In a cholera outbreak investigation, the attack rate in a community is calculated. Among 400 people who attended a feast and drank contaminated water, 80 developed cholera; among 600 who did not drink the water, 6 developed cholera. The Relative Risk (RR) of cholera associated with drinking contaminated water is approximately:

• A 8.0
• B 13.3
• C 4.0
• D 20.0 ✓

Explanation

Attack rate in exposed (drank contaminated water) = 80/400 = 0.20 (20%). Attack rate in unexposed = 6/600 = 0.01 (1%). RR = 0.20/0.01 = 20.0. In outbreak analysis, the attack rate ratio (RR) quantifies how many times more likely exposed attendees are to develop the disease than unexposed individuals. An RR of 20 provides very strong epidemiological evidence implicating contaminated water as the vehicle of transmission. Case-control studies using odds ratios are an alternative when attack rates cannot be computed.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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