Emerging Infectious Diseases Flashcards
Epidemiology and control
What is epidemiology?
Study of distribution and determinants of health-related states or events in specified poupulations and the application of this study to the control of health problems
> Concerned with populations, not individuals
Define Risk Factors, Exposure, Outcome, Intermediate outcomes, Observational studies, Interventional studies
Risk Factors: any attribute, characteristic or exposure of an individual that increases the likelihood of developing a disease
Exposure: Any factor that may be associated with an outcome of interest
Outcome: Describes a disease, health event (infection/onset of new symptoms) or death
Intermediate outcomes: Outcomes that do not represent a definite endpoint
Observational studies: Investigator does not intervene with the population but only observes and assesses the strength of the relationship between an exposure and disease variable (can be descriptive –> frequency of disease in population) or analytical (investigate associations between risk factors and disease)
What are outcome measurements?
Use to determine health related outcome
Incidence: rate of new,or newly diagnosed, cases of the disease
- Number of new cases occuring within a period of time
- Quality of data depends upon accuracy of diagnosis and reporting
- Can be further categorised into different subsets of the population i.e. gender, racial origin, age group etc
Prevalence: actual number of cases in a population with the disease either during a period of time of particular date in time (point prevalence)
- Most meaning fully reported as the number of cases as a fraction of total populationat risk
What is the interaction between incidence to prevalence
Relationship between incidence and prevalence depends on the history of the disease state being reported
- If disease has high incidence but high rate of disease resolution = wont contribute to growth of prevalence –> Influenza
- If disease has low cure rate, but treatment prolongs suvival, the incidence contributes to continuous growth of prevalence –> HIV
- Prevalence will grow until mortality equals or exceeds the incidence rate
What is the interaction between incidence, prevalence, mortality and treatment in HIV infection
- Person is infected with HIV, prevalence and incidence grows exponentially
- As more people become infected in the population, proportion, proportion of ‘not HIV infected’ people decreases
- With fewer ‘Not-HIV infected people’ –> incidence of disease is reduced but the prevalence continues to rise
- Survival time of HIV is 6-8 years, AS MORTALITY INCREASES, PREVALENCES DECREASES
- However, if HIV treatment is introduced into a population, the patient survival is prolonged and the prevalence will continue to grow until the mortality rate equals the rate of incidence
There are three types of obervational studies
A) Cross-sectional study
B) Cohort study
C) Case-control study
Explain what they are and the pros and cons of each one
Cross-sectional studies
>Measures the frequency of an outcome and/or exposure(s) in a defined population at a particular point in time
>Descriptive (measure disease burden) or analytical (compare frequency of exposed vs unexposed to a risk factor)
Pros
- Useful to determine the scale of a problem
- Cheap and quick to run
Cons
- Difficult to differentiate between exposures causing disease and those improving survival
Case control studies
- Identify people with outcome (cases) and a representative group without the outcome (controls)
- Cases and controls then compare to differences in past exposure
- Always analytical studies
- Neither disease incidene, nor prevalence is measured
- Exposure is determined when the outcome has occurred
Pros
- less epensive or time consuming as cohort or interventions study
Cons
- When exposure is rare, case-control studies are impractical
Cohort study
- Follows group of people that initially dont have the outcome of interest and then determine whether they develop the diease (descriptive)
- classifies people at the start of the study as exposed or un-exposed to a certain risk factor (analaytical)
- Both groups followed over time and occurence of disease is compared between exposed and unexposed
Pros
- Provide robust evidene of casual associations
Cons
- Inefficient when investigating rare diseases
- Loss to follow-up
- Expensive in cost and time
There is one type of experimental/interventional study
A) Intervention trial (randomized control trial)
Explain what it is and the pros and cons
- Disease free and exposure-free people are allocated on exposure (intervention) or no exposure (no intervention)
- Groups are followed over a period of time and outcome frequency compared
- randomized control studies are sub type
Pros
- Provide strong evidence of casual associations
- Equal numbers in intervention and no intervention groups (random allocations)
Cons
- Loss to follow-up
- Expensive in cost and time
Define the following terms
For infection, define: latent period, infectious period, non-infectious period
For infectious disease, define: Incubation period, symptomatic period, non-disease period
Latent period: time between point of infection and when the person becomes infection
Infectious period: infected individual is able to transmit the infectious agent
Non-infectious period: infect person recovers or dies
Incubation period: time between the point of infection and when the person develops symptoms
Symptomatic period: infected individual shows signs of symptoms
Non-disease period: infected person recovers or dies
What is the basic reproduction rate (RO) ?
Number of new cases a single infected person can generate, over the course of infectious period of a disease
- Helps determine whether or not an infectious disease can spread through a population
RO=1 Cases one new case
RO >1 Infection will spread in a population
RO <1 Infection will be controlled and eradicated
- Larger RO = harder to control epidemic
Basic reproductive rate depends: duration of infection (d), number of contacts per time (c), and transmission of probability (p)
Ro = c * p * d
What is the net reproduction rate (R)
Average number of new cases (secondary ) in a population where not all individuals are susceptible
- Depends on the RO and the proportion of susceptible individuals (x)
- R = RO * x
- The lower the proportion of susceptible individuals, the lower the probability that an infectious person will be in contact with a susceptible person
- If x is low enough, R will be <1 and the disease will be eradicated
What is Herd Immunity (H) and Vaccination Efficacy(VE)?
*provide the formulas in your answer
herd immunity (H) is the proportion of the population who are immune to an infection
- HI threshold is proportion of the population that need to be immune for a disorder to die out (R<1 ):
- HI = RO - 1 / RO
- Vaccines make individuals less susceptible to infection andprotect unvaccinated people through increased herd immunity
vaccine efficacy measures the effect with evaluating vaccines and is most often determined in randomized indibidual trial
- VE = incidence of in unvaccinated - incidence of disease in vaccinated / incidence of disease in unvaccinated individuals
- VE = real world view of how well a vaccine performs in a population
