Question

An influenza vaccine shows 90% efficacy in a randomized controlled trial but only 85% performance when used in the routine community programme. Which value represents the effectiveness of the vaccine?

• A. 85% (performance in real-world field conditions)
• B. 90% (performance in the ideal trial setting)
• C. 5% (difference between the two values)
• D. 87.5% (average of the two values)

Hint:

Effectiveness = real-world/field; efficacy = ideal trial.

Answer 1

The Correct Option is A

Step 1: Distinguish efficacy from effectiveness. Efficacy is how well a vaccine works under ideal, controlled conditions such as a well-run randomized controlled trial. Effectiveness is how well it works under real-world, routine field conditions in the actual population programme.

Step 2: Map the numbers. The 90% comes from the controlled clinical trial → that is the efficacy. The 85% comes from routine community use → that is the effectiveness.

Step 3: Why effectiveness is usually lower. Real-world use introduces cold-chain breaks, missed/late doses, a broader and less healthy population, and variable circulating strains — all of which reduce the observed benefit below the trial efficacy.

Step 4: Why the others are wrong. 90% is the efficacy (trial), not effectiveness. 5% is merely the arithmetic gap between the two. 87.5% is a meaningless average; effectiveness is a directly measured field value, not an average of efficacy and itself.

Key fact: Effectiveness = real-world field performance = 85% (efficacy = 90% is the trial value).