Epidemiology

The study of the spread and control of infectious disease

1. Infectious disease occurrence in populations
   1. Epidemic: Outbreak of a disease affecting many people in a short time period (cholera)
      • Index case
      • Outbreak
      1. Common source epidemic (single source outbreak) (Leptospirosis in triathletes)
      2. Propagated epidemic: 2014 Ebola outbreak
   2. Endemic: Disease which is present at a steady level in a population (gonorrhea)
   3. Pandemic: Disease which has spread globally (AIDS)
   4. Sporadic disease: occasional outbreaks in a population (tetanus)
2. The spread of a disease depends on many variables
   1. Pathogen virulence
   2. Mode of transmission
   3. Host population resistance
   4. Incubation period of the disease
3. The tools of an epidemiologist
   1. Surveys, public records, census data
   2. Field studies: questionnaires, interviews, lab tests
   3. Statistical analyses
      • Incidence rate (Morbidity)
        • Which of the following diseases had the highest incidence in the US for 2010: HIV, malaria, syphilis, Lyme Disease, STEC E. coli?
      • Prevalence rate 
      • Mortality rate
      • Demographics
        • Gender
        • Race
        • Age
   4. Modeling studies (diagram >>>)
      • # Infected over a period of time (dI/dt)
      • # susceptibles: unvaccinated
      • Removals: immunity, vaccination, death
4. Studying an epidemic
   1. Descriptive epidemiology
   2. Field epidemiology
   3. Surveillance epidemiology
   4. Hospital epidemiology: nosocomial infections (72,000 deaths in 2015)
      • Immunocompromised patients
      • Invasive procedures (e.g. SSI)
      • Antibiotic resistance

Disease Detectives & The Infectious Disease Cycle

Epidemiology of an infectious disease

1. Identify the Pathogen (e.g. 1993 Hantavirus outbreak)
   1. Signs & Symptoms
   2. Role of Clinical Laboratory
   3. Diagnosis

   This is not always immediately possible (AIDS)

2. Locate the Source (e.g. 1854 cholera outbreak)
   1. Reservoir
   2. Zoonosis
   3. Carrier
      • Healthy
      • Incubatory
      • Active
      • Convalescent

   Controlling the source of an epidemic can stop the spread of an epidemic (e.g. cholera )

3. Establish Method of Transmission (2013 H7N9 avian influenza outbreak)
   1. Airborne
   2. Contact
   3. Vector
   4. Vehicle (fomite)

   Control of disease transmission may work when other methods won't (Malaria)

4. Determine Role of Host Susceptibility (e.g. AIDS epidemic)
   1. Virulence of pathogen
   2. Host immune system

   Vaccination can limit the spread of disease completely (smallpox)

5. Find Exit Route
   1. Active escape
   2. Passive escape

Disease Prevention

1. Public Health Education
2. Epidemiology 
3. Managing outbreaks
   • Quarantine: Ebola
4. Identifying risk factors
5. Eliminating reservoirs
6. Reducing transmission
7. Vaccination:
   1. Active immunization
      • Attenuated vaccines
      • Inactivated vaccines
      • Toxoids and subunit vaccines
      • DNA/RNA-based vaccines
   2. Passive immunization
      • post-exposure prophylaxis (rabies)
      • symptomatic treatment
      • protection of immunodeficient individuals
      • anti-toxins (botulism, tetanus)
   3. Epidemiological case studies
      • The 2015 Measles outbreak
      • Polio in 2014
   4. Effectiveness of vaccination
      • protection of the individual: efficacy
      • protection of the community: herd immunity
        • Zika virus: 63%?
        • Measles: 95%
        • Polio: 80%
        • COVID-19: ?
      • depends on R₀ (R-naught)
        • the number of secondary infections resulting from one infected individual in a susceptible population
        • a measure of contagiousness
        • e.g. Measles = 12-18
      • When portions of a population are no longer susceptible, we use the effective reproductive rate R
        • R = s*R₀
        • s = proportion of population that is susceptible
        • If R < 1, the epidemic will end

Links/References

1. https://plus.maths.org/content/maths-minute-r0-and-herd-immunity