10 - Prevention

Question 1

Primary (1o) Prevention

Answer 1

the protection of health by personal and community-wide efforts with a focus on the whole population
• Objectives:
– To prevent new cases of disease occurring and therefore reduce the incidence of disease
• Where and How?:
– Population-level
– Individual level

Question 2

1o Prevention at Population-level

Answer 2

• By reducing exposure to causal (risk) factors
- e.g., reducing smoking initiation in teenagers
• By adding a factor that prevents disease
- e.g., vaccination, water fluoridation
• Usually requires policy and/or legislation
- Smoking = tobacco taxes, restrictions on smoking indoors
- Physical activity = structural changes to the environment– sidewalks, walking paths, bike lanes (Town planning)
• Primary prevention at the population-level works best when it is driven by changes in societal attitudes
- e.g., drinking and driving, bikes lanes

Question 3

1o Prevention at Individual-level

Answer 3

• By removing or lowering risk factors in at-risk patients
– Occurs at the patient-physician level
– Rationale behind the periodic health exam (PHE)
• e.g., smoking cessation counseling
• e.g., risk factor screening in at-risk patients (BP, BC, Physical inactivity, abdominal obesity)
• Distinction between primary prevention and secondary prevention hinges on the presence of existing disease
• Primary prevention = no existing disease

Question 4

Secondary (2o) Prevention

Answer 4

measures available for the early detection and prompt treatment of health problems
• Objectives:
– To reduce the consequences of disease (death or morbidity) by screening asymptomatic patients to identify disease in its early stages and intervening with a treatment which is more effective because it is being applied earlier.
– It cannot reduce disease incidence
• Where and how do we screen?:
– Population-level or mass screening
– Individual-level screening or case finding

Question 5

Screening – two different approaches

Answer 5

• population level screening

- individual level screening

Question 6

• Population-level screening

Answer 6

– National level policy decision to offer mass screening to a whole sub-group of a population
• e.g., mammography screening
• e.g., Vision and hearing screening in primary schools

Question 7

• Individual-level screening

Answer 7

– Occurs at the individual patient-physician level
– Also refereed to case finding
• e.g., BP screening every time you visit GP
– Focus is on identifying existing disease in patients who don’t know they have it.

Question 8

Tertiary (3o) Prevention

Answer 8

• measures available to reduce or eliminate long-term impairments and disabilities, minimize suffering, and promote adjustments to irremediable conditions
• Objectives:
– To reduce the consequences of disease (esp. suffering) by treating disease and/or its direct complications in symptomatic patients.
• A proactive approach to medical care– may involve rehabilitative and/or palliative care
• Examples
• education about disease management (asthma)
• regular foot exams in diabetics
• pain management in hospice patients

Question 9

Screening

Answer 9

• Objective: to reduce mortality and/or morbidity by early detection and treatment.
• Effective screening involves both diagnostic and treatment components

Question 10

Pre-clinical Phase (PCP)

Answer 10

period between when early detection by screening is possible and when the clinical diagnosis would normally have occurred.
• Prevalence of PCP indicates how much early disease there is to detect

Question 11

Lead Time

Answer 11

Lead time = amount of time by which diagnosis is advanced or made earlier

Question 12

Sensitivity (Se)

Answer 12

the proportion of cases with a positive screening test among all individuals with pre-clinical disease
– Want a highly Se test in order to identify as many cases as possible…

Question 13

Specificity (Sp)

Answer 13

the proportion of individuals with a negative screening test result among all individuals with no pre-clinical disease
Imperfect Sp affects many (the healthy), whereas an imperfect Se affects only a few (the sick)

Question 14

Assessing the feasibility of screening

Answer 14

• Burden of disease: Effectiveness of treatment without screening
• Acceptability: Convenience, comfort, safety, costs (= compliance)
• Efficacy of screening: Test characteristics (Se, Sp), Potential to reduce mortality
• Efficiency: Low PVP, Risks and costs of follow-up of test positives, Cost-effectiveness
• Balance of risks (harms) vs. benefits

Question 15

What makes an infection controllable?

Answer 15

• Relationship between symptoms and infectivity
• Healthcare can only respond to symptoms
– Quarantine, treatment, etc
Pathogens that are infectious following symptom onset are easiest to control

Question 16

Issues surrounding vaccination

Answer 16

1• Reasons for vaccinating
2• Efficacy / Effectiveness
3• Logistical constraints (limited resources)
4• Ethical considerations
5• Behavioural aspects

Question 17

1. Reasons for vaccinating

Answer 17

• Direct benefit
– protection individuals against infection
• Indirect benefit
– invoking herd immunity
– don’t need to vaccinate everyone
– models can help estimate how much immunity in population is needed

Question 18

2. Efficacy / Effectiveness

Answer 18

• Not all vaccines are 100% effective or immediately effective
• Can be estimated using RCT or observational studies (confounding bias may be an issue)

Question 19

3. Logistical constraints (limited resources)

Answer 19

• Henderson ascribes success of smallpox eradication campaign to several factors:
1. Coordinated response across many countries
2. Vaccine could be kept at 37°C for a month, didn’t need refrigeration
– difficult to do in developing countries
3. Cases were highly symptomatic (rash) and easily identified
4. People in affected areas didn’t travel far, so had high success in finding further cases around an index case

Question 20

4. Ethical considerations

Answer 20

• Limited vaccines mean limited vaccinations
• Who should get vaccinated?
– those at high risk of complications if infected (immuno-compromised for e.g.)?
– groups of the population responsible for transmission? (e.g. school children)
– other targeting? (e.g. large households)

Question 21

5. Behavioural aspects

Answer 21

• The intension of individuals to seek treatment or follow control advice may change during an epidemic

Question 22

Identifying optimal control options

Answer 22

– Minimize infection
– Minimize transmission
– Minimize death
– Minimize other impacts of disease
– Time to epidemic end
– Reduce epidemic peak
– Minimise social or economic impact
– Eradication