As we'll see in section today, a vaccine does not have to be totally perfect in order to prevent transmission of a disease, because when a disease enters a community its spread from one person to another.
In 1991, according to Ceri Peach, an Oxford University geographer, 60% of Pakistanis and 70% of Bangladeshis would have to move in order to spread themselves evenly across the city's wards.
The commonly used index of segregation, which measures the number of people who would have to move in order to spread themselves evenly over a city, shows that every large ethnic minority group became less segregated between 1991 and 2001.
