Which calculation yields the attack rate among neonates in the NICU outbreak?

• A. 18/22 × 100
• B. 11/20 × 100
• C. 6/15 × 100
• D. 18/20 × 100

Answer: (A)

Attack rate measures how many people in a defined at‑risk population develop the disease during a specific outbreak period, expressed as a percent. It uses the number of new cases divided by the total population at risk at the start, multiplied by 100.

In this NICU outbreak, 18 neonates became ill out of 22 at risk, so the attack rate is 18 divided by 22, times 100, which is about 81.8% (roughly 82%). This reflects the proportion of the neonate population that became ill during the outbreak.

Using a denominator like 20 or 15 would imply a different at-risk population than the one described, leading to 55%, 40%, or 90%, which don’t match the actual outbreak scenario in the NICU.