CM8.4-5 | CM8.4-5 | Epidemic Control Planning — Summary & Reflection

KEY TAKEAWAYS

Epidemics occur when disease cases clearly exceed expected levels — through common-source (sharp single-peak epidemic curve) or propagated (multi-modal curve) mechanisms. The R0 determines epidemic growth potential and defines the herd immunity threshold (HIT = 1 − 1/R0); for measles R0 ≈ 15, HIT ≈ 93%. Key epidemiological measures for quantifying outbreaks include: attack rate (cases/exposed × 100), secondary attack rate (new household cases/susceptible contacts × 100), and case fatality rate (deaths/cases × 100). The 10-step outbreak investigation (verify diagnosis → confirm outbreak → case definition → case finding → descriptive epidemiology → hypotheses → test hypotheses → control measures → evaluate → communicate) provides the systematic framework. Control targets all three chain-of-infection components: source elimination, host protection (vaccination, chemoprophylaxis), and transmission interruption. The Epidemic Diseases Act 1897 (amended 2020) provides the legal authority for epidemic control measures. The PHC physician leads the first 24–48 hours of response: notify IDSP, collect specimens, create a line list, implement provisional control, and begin active case finding — without waiting for laboratory certainty before acting.

REFLECT

Return to the opening scenario: eleven gastrointestinal cases from a hamlet after a shared school meal. Using the 10-step framework: (1) What case definition would you write? (2) What food-specific attack rate calculation would identify the most likely vehicle? (3) What control measures would you implement on Day 1 before culture results return? (4) If new cases continue to appear a week after the suspected food source was removed, what hypothesis would you now form, and what new investigation would you design? How would a rising case fatality rate change your response?