The process was guided by systematic development protocols for DAs [25,26] and involved two main phases: (1) drafting the main content and (2) iterative web application development. Our user-centered approach involved engaging stakeholders across different phases. The flowchart in Figure 1 provides an overview of the phases and the involvement of stakeholders.

We formed an advisory group of stakeholders to provide guidance and input throughout the development process. On the basis of the recommendations of Elwyn et al [26], we aimed for a stakeholder group of 6 to 10 members with balanced representation across our stakeholder groups. These groups included the target group (parents), CYH professionals providing vaccination services, medical advisers from the NIP hosted by the National Institute of Public Health, and experts in communication and DA development. Additionally, subject matter experts were consulted on an ad hoc basis whenever specialized expertise was needed. The stakeholder group members and experts were purposefully selected through the research team’s professional network.

During the first phase, we based the content of the PDA on the results of a needs assessment. This consisted of a review of relevant literature and interviews to identify parents’ questions, information needs, and decision-making needs. We searched for relevant literature in the PubMed database using the keywords “childhood vaccination,” “information needs,” “decision-making needs,” and “parents” and searched for additional literature using the snowball method. After reviewing the search results, we prioritized key papers that contained relevant information and were applicable to our setting. We extracted the described information and decision-making needs.

Additionally, we conducted brief informal interviews with parents visiting a CYH clinic to explore their information and decision-making needs. Parents were approached at 2 different clinics before their appointment and received verbal information about participation. When parents agreed, a brief informal interview was conducted in person at the respective clinic by a member of the research team (DB-V) and took 10 to 20 minutes. During the interviews, notes were taken, no recording was made, and no personal identifying details were collected. On the basis of the notes, parental needs were summarized, and interviews were continued until no new topics came up. The findings from the literature and interviews were triangulated with a CYH clinician who was highly experienced in counseling parents with vaccine hesitancy. During the triangulation process, the identified needs were grouped into main topics and prioritized. We chose this approach because we aimed to provide a concise and clear overview of the most important topics and to cover the main needs rather than answer every detailed need. Some needs were highly individual and competed with those of other individuals, and integrating these would compromise the accessibility and readability of the DA. The main topics were subsequently reviewed by the advisory group and then used to inform the first draft of the PDA’s content. We also involved a health literacy expert and a CYH clinician as subject matter experts to thoroughly review the first version of the PDA draft; they were both recruited through the research team’s professional network.

In the second phase, a company was contracted to undertake the technical realization of the draft content into a prototype web application. As we used a software template previously developed for a vaccination DA, we collaborated with the same design company and its user experience (UX) professional and could thereby benefit from their expertise in this field.

The prototype was evaluated by panels of both parents and professionals and adapted in iterative cycles. We used different routes to engage panelists as we sought to include a variety of perspectives. We invited professionals involved in routine childhood vaccination care through a newsletter call at 2 different regional CYH organizations and selected both physicians and nurses. Additionally, we invited parents through a call in a childcare service newsletter and through collaboration with an existing low socioeconomic parent panel [27].

First, we focused on general use and navigation through the app and invited a broad group of panelists to conduct a robust evaluation of the prototype. Panelists reviewed the prototype individually online via Microsoft Teams, with a member of the research team taking notes. These sessions took approximately 30 to 40 minutes. In this cycle, we verbally administered the System Usability Scale (SUS) to assess the level of usability [28]. We analyzed session notes to identify issues with the PDA’s design, content, and structure and discussed them within the research team. Additional review sessions with new panelists were planned until no new issues were identified. On the basis of the panelists’ experiences with the prototype, we formulated adaptations to improve the design, structure, and content of the PDA. These were subsequently discussed with the advisory group and implemented. From the second cycle of review sessions onward, we focused on fine-tuning the suitability of the prototype for users with lower health literacy skills. The prototype was evaluated in person at a more detailed level with members from the low socioeconomic parent panel who had low or medium levels of educational attainment. These sessions took approximately 60 minutes. Similar to the previous cycle, we analyzed session notes, recommended adaptations, and reviewed the adaptations with the advisory group before implementation. We planned a final round of test sessions to evaluate if the adaptations had the expected result.

We assessed the available literature from 2012 to 2022 addressing the needs of parents when making decisions about childhood vaccination. Additionally, interviews with 18 parents conducted between May and June 2023, along with triangulation with a CYH clinician, confirmed and added to the findings of the literature assessment.

We grouped the identified needs into the 5 topics shown in Textbox 1: disease descriptions, vaccination schedule, practical information, factual, balanced, reliable information, and discussion and deliberation.

