Study design and setting
This cross-sectional study was conducted at the 18-month child health check-up in Kanazawa city, Japan, between December 2017 and July 2018. Kanazawa city is the largest city in Japan’s Hokuriku region. In 2018, the total population was about 465,000 people and there were around 3,800 births. The Japanese government provides a nationwide continuum of maternal, newborn, and child healthcare, the central component of which is the maternal and child health handbook.23 When pregnant women register, the local government provides them with a maternal and child health handbook; all services then start and follow a standard schedule. Pregnant women bring the handbook to their maternity clinic of choice for antenatal care, and their doctor records antenatal care results for them and their fetus in the handbook. After giving birth, all data regarding childbirth are recorded in the handbook and mothers receive a birth certificate from the doctor that they take to local government to register the birth. Local governments then provide all registered infants with health information, healthcare advice, and vouchers for child health check-ups and immunizations that are included in the NIP. The main contents of the maternal and child health handbook are described in detail elsewhere.24 Child health and development check-ups (birth to age 6 years) and immunization status are recorded in the handbook, regardless of whether they are included in the NIP. Local governments provide health check-ups for all children aged 18 months under the Maternal and Child Health Act. Kanazawa city has three welfare and health centers that provide health check-ups for all children aged 18 months. The annual reports of the health and welfare services for children in Kanazawa city indicate that the participation rate in health check-ups for children aged 18 months was around 98.9%.25
At the time of the children’s health check-up, trained community nurses informed parents about this study and obtained consent for study participation. Then parents were interviewed using a standardized questionnaire that investigated factors that potentially influence rotavirus vaccine uptake. The questionnaire covered parents’ perceptions about RVGE and the rotavirus vaccine, recommendations received or information obtained about rotavirus vaccine, and socioeconomic factors (e.g., number of household members and siblings, parental age and education level, mothers’ employment). In addition, data were collected on the child’s birth weight, birth order, underlying diseases (food allergies, atopic dermatitis, nasal allergy, asthma, heart disease, and respiratory disease), and daycare service use. Immunization status was ascertained from the maternal and child health handbook, in which all vaccination history must be recorded in Japan as per the Maternal and Child Health Act.
For a significant effect size of 50%, and assuming vaccination uptake of 50%, a percentage of factors associated with vaccination among unvaccinated subjects of 20%, two-sided significance of 0.05, and power of 0.8, this study required 1,124 participants. We obtained written informed consent from all participating parents. The Institutional Review Board associated with Saga University approved this study (No. 30-33).
Definition and estimation for coverage of rotavirus vaccine
Ensuring that vaccination indicators (e.g., vaccine coverage and uptake) are clearly and consistently defined is important for effective communication of outcomes.26 According to the US Centers for Disease Control and Prevention, childhood vaccination coverage is defined as the percentage of children in the target population who received a dose of a recommended vaccine.27 Vaccine uptake is most commonly defined as the absolute number of people who received a specific dose; that is, the numerator in the vaccine coverage calculation. In Japan, rotavirus vaccine is recommended to start at 2 months of age and should be completed before 6 months; therefore, all children in this study had passed the recommended age range. We defined vaccine uptake as the number of children who had received a specific dose of rotavirus vaccine. To estimate rotavirus vaccine coverage, we estimated the target population using the following formula. Estimated total number of target population = number of children (parents) who completed the interview for this study / (answered proportion × proportion of children that participated in Kanazawa city health check-ups, 98.9%). Then, we estimated rotavirus vaccine coverage as rotavirus vaccine coverage (%) = number of children who had received at least one dose of rotavirus vaccine / estimated total number of target population × 100.
Evaluating equivalent household income and relative poverty rate
We recorded annual household income using five categories: less than \2,000,000; \2,000,000–3,999,999; \4,000,000–5,999,999; \6,000,000 or more; and unknown. To estimate the mid-point of an open-ended income category, we used formulas described by Parker et al.28 These formulas are based on Pareto’s law of income distribution, which states that the logarithm of the percentage of units with an income in excess of a certain value is a negatively sloped linear function of the logarithm of that value. According to this theory, the median income for the top-coded category = 10^(0.301/v)*(X), where X = lower value of the top-coded/open-ended category and v = c−d/b−a. Where a = the log of the lower limit of the interval preceding the top-coded/open-ended category; b = log of the lower limit of the top-coded/open-ended category; c = the log of the sum of the frequencies in the top-coded category and the category preceding it; and d = the log of the frequencies in the top-coded category.
We equivalized household income using the square root of household scale, which means that household income was divided by the square root of household size.29 For example, a household of four persons has needs twice as large as one composed of single person. The relative poverty rate was defined as the proportion of children whose equivalent household income was less than 50% of the median of all surveyed children, following the definition of the Organisation for Economic Co-operation and Development (OECD).30
Statistical analysis
We first assessed correlations in parents’ perception of rotavirus vaccine and RVGE, socioeconomic factors, and children’s characteristics between vaccinated and unvaccinated children. Odds ratios (OR) and 95% confidence intervals (CI) were calculated using a univariate logistic regression model. Next, multiple logistic analysis was performed to investigate correlations between rotavirus vaccination status and potentially influential factors, adjusted for possible confounders. For equivalent income and relative poverty rate analyses, we excluded cases where data for household income and household size were not available, and performed additional analyses using complete data. Median household equivalent income was compared by rotavirus vaccine uptake status using the Mann-Whitney U-test. We evaluated associations between relative poverty and rotavirus vaccine uptake and uptake of other self-paid vaccines using multiple logistic regression analysis with adjustment for possible confounders. All analyses were conducted using SAS version 9.4 (SAS Inc, Cary, NC).