I am doing my thesis on assessment of immunization status in children in District swat. I will be using multistage stratified sampling technique.This will be a house hold survey.
You can also use Lots Quality assessment Sampling (LQAS)which is also used to assess immunization coverage then you can focus on the children aged 12-23.You can get demos on youtube
Tagbo, B.N., Eke, C.B., Omotowo, B.I., Onwuasigwe, C.N., Onyeka, E.B. and Mildred, U.O. (2014) Vaccination Coverage and Its Determinants in Children Aged 11 - 23 Months in an Urban District of Nigeria. World Journal of Vaccines, 4, 175-183. http://dx.doi.org/10.4236/wjv.2014.44020.
My take on this question is that immunization assessment can be done via survey method which I think will be very difficult to conduct and is a costly and time consuming exercise. You should contact EDO office of District Swat and get data from DHIS. You can also get data from district coordinator EPI District Swat, he must be in the District health office.