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

<p> YELLOW FEVER:- Zoonoses disease caused by arbovirus. Affects principally monkeys and other veebrates and is transmitted to humans by culicine mosquitoes. Reservoir- mokeys and forest mosquitoes., in urban areas reservoir is man besides Aedes aegypti. A temperature of 24 deg celcius or more required for its multiplication. Period of communicability- Blood of patients infective upto first 3-4 days Aedes aegypti index:- This is a house index. Defined as the percentage of houses and their premises, in a limited well defined area, showing actual breeding of Aedes aegypti larvae. This index should be kept below 1. {Reference: park&;s textbook of preventive and social medicine, 23rd edition, pg no.282}</p>