Lecture 27 - Epidemiology and Surveillance of Viral Infections Flashcards
Definition of sporadic outbreaks
Occasional cases of a viral infection
Definition of endemic
Present in the community at all times, but at a relatively low frequency
Definition of an epidemic
A sudden, severe outbreak in a region or group
Stage 1 crossover event
Only transmitted between animals
Stage 2 crossover event
Only transferred from infected animal to a human (EG: rabies)
Stage 3 crossover event
Limited outbreak. From animal reservoir to humans, with some low-level human-human transmission (EG: ebola)
Stage 4 crossover event
Long outbreak. From animals to humans, with many cycles of human-human infection (EG: Dengue)
Stage 5 crossover event
Only human-human transmission
Example of an endemic virus
HSV
Difference in epidemiology of diseases that affect different age groups
Diseases that give lifelong immunity often are diseases of the young
Diseases that only give partial immunity often have a more even age distribution
Factors leading to a pandemic
1)
2)
3)
1) Novelty
2) Susceptibility (if healthy adults are over-represented among the sick or dead)
3) Transmissibility
Factors involved in epidemiological modelling
1)
2)
3)
1) Explicitly considers the causal processes involved in infection and transmission
2) Recognises interdependence of factors
3) Means of synthesising data from basic science and population studies
SIR paradigm
Susceptible->infectious->recovered
Infectious infects susceptible based on ‘beta’ coefficient (measure of innate infectiousness of virus)
Example of a population at particular risk of viral infection
Indigenous Australians have a restricted T cell repertoire against influenza
Host determinants of susceptibility 1) 2) 3) 4) 5)
1) Age
2) Underlying conditions
3) Immune status
4) Pregnancy
5) Race
Virus determinants of natural history
1)
2)
3)
1) Latency
2) Infectiousness (pre/asymptomatic, duration of infectiousness, mode of transmission)
3) Induction of immune response (temporary, strain specific or not)
Population determinants of spread 1) 2) 3) 4) 5)
1) Birth rate
2) Household size
3) Social and employment networks
4) Population density and connectedness
5) Population mobility
Environmental determinants of spread 1) 2) 3) 4) 5)
1) Seasonality
2) Sanitation
3) Proximity to vector populations
4) Proximity to reservoir animal populations
5) Natural disasters
New method of disease surveillance
Symptom surveillance
Symptom surveillance
Identify specific sets of symptoms, rather than particular diseases.
Reliant on clinicians reporting out-of-the-ordinary symptoms
Advantages of symptom surveillance
1)
2)
3)
1) Not restricted by individual disease diagnosis
2) Year-on-year disease burden measured, out of the ordinary disease activity is detected.
3) Uses threshold detection algorithms to detect such spikes in incidence.
Method of disease surveillance that can detect symptomatic and asymptomatic infection
Serosurveillance
Social impact assessment example
See if number of days of work missed increases over a certain period, and if this correlates with what might be an epidemic
Type of study that can identify less-serious infections
Household studies. Hospitalisations only represent the most-severe cases
Intervention endgames
1)
2)
3)
1) Containment
2) Transmission reduction
3) Mitigation
Importance of infection mitigation
Reduce burden on public health system
Why is it hard to identify initial infection source of ebola?
Incubation period is 2-21 days.
Ebola patient zero
Thought to be a 2 year old boy, infected mother, sister, grandmother.
When is containment of a virus feasible?
When virus has low transmissibility, high visibility.
When labour is available (very labour-intensive)
Effect of transmission reduction
1)
2)
3)
1) Slows down epidemic
2) Can buy time for effective intervention
3) Reduces peak burden on healthcare system, but can prolong time of burden