The findings of the needs assessment were reviewed by the advisory group. This group was permanently composed of 7 members, including experts, parents, CYH professionals, and representatives of the National Institute of Public Health. This group endorsed the guiding principles and identified needs but did not support the request to omit details of illness that can be prevented by vaccination. Members also wanted to include upcoming changes to the NIP and recommended referring parents to health care professionals and/or their CYH centers if they had questions after using the PDA.

We based the main content of the PDA on the identified parental needs, evidence-based information about vaccination, and how to communicate about vaccination. We designed 2 modules. One module provided information to support an informed choice, and the other module promoted reflection on values and the views of other parents to support a deliberate choice. The initial draft covered all 5 topics identified in the needs assessment (Textbox 1). In November 2023, a health literacy expert suggested readability improvements, a reduction in the volume of information, and visual elements to support the text. At the same time, the content was reviewed by a CYH clinician for accuracy, tone, and alignment with practice. Table 1 shows illustrative examples of adaptations following their advice.

A significant challenge was how best to present probabilities. A general recommendation for DAs is to provide numerical information in a consistent format [35]. This was difficult for the childhood vaccination PDA, as the range of probabilities varies for each of the 12 diseases and depends on many other factors, such as overall population vaccination coverage and the child’s age. Presenting probabilities using scenarios of best, worst, and most likely outcomes was a possible solution [36]. In the first draft of the PDA, we included both numerical and scenario-based information so that we could seek guidance from our stakeholders.

The hired company created a prototype, and a UX professional supported us in designing a layout that was appealing, professional, user-friendly, and easy to navigate. Illustrative examples of subsequent adjustments are shown in Table 1. Additionally, we added a read-aloud feature to support better text comprehension.

We subjected the prototype draft to iterative cycles of testing, review, and adaptation (Figure 1). During the first cycle, we focused on evaluating usability and content through online testing sessions with panelists, including parents and professionals. We observed how they used the prototype through screen sharing and encouraged them to think aloud. This gave us insights into how users navigated the PDA. Their overall impression was positive. They found the scenario-based format of risk representation (best-case and worst-case scenarios) clear and concise, although some preferred numerical risks. Panelists also found the overview of considerations provided in the decision support module helpful.

The level of usability according to the SUS ranged from 68 to 93 (out of 100), with an average of 78, indicating good usability [28]. In subsequent test cycles, we focused on parents with lower health literacy skills. We evaluated their UX using in-person test sessions and observed how they used the prototype. These parents found some words confusing, and we made changes to address this issue.

After each cycle, we discussed adaptations with the advisory group. Table 2 shows the characteristics of the panels, illustrative examples of comments and feedback provided by the panels, and subsequent adaptations made in each cycle. A key editorial decision involved reducing the amount of information, categorizing the information into essential and additional topics (refer to Final PDA Prototype section), and simplifying the navigation within the decision support module.

We considered the prototype final when the panel sessions did not yield any new feedback and when users could navigate the PDA easily. The Dutch Foundation for Easy Reading assessed the final content and awarded a certificate recognizing its use of plain and ordinary Dutch. The final PDA prototype consists of the following components.

The PDA begins with a welcome page that explains what the PDA is, who it is intended for, and how to use it (Figure 2A). From the navigation bar, users can navigate to the different components.

This module features a tiled architecture that organizes topics into 2 categories: essential information titled “This is what most parents want to know” and additional information titled “You can find more information here” (Figure 2B). Each tile links to a question and its answer, presented in the form of text and/or visual information. The essential topics cover the individual and societal aspects of vaccination, the diseases that vaccines are designed to prevent, how to find reliable information, and vaccine safety. The additional topics include how vaccines work, the vaccination schedule, the advantages and disadvantages of vaccination, and details about the vaccine invitation.

The main page of the decision support module features an explanation of the different steps in the decision-making process and a video of a clinician’s experience in assisting parents through this process. This page links to “The choice of others,” with perspectives of other parents on vaccination, along with an icon array visualizing vaccination coverage. Within this module, users can also access the value clarification exercise (“My Choice”). This exercise allows users to reflect on their values and considerations related to their vaccination decision. Users are invited to agree or disagree with various considerations regarding their vaccination decision and can also add their own. Figure 3A shows an example of a consideration. At the end of the exercise, a customized overview of considerations relevant to each user is generated (Figure 3B). This overview can serve as a basis for a conversation with a health care professional.

To clarify who designed the PDA, we included an information page that introduces the project team members and briefly explains the evidence on which the PDA is based.