Study design and study population
This study, a school-based cross sectional survey was conducted in Anambra state, Nigeria, from February to August, 2015. Anambra state was selected due to its accessibility and proximity to the researchers. The state is located at latitude 6.20°N and 7.00°E with total area of 4844 km and population of 4.1 million . The main indigenous ethnic group in Anambra state is Ibo and there is also a small population of Igala. The state contains numerous thickly populated villages and small towns. Anambra state is rich in natural gas, crude oil, and bauxite and ceramic and has almost 100 % arable soil. Literacy level in the state is quite high. The inhabitants are mainly business people, government workers and students. According to the State Ministry of Education record, there are 254 public secondary schools and 166 private secondary schools in the state.
Ten secondary schools (five schools in Onitsha, three schools in Ekwulobia and two secondary schools in Isuofia) all in Anambra state were purposively selected for the study. Onitsha is an urban city while Ekwulobia and Isuofia are rural cities. Five schools were privately owned while the other five were public schools. The reason for selecting schools from both urban and rural area as well as from both private and public schools was to ensure inclusion of persons from all socioeconomic strata. Eligibility for participation (i.e. to be given a questionnaire) was (1) female students aged between 9 and 12 years old and (2) their mothers being able to read and write in English language.
Using Anambra state population size of 4.1 million , confidence level of 95 % and margin of error of 3 %, 385 respondents (approximately 40 respondents/school) were determined to be appropriate for the survey. Fifty questionnaires were distributed to each of the 10 secondary schools to be distributed randomly to eligible girls.
The questionnaires were given to school teachers who distributed them to girls aged 9–12 years to take home to their mothers. The mothers were requested to return the completed questionnaire via their child back to the school teachers within 3–7 days. Contingent valuation approach using the payment card technique was used to estimate the average maximum WTP among the survey participants.
Willingness-to-pay for HPV vaccine assessment
A 23-item self administered questionnaire was developed for the WTP assessment. The questionnaire consisted of three sections. The first section included general information and socio-demographic characteristics such as age, number of daughters, level of education etc. The second section assessed awareness of HPV, genital warts, cervical cancer as well as HPV vaccines. Five questions examining causes of genital warts and cervical cancer were used to assess the knowledge of those aware of the diseases and HPV vaccine (i.e. knowledge index score). The third section presented facts about HPV and contained the payment card used to assess mothers’ WTP for HPV vaccine.
Vaccine rejection was measured based on the response to the following question: “If the vaccine is not free, and you have to pay ‘out of pocket’ by yourself, will you vaccinate your daughter against HPV”? The follow-up question was used to assess willingness to pay (WTP) of “vaccine acceptors”. The question reads as follows: “If so, from the scale below mark ‘x’ on the maximum amount you will pay (in Naira) to have your daughter vaccinated against HPV”. The parents who answered “no” or indicated zero in the payment card were classified as “vaccine rejecters”, while the ones who answered “yes” and indicated a positive value in the payment card were classified as “vaccine acceptors”. Offered WTP values in the payment card ranged from zero to more than 12,000 Naira (equivalent to US$ 60). The maximum price offered reflects the Nigerian market price for the vaccine. The maximum amount they were willing to pay was considered as their perceived monetary benefit of the vaccine. This is in accordance with welfare economic theory which states that the benefit to an individual of a service or intervention is defined as the individual’s maximum willingness to pay for the service or intervention.
The research design and procedure were approved by the ethical clearance committee of Nnamdi Azikiwe University Teaching Hospital Nnewi, Anambra state. The study secured written informed consent from the respondents. Anonymity of participants’ data was maintained by not including participants’ names.
Responses to the willingness-to-pay (WTP) question were grouped into two categories: ‘vaccine acceptors’ versus ‘vaccine rejecters’. The response to WTP question served as the dependent variables in multivariate binary logistic regression. The explanatory or independent variables were re-categorized into the following variables:
Socio-economic data: place of residence (urban or rural); age of respondents (three dummy codes for 31–40 years, 41–50 years, and >50 years); household size (three dummy codes for 4–6 persons, 7–9 persons and ≥10 persons); occupation; average household income (4 dummy codes for US$ 251–502 versus others, US$ 503–1256, US$ 1257–2512, and >US$ 2513); whether respondent is religious; whether respondent is a catholic; and whether respondent is a protestant.
Awareness of HPV infection: ever diagnosed of infection, ever diagnosed of genital warts, and knowledge of HPV infection and consequences (summarized by differentiating those that answered all questions on knowledge of HPV infection correctly from those that did not).
Data were initially coded and transferred to Microsoft Excel (Microsoft Office 2010). Further re-categorization of data (i.e. creation of dummy variables) was done in Microsoft Excel before importing the data to SPSS (Version 20). Multivariate binary logistic regression used the backward conditional as enter method and was performed with SPSS version 20. A two-tailed significance value of 0.05 was used.
Estimation of average cost per vaccinated girl
Cost per vaccinated girl (CVG) was estimated by adjusting cost of HPV vaccination delivery in Tanzania to the Nigerian setting . We adjusted the Tanzanian HPV delivery cost estimates by modifying cost items—social mobilization/information, training, procurement (except for vaccine cost), vaccination, cold storage, and administration/supervision—based on the difference in local purchasing power between Tanzanian and Nigeria. Difference in local purchasing power was computed with a web-based cost of living calculator . In accordance with recent recommendations, vaccine procurement cost was modified to reflect the cost of two doses at $4.50 per dose instead of 3 doses at $5 as per original study . Vaccine price of US$ 4.50 reflects the price being offered by Gavi for countries eligible for support, while vaccine price of U$ 13 represents the lowest public sector price offered by HPV vaccine manufacturers . Except for the vaccine cost, all other costs were inflated from 2012 (i.e. the year of publication) to 2015 US$ value. This was done by converting adjusted cost in US$ to naira equivalent, inflating to 2015 value using the consumer price index, and then converting back to US$. We used exchange rates published by Central bank of Nigeria  and consumer price index published by World Bank .