To prevent Yellow Fever Aedes aegypti index should be less than?

Aedes aegypti index is a house index and defined as the percentage of houses and their premises,in a limited well defined area, showing actual breeding of Aedes aegylti larvae. This index should not be more than 1% in towns and seapos in endemic areas to ensure freedom from yellow fever. Reference; Park&;s Textbook of preventive and social medicine, 24th edition.Pg no. 301