Polio will never be eradicated in Pakistan until a way is found to persuade poor Pashtuns to embrace the vaccine, according to a study released by the World Health Organization.

A survey of 1,017 parents of young children found that 41 percent had never heard of polio and 11 percent refused to vaccinate their children against it. The survey was done in Karachi, Pakistan’s largest city and the only big city in the world where polio persists; it was published in the agency’s November bulletin.

Parents from poor families “cited lack of permission from family elders,” said Dr. Anita Zaidi, who teaches pediatrics at the Aga Khan University in Karachi. Some rich parents also disdained the vaccine, saying it was “harmful or unnecessary,” she added.

Pashtuns account for 75 percent of Pakistan’s polio cases even though they are only 15 percent of the population. Wealthy children are safer because the virus travels in sewage, and their neighborhoods may have covered sewers and be less flood-prone.