33 Epidemiology and control of infectious diseases Flashcards
Epidemiology is study of population. Important to look at who, when, where when investigating an outbreak
Define prevalence
Define incidence
Prevalence is number of existing cases in population at specific given time. Influenced by incidence, and duration of diseases
Incidence is number of new cases occurring during a specific time period
What types of studies are used in scientific investigation
Cross-sectional study
Case-control
Cohort
Intervention
What are cross-sectional studies, and what are they useful for?
Observational study. Measure prevalence of a disease at specific point in time.
e.g what proportion of population has past infection lyme disease
Cheaper, and easy to do.
Sometimes difficult to tell what relationship is between outcome and exposure.
What are case-control studies, and what are they useful for?
Identify people with outcome (cases) and group without outcome (controls).
both of these groups then comapred with regards to previous history of exposure e.g smoking
good for investigating rare diseases
What are cohort studies, and what are they useful for?
Follow group of people who do not initially have the outcome of interest, and determine whether they develop disease after receiving different exposures
Measures incidence
Good at investigating rare exposures e.g radiation exposure causing disease
What are intervention studies, and what are they useful for?
Two groups allocated to either exposure or no exposure. Followed over period of time to see who develops disease.
Gold standard and provides strong evidence
Randomisation
Placebo-controlled
Double-blind
Expensive and time consuming
Infectiousness of an agent depends on many factors.
What are they?
Time between acquiring infection, and becoming infectious
Duration of infectiousness
Probability of transmission
Contact between susceptible people
Susceptibility of population
What is meant by “latent period of infection”
What is meant by “average generation time of infection”
Period between infection occurring, and becoming infectious
e.g measles is latent for 6-9 days, followed by infectious period of 6-7 days
Measles symptoms occur after becoming infectious, so isolating after this is futile.
Sum of latent period of infection, plus infectious period
These both reference the infective agent, and not the patient
What is meant by incubation period?
Time from infection, to symptoms developing
Incubation period and latent period can both be different.
What is meant by R0?
What if R0 is >1 or <1
Basic reproductive rate is average number of infected secondary cases produced by each infectious cases in susceptible population. Termed basic, as assumes fully susceptible population.
R0>1 - each infection causes more than 1 further infection
R0<1 - each case leads to less cases, so disease will run its course
what is R0 dependent on?
- Duration of infectiousness
- Number of contacts per unit time
- Transmission probability
This gives basic reproductive rate, which assumes fully susceptible population.
Net reproductive rate is depending on R0 multiplied by proportion of susceptible individuals.
How does R0 relate to herd immunity?
Proportion of population immune to infection is called herd immunity. This is threshold in population that is needed to be immune in order for disease to die out R0<1
Allows us to estimate how much vaccine coverage required to control an infecitous disease.
Measles has high R0 (15-17), so need >92% for coverage
Rubella has lower R0 (7-8), so need only 85% coverage
How do we measure vaccine efficacy?
compare incidence of disease in unvaccinated and vaccinated individuals
Notifiable diseases should be reported to Health Protection Team. What role do they have?
Identify source Offer chemoprophylaxis Vaccination Education Closing down premises
What is schedule 1 and schedule 2 of Health Protection list of notifiable diseases?
Schedule 1 - notifiable diseases
Schedule 2 - notifiable organisms
