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CM8.4-5 | CM8.4-5 | Epidemic Control Planning — SDL Guide (Part 2)
Evaluating Epidemic Control: Rates, Curves, and Effectiveness
Quantitative epidemiological measures allow the PHC physician to objectively describe the severity of an outbreak, identify high-risk groups, and evaluate whether control measures are working.
Attack rate (AR):
AR = (Number of cases / Total number exposed or at risk) × 100
The AR tells you what proportion of the exposed population developed disease. A very high AR (e.g. 70% after eating contaminated food) suggests a highly virulent or large-dose pathogen, or highly efficient exposure. Comparing AR by food item (food-specific AR) identifies the likely vehicle in food-borne outbreaks: the food item with the highest AR and the widest difference between those who ate vs did not eat it is the most likely vehicle.
Secondary attack rate (SAR):
SAR = (New cases among close contacts / Total susceptible close contacts) × 100
The SAR measures household transmissibility — how efficiently the pathogen spreads from an index case to susceptible household members within the incubation period. High SAR (>50%) indicates efficient person-to-person transmission; low SAR (<10%) suggests limited secondary spread. SAR is used to: assess transmission potential, guide quarantine decisions, and evaluate whether household-level control measures (mask use, separate sleeping, hand hygiene) are reducing spread.
Case fatality rate (CFR):
CFR = (Deaths from disease / Total confirmed cases) × 100
The CFR measures case lethality — the risk of death given that disease has occurred. CFR varies by pathogen (cholera CFR with treatment <1%; untreated >50%; Ebola CFR ≈25–50%), by age group (influenza CFR much higher in elderly), and by quality of care received. A rising CFR during an outbreak may indicate that severe cases are now predominating, that treatment capacity is overwhelmed, or that a more virulent variant has emerged.
Using the epidemic curve to evaluate control:
The most powerful visual tool for evaluating whether control measures are working is tracking the epidemic curve over time. After a specific control measure is applied (e.g. the contaminated water source is closed, or mass vaccination begins), expect the following:
- For common-source outbreaks: an immediate drop in new cases after the source is removed (no new cases within one incubation period confirms the source was eliminated)
- For propagated outbreaks: a gradual decline in Re below 1.0, visible as a flattening and then declining epidemic curve; may take multiple incubation periods to see the effect of vaccination or social distancing
A rising CFR or rising case count despite control measures is a warning signal — it may indicate that the hypothesised source was wrong, that the case definition is missing atypical cases, or that there is a second transmission route that has not been controlled.
CLINICAL PEARL
Start control before the investigation is complete. The single most common error in outbreak response is waiting for laboratory confirmation before implementing control measures — a decision that allows hundreds of additional exposures. The principle is: implement the most plausible control measure as soon as the preponderance of epidemiological evidence points to a specific source or transmission route. Close the suspected water source now, isolate cases now, and verify the hypothesis simultaneously. If you later discover the hypothesis was wrong, you can reopen the water source — but you cannot un-expose the 200 people who drank contaminated water while you waited for culture results. Public health action operates on probability, not certainty.
The PHC Physician's Role in Epidemic Response
When an epidemic occurs in a PHC's catchment area, the medical officer has both the legal authority and the operational responsibility to lead the initial response. The District Rapid Response Team (RRT) will eventually arrive, but the first 24–48 hours are often managed by the PHC alone.
Immediate (Day 0–24 hours):
- Verify the diagnosis: clinically assess cases, collect appropriate specimens (stool, blood, swabs) before antimicrobial treatment where possible; send to district laboratory with IDSP L-form
- Create a working case definition and line list (at minimum: name, age, sex, onset date, symptoms, village, suspected exposure)
- Notify: file IDSP P-form immediately; call District Surveillance Officer; notify Block Medical Officer of Health (BMOH)
- Initiate provisional control measures based on most likely transmission route: water safety (chlorination, boiling advisory) for suspected faeco-oral; isolation and contact tracing for suspected person-to-person spread; vector control for suspected vector-borne
Short-term (Days 1–7):
- Lead or participate in active case finding — village-level search for additional cases using the case definition
- Conduct a rapid food history or water exposure assessment (food-specific attack rate calculation for food-borne)
- Coordinate with ASHA workers for household contact tracing and surveillance
- Implement specific control measures per disease (mass ORS distribution for diarrhoeal outbreaks, emergency measles vaccination for measles, vector source reduction for dengue)
- Daily situation reports to DSO including: total cases, new cases in last 24 hours, deaths, hospitalised, control measures implemented
Communication:
- Hold a brief community meeting to: dispel rumours, communicate the known facts (cause, symptoms, treatment, prevention), and mobilise community cooperation (e.g. boiling water, reporting new cases)
- Coordinate with local media only through the District Health Officer; do not make statements about disease aetiology before laboratory confirmation
Post-outbreak:
- Complete and submit the outbreak investigation report (standard format: background, case definition, line list, epidemic curve, attack rate analysis, hypothesised cause, control measures, lessons learned)
- Identify systemic lapses that enabled the outbreak (surveillance gap, water supply failure, vaccination gap) and recommend structural corrections