterborne illness from a contaminated well)
  • Propagated outbreaks involve person-to-person transmission and may show multiple waves or peaks of cases over time (measles in an unvaccinated population)

Outbreak Investigation Steps

Establishing the Existence of an Outbreak

• Outbreak investigations aim to identify the source, mode of transmission, and extent of a disease outbreak in order to implement effective control measures
• The first step in an outbreak investigation is to establish the existence of an outbreak by comparing the observed number of cases to the expected baseline incidence of the disease
• A case definition is developed to standardize the criteria for identifying and counting cases, including clinical symptoms, laboratory confirmation, and epidemiological factors

Conducting Descriptive and Analytical Epidemiology

• Descriptive epidemiology is conducted to characterize the outbreak in terms of person, place, and time, using tools such as epidemic curves, attack rates, and maps
• Hypothesis generation involves identifying potential risk factors, exposures, or sources of the outbreak based on the descriptive epidemiology and available evidence
• Analytical studies, such as case-control or cohort studies, are conducted to test the hypotheses and identify significant associations between exposures and disease outcomes
• Environmental investigations, such as inspections, sampling, and testing, may be conducted to identify potential sources or reservoirs of the pathogen

Implementing Control Measures and Communication

• Control measures are implemented based on the findings of the investigation to prevent further spread of the disease, such as isolation, quarantine, or recall of contaminated products (closing a restaurant, vaccinating exposed individuals)
• Communication and coordination with public health authorities, healthcare providers, and the public are essential throughout the investigation to ensure timely and effective response
  • Regular updates and guidance should be provided to stakeholders and the media
  • Risk communication strategies should be employed to address public concerns and promote adherence to control measures

Epidemiological Study Types

Observational Studies

• Descriptive studies, such as case series or cross-sectional surveys, are often used initially to characterize the outbreak and generate hypotheses about potential risk factors or exposures
  • Case series describe the clinical and epidemiological characteristics of a group of cases (symptoms, demographics, exposures)
  • Cross-sectional surveys assess the prevalence of disease and exposures in a population at a single point in time
• Case-control studies compare the exposure history of cases (individuals with the disease) to controls (individuals without the disease) to identify significant associations and estimate the strength of the association using odds ratios
• Cohort studies follow a group of individuals over time to compare the incidence of disease among those exposed to a risk factor versus those unexposed, allowing for the calculation of relative risks and attributable risks
• Ecological studies compare disease rates or exposures at the population level, such as comparing outbreak rates across different geographic areas or time periods

Laboratory and Environmental Studies

• Molecular epidemiology studies use laboratory techniques, such as DNA fingerprinting or whole genome sequencing, to identify and compare the genetic characteristics of pathogen isolates from cases and potential sources
  • Pulsed-field gel electrophoresis (PFGE) is a common method for bacterial subtyping
  • Whole genome sequencing provides high-resolution data for outbreak strain identification and transmission tracking
• Environmental studies, such as traceback investigations or environmental sampling, aim to identify the source or reservoir of the pathogen in the environment or food supply chain
  • Food safety inspections and testing of suspected food items
  • Environmental sampling of water sources, animal reservoirs, or contaminated surfaces

Case Definitions and Contact Tracing

Developing and Applying Case Definitions

• Case definitions establish standardized criteria for identifying and counting cases of the disease under investigation, ensuring consistency and comparability of data across different settings or jurisdictions
  • Clinical case definitions specify the signs, symptoms, and diagnostic criteria required for a case to be considered confirmed, probable, or suspected (fever, rash, and positive laboratory test for measles)
  • Epidemiological case definitions may include additional criteria, such as exposure history, risk factors, or demographic characteristics (travel to an endemic area, occupation)
• Clear and specific case definitions are essential for accurately measuring the extent and severity of an outbreak, as well as for guiding case finding and public health interventions
  • Consistent application of case definitions allows for reliable tracking of the outbreak over time and across jurisdictions
  • Case definitions may be refined as more information becomes available during the investigation

Conducting Contact Tracing

• Contact tracing involves identifying, assessing, and managing individuals who have been exposed to a confirmed case of the disease, in order to prevent further transmission
  • Close contacts are typically defined based on the mode of transmission and the duration and proximity of exposure to the case (household members, sexual partners, healthcare workers)
  • Contacts are monitored for the development of symptoms and may be subject to quarantine, testing, or prophylactic treatment depending on the disease and the level of risk
• Contact tracing is a key strategy for interrupting chains of transmission, particularly for diseases with person-to-person spread or long incubation periods (COVID-19, tuberculosis, sexually transmitted infections)
• Effective contact tracing requires rapid case identification, thorough case interviews, and efficient communication and coordination with public health authorities and healthcare providers
  • Timely notification and follow-up of contacts is critical for preventing secondary cases
  • Digital tools and mobile applications can facilitate contact tracing efforts, particularly in large-scale outbreaks
• Contact tracing data can also provide valuable information about the transmission dynamics and risk factors associated with the outbreak, informing targeted control measures and future prevention strategies