Week 9 - Public Health Flashcards
What is Attack Rate (AR)?
- type of incidence used in infectious disease outbreak investigation
- Used for diseases observed in a population for a short period of time
How to calculate?
- AR = ill / (ill + well) * 100
What is the different between Attack Rate (AR) and Cumulative Incidence?
Difference between AR and cumulative incidence:
- for cumulative incidence, we don’t include ill individuals in denominator because we say they aren’t at risk
What is an Epidemic curve?
- graph of cases according to time of onset of illness
- Shape can reveal type of outbreak
- (common source, point source, propagated outbreak)
What is a Common Source Outbreak?
- Common disease-causing agent
- (e.g. contaminated water supply)
- Individuals exposed either continuously or intermittently
What is a Point Source Outbreak?
- Group of individuals exposed almost simultaneously to disease source
- Circumscribed in place and time (e.g. food-borne outbreak)
- Period of exposure is brief and all cases occur within one incubation period
What is a Propagated Outbreak?
- Person-to-person spread (e.g. STI, measles)
- Last longer than common source outbreaks and have multiple waves
- Each wave taller than previous
- Waves are 1 incubation period apart
What are the 10 Steps in an outbreak investigation?
-
Establish the existence of the outbreak
- Need to define population at risk and compare current incidence to usual incidence
- Determine “excess” frequency of cases
- Epidemic threshold curve: epidemic occurs when frequency curve crosses threshold
-
Verify diagnosis
- Lab and clinical tests confirm diagnosis and identifies agent responsible
- Construct working case definition
- Find cases, count cases
- Descriptive epidemiology
- Develop a hypothesis about the exposure
- Evaluate hypothesis
- Implement control and prevention measures
- Communicate findings
- Maintain surveillance
What are the 2 types of study designs can be used in outbreak investigation?
Retrospective cohort:
- when population at risk is well defined
- E.g. when an outbreak follows a picnic, we know population at risk
- Attack rate
Case-control:
- not sure about population at risk
- odds ratio
Attack rate ratio is similar to which measure of association?
Relative risk
- use incidence to calculate
- Iexp / Inon-exp
- AR is similar to incidence
AR ratio is ARexp / ARnon-exp, very similar to RR
What are the 4 Steps in Investigating Clusters?
- Initial ascertainment of cluster
- Assessment of excess occurrence
- Determination of feasibility of etiologic study
* (study that tries to identify cause) - Conduct etiologic investigation
Note:
At end of each step, can decide to end investigation or take action and proceed to next step
What are the 2 types of Public Health Surveillance?
Public Health Surveillance
- Ongoing, systematic collection, analysis, interpretation, and dissemination of data regarding a health-related event
- Used to reduce morbidity and mortality and improve health
2 Types:
- Passive: health data shared by healthcare providers with public health agencies (passive for health department)
- Active: public health agencies routinely contact data sources to acquire reports
Why do we need public health surveillance?
We need PH surveillance because:
- Assessment of status of health in population (magnitude of problem)
- Establish baseline of a condition
- Understand trends and patterns of disease
- Information to design and plan PH programs
- Set research priorities
What are the 3 levels of prevention?
-
primary
- maintenance of health so that disease process never starts
- (before pathological onset)
-
secondary
- goal is to reduce expression and severity of clinical disease through identification of asymptomatic individuals
- (pre-clinical period, between pathological onset and onset of symptoms)
-
tertiary
- goal is to block or slow progression of disease and reduce impairments or disabilities, thereby improving quality of life or survival
- (clinical phase, after diagnosis)
Characters of Infectious Disease Agents
Infectivity:
- characteristics of the infectious agent that embodies capability to enter, survive, and multiply in the host
- How “good” agent is at establishing itself inside acceptable host
- Measure by calculating infective dose:
- theoretical number of organisms required to establish an infection
-
ID50:
- minimum number of agents required to cause infection in 50% of hosts
Pathogenicity Vs. Virulence
Pathogenicity:
- ability of agent to cause disease (symptomatic disease) in infected host
- Proportion of individuals with symptomatic disease
Virulence:
- severity of the disease, measured by the number of severe or fatal cases
- Case fatality rate often use to measure virulence
Types of Defense Mechanisms
Nonspecific:
- includes skin, gastric juices, mucosal surfaces, non-specific immune system
Specific:
- immunity against a specific agent, particularly Ab production
Active: in response to IDA, body produces Ab
- Natural: getting infection and allowing immune system to produce Ab
- Artificial: vaccination (vaccine-induced immunity)
Passive: introduce pre-formed Ab into body
- Natural: mother to fetus transmission (transplacental)
- Artifical: injected pre-formed immunoglobulin (e.g. tetanus, rabies)
Routes of Tranmission
-
Vertical
- mother to child
-
Horizontal
- transmitted among individuals of same generation
-
Direct
- spread through person-to-person contact (skin, body fluids/secretions) or droplet tranmission (relatively close proximity required)
- Ex: HIV, HepB, Ebola (person to person)
- Ex; flu, pertussis, smallpox (droplet)
- spread through person-to-person contact (skin, body fluids/secretions) or droplet tranmission (relatively close proximity required)
-
Indirect
- spread through an intermediary source (can be vehicle or vector)
- Vehicle-borne: water-borne (cholera), food-borne (salmonella), air-borne (measles, chickepox), blood-borne (Hep B, HIV, Ebola), formite-borne (rhinovirus)
- Vector-borne: living insects/animals involved with transmision of the disease (e.g. malaria, WNV)
- spread through an intermediary source (can be vehicle or vector)
Important Time Periods in Tranmission of Infections
Incubation period:
- period of time between onset of infection and appearance of symptoms
- May or may not be infectious during this time
Infectious period:
- period of time during which an individual is infected & infectious
Latent period:
- period of time during which individual is infected but not infectious
- precedes the infectious period
Generation time:
- time interval between onset of infection and end of infectious period
What does it mean when Latent period > Incubation period?
-
Symptoms appear before an individual becomes infectious
- Easier for transmission control (can quarantine)
- Harder for secondary prevention
- (less time, because incubation period is short)
What does it mean when Latent period < Incubation period?
- Person is infectious before symptoms appear
- Challenging for infection control (transmitting but aren’t aware you have infection)
- If it can be detected serologically, more time for secondary prevention (since incubation period is long)
Herd Immunity and R0
Critical proportion:
- proportion of the population that needs to be immune in order to achieve herd immunity
- p* = 1 – (1/R0)
- Relationship between p* and R0 is a direct relationship (subtracting it from 1 makes it direct, not inverse)
Efficacy of a vaccine/Coverage:
- p* = coverage * efficacy of vaccine
- If we know efficacy of vaccine and critical proportion of population that needs to be immunized, we can calculate coverage
What is the SEIR model?
- susceptible
- exposed
- infected
- recovered
We model in order to predict spread of infection in a susceptible population
