Incidence, Prevalence & Morbidity Measures
A high-yield STEP 1 lesson distinguishing incidence (new cases, risk, acute disease, causation) from prevalence (all existing cases, burden, chronic disease), anchored on P ≈ I × D and the classic board traps of prolonged-survival treatments raising prevalence while vaccines lower incidence first.
Two Different Questions
Incidence and prevalence answer different questions. Incidence measures how fast NEW disease appears — new cases arising in a previously disease-free population at risk over a defined period. It captures risk and rate of onset, so it is the measure for acute disease and for studying causation (cohort studies). Prevalence measures how much disease exists right now — all cases (new + pre-existing) present in the total population at a given time. It captures disease burden, making it the measure for chronic disease and health-resource planning.
Picture a bathtub: the faucet is incidence (new cases flowing in), the water level is prevalence (everyone currently living with disease), and the drain is exit from the pool by cure or death. Anything that opens the faucet, plugs the drain (longer survival), or is simply chronic raises the water level. At steady state with low prevalence: Prevalence ≈ Incidence × Duration.
- Incidence = new cases ÷ population at risk, over a period → measures risk/rate
- Prevalence = all existing cases ÷ total population, at a point → measures burden
- Incidence rate uses person-time in the denominator; cumulative incidence (risk) uses persons at risk at the start
- Prevalence ≈ Incidence × Duration (steady state, low prevalence) — memorize
- Chronic disease (long duration): prevalence >> incidence (diabetes, HTN, HIV on treatment)
- Acute / short-duration disease: prevalence ≈ incidence (common cold, influenza) — fast turnover keeps the pool small
- Point prevalence = a single moment; period prevalence ≈ point prevalence at start + new cases (incidence) during the interval
- Attack rate = cumulative incidence during an outbreak (food poisoning, epidemics)
- Use incidence for etiology/causation; use prevalence for healthcare planning
Incidence vs Prevalence
| Feature | Incidence | Prevalence |
|---|---|---|
| Cases counted | New only | All (new + existing) |
| Denominator | Population at risk | Total population |
| Time frame | Over a period | Point (or period) |
| What it measures | Risk, rate of onset | Burden of disease |
| Best for | Acute disease, causation | Chronic disease, planning |
| Classic study design | Cohort | Cross-sectional |
| Longer disease duration | No change | Increases |
| Better tx that doesn't cure | No change | Increases |
Prevalence pool = inflow (incidence, in-migration of cases) − outflow (death, cure, out-migration).
INCREASES prevalence:
- Longer disease duration
- Better treatment that prolongs life without curing (classic board trap)
- Higher incidence (more new cases)
- In-migration of cases / out-migration of healthy people
- Improved or earlier diagnosis/screening (detects previously undiagnosed/subclinical cases)
DECREASES prevalence:
- Cure or faster recovery (shorter duration)
- High case-fatality / rapid death (shorter duration)
- Effective vaccine or prevention (lowers incidence first)
- Out-migration of cases / in-migration of healthy people
Key trap: a therapy that prolongs survival but does not cure raises prevalence while leaving incidence unchanged. A vaccine lowers incidence first; prevalence falls only later.
A new combination therapy for a chronic viral infection markedly prolongs patient survival but does not eradicate the virus. Ten years after its introduction, community surveys show many more people living with the infection, though the rate of new infections is unchanged.
What happened to incidence and prevalence?
- Prevalence ↑ — patients survive longer, so disease duration rises and the prevalence pool enlarges (P ≈ I × D).
- Incidence unchanged — the drug treats existing disease; it does not prevent new infections.
Next-step reasoning: Whenever a treatment extends life without curing (e.g., HAART for HIV), expect rising prevalence with stable incidence. Do NOT conclude the epidemic is worsening — the extra prevalent cases reflect better survival, not more transmission.
At a wedding, 40 of 100 guests who ate the potato salad develop vomiting and diarrhea within 6 hours. Investigators want the measure that quantifies how many exposed people became ill during this outbreak.
Best measure — Attack rate = new cases ÷ population at risk during the outbreak = 40 / 100 = 40%.
- The attack rate is a form of cumulative incidence applied to an epidemic/outbreak over a limited window.
- Comparing attack rates across foods (potato salad vs cake) helps identify the source — the item with a high attack rate among eaters and low rate among non-eaters is the likely culprit.
This is incidence, not prevalence: it counts new cases arising in an at-risk group over a defined time.
- PID → P = I × D: Prevalence = Incidence × Duration. Know any two, solve the third.
- iNcidence = New (both have an 'N'): incidence always counts new cases.
- Prevalence = pre-existing + new = everyone with the disease right now.
- Chronic ⟶ high prevalence (long duration fills the pool); rapidly fatal ⟶ low prevalence (short duration drains it).
- Bathtub: faucet = incidence, water level = prevalence, drain = cure + death.
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