According to international Health Regulation, there is no risk of spread of yellow fever if the Aedes aegypti index remains below –

**Core Concept**
The Aedes aegypti index is a measure of the density of the Aedes aegypti mosquito population in a given area. According to international health regulations, the Aedes aegypti index is used to assess the risk of yellow fever transmission. A low index value indicates a lower risk of yellow fever spread.

**Why the Correct Answer is Right**
The World Health Organization (WHO) and the International Health Regulation (IHR) define the threshold for risk of yellow fever transmission based on the Aedes aegypti index. The index is calculated as the number of Aedes aegypti mosquitoes per 100 houses or per 100 people. A value below 1 indicates a low risk of yellow fever transmission, while a value above 1 indicates a higher risk. This is because Aedes aegypti is the primary vector for yellow fever virus transmission.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not provide a specific threshold for the Aedes aegypti index.
**Option B:** This option is incorrect because it is a different threshold for the Aedes aegypti index, not the correct one according to IHR.
**Option D:** This option is incorrect because it is a higher threshold than the correct answer.

**Clinical Pearl / High-Yield Fact**
The Aedes aegypti index is a critical parameter for assessing the risk of yellow fever transmission. Healthcare workers and travelers to areas with high Aedes aegypti indices should take necessary precautions to prevent mosquito bites and vaccination against yellow fever.

**Correct Answer: C. 1**