Given the large-scale impact, timing, and unpredictability of the COVID-19 pandemic and the threat this has posed to the health care system's routine capabilities, the battle against COVID-19 is still ongoing. The development of effective vaccines was highly anticipated, and several are now available. The timeliness of raising vaccine acceptance and uptake among the public is essential to overcoming COVID-19. A sizable proportion of the population must be vaccinated to achieve herd immunity and prevent the continued spread of the virus. When COVID-19 vaccination was first implemented, the government of the Republic of Korea specified that herd immunity would be achieved by November 2021 and proposed the goal of vaccinating 70% of the population by the end of October 2021 [1]. Recent studies have shown that third shots (boosters) of COVID-19 vaccines effectively provide additional protection from infection, and some countries are now offering fourth doses [2].

The decision to receive a vaccine is complex and reflects one’s values, attitudes, and experiences and the information one receives about vaccination. Although vaccine implementation is more effective when deployed as soon as possible, vaccine hesitancy is a substantial threat to the success of vaccination programs [3-5]. A well-known aspect of COVID-19 vaccination is that it may induce side effects. Data on the short-term and long-term side effects of vaccination have been reported [6,7], raising concerns regarding these side effects [8]. Although possible severe side effects are comparatively rare, the risk of their occurrence may considerably influence one’s decision not to receive the vaccine, thereby leading to vaccine hesitancy [9,10].

For patients with underlying diseases, the decision process becomes even more complex, ultimately leading them to avoid vaccination owing to the risk of harming their health [10,11]. In Korea, individuals with underlying diseases are classified as high risk and selected as the priority demographic for vaccination [1]. Previous studies have shown inconsistent COVID-19 vaccination intentions among populations with medical conditions [12,13]. Korea Centers for Disease Control and Prevention data from 2021 show that patients with chronic diseases had a lower vaccination rate than the general population of Korea. More recent data from May 2022 show that although the vaccination rate among those aged 40 to 59 years in the general population is 96.7%, that among patients with chronic disease is approximately 2% to 7% lower (94.7% among patients with hypertension, 93.7% among patients with diabetes, and 90.3% among patients with solid organ malignancy) [14]. A study in Brazil showed that concurrent malignancy, fibromyalgia, hydroxychloroquine use, and recent corticosteroid pulse therapy were independently associated with higher odds of COVID-19 vaccine hesitancy [13]. Conversely, a Japanese study revealed that a higher COVID-19 vaccine rate was positively associated with underlying disease, as patients with underlying diseases perceived themselves as being more vulnerable to COVID-19 [12].

Communication interventions such as decision aids (DAs) may yield promising results in addressing this issue. The International Patient Decision Aid Standards (IPDAS) Collaboration defines DAs as “tools designed to help people participate in decision making about health care options” [15]. They provide information on different health care options and help patients by clarifying and communicating personal values while guiding them through the decision-making process [16]. DAs aim to include high-quality epidemiological evidence on the benefits, risks, and associated probabilities of outcomes [17]. Their most common forms are web-based tools, pamphlets, and videos, and they fall within the broader field of shared decision-making—a process by which patients and clinicians work together to (1) clarify treatment goals and (2) review information about the available options and their outcomes to reach a mutual agreement [18,19]. DAs have improved patients’ perceptions of the decision-making process and increased their knowledge about the options that align with their values [20]. Although they effectively assist patients in making health treatment and screening decisions, less is known about the impact of DAs on vaccination uptake. A study that examined the impact of DAs on health care personnel’s decisions regarding immunization against influenza [21] and hepatitis B [22] found that DAs could be good tools for addressing the phenomenon of vaccine hesitancy [19,23].

Several intervention studies attempting to increase the acceptance of COVID-19 vaccination via informational messages, behavioral nudges, web-based interventions, and videos appear in the academic literature [24-28]. However, interventions targeting patients with underlying medical conditions could not be uncovered in the literature. Therefore, the implementation of interventions with proven effectiveness targeting patients with underlying medical conditions are highly needed [29].

This study aims to evaluate the effectiveness of a communication tool for patients with underlying medical conditions for deciding on whether to receive a COVID-19 vaccine. This study develops a web-based communication intervention called “SMART,” a COVID-19 vaccination DA, and a vaccine-related information program tailored to the target the population with underlying diseases.

A randomized controlled trial (RCT) will be conducted and reported according to the CONSORT-EHEALTH (Consolidated Standards of Reporting Trials of Electronic and Mobile Health Applications and Online Telehealth) checklist, and the protocol timeline is shown in Table 1. This study is a 3-arm prospective RCT enrolling >450 participants with underlying medical conditions (Figure 1). The participants who complete their baseline web-based questionnaire will be recruited into the study and randomly assigned to 3 equal groups, stratified by their underlying disease, sex, age, and willingness to receive a COVID-19 vaccine (willingness and hesitation). The participants will be randomized to receive (1) standard information about COVID-19 vaccination (control group), (2) COVID-19 vaccination DA (intervention group SMART-DA), or (3) COVID-19 vaccination DA with targeted information provision (intervention group SMART-DA-α). Each group is described in detail in the following sections. After 1 week, we will follow-up with the participants and use an intention-to-treat (ITT) approach to determine the effects of their COVID-19 communication intervention on COVID-19 vaccination. There is no expected harms or unintended effects in each group.

Recruitment will be conducted on the web to reflect the web-based nature of the intervention. The intervention study will be conducted via a web-based platform from a research company. The company will recruit participants by sending study invitations via emails or SMS text messages containing general information about the study (eg, the study aims and consent statement), and a URL to access the platform to survey panel members who meet the inclusion criteria. The inclusion criteria are as follows. First, the participants must have received a diagnosis of at least one of the following diseases: hypertension, diabetes, cancer, angina, myocardial infarction, or stroke. Second, the participants must be aged between 18 and 70 years. Third, the participants must have a fluent in the Korean language and be a resident of the Republic of Korea. Fourth, the participants must have access to the internet and the necessary internet literacy skills to use the interventions. Finally, those who have completed the COVID-19 vaccination (those who have received 2 or more doses and those who have been revaccinated regularly) will be excluded. Upon meeting all the inclusion criteria of the study, the participants will provide their informed consent electronically. The informed consent will appear on the study’s first page, and the research company will ensure the confidentiality of all the participants. The participants will be informed that they may discontinue participation in the research study at any time without any penalty or loss of benefits. All the researchers in this project will be given access to the cleaned data sets, and all data sets will be password protected. To ensure confidentiality, the data dispersed to the project team members will be blinded of any identifying participant information.

After the intervention, the participants will complete follow-up surveys, 3 and 7 days after the intervention, amounting to a total of 3 surveys during the study period. The participants in all the study groups will receive automated text message notifications to complete their surveys to improve study retention.

The participants will be able to earn up to KRW 15,000 (approximately US $13) in gift cards for completing the study’s requirements. These incentives include a gift card worth KRW 5000 (US $4.01) for completing the initial baseline survey and engaging with the SMART-DA or SMART-DA-α intervention or control group material on the web; a gift card worth KRW 5000 for completing the 3-day survey; and a gift card worth KRW 5000 for completing the 7-day survey. All the incentives will be sent to the participants via their mobile phones.

We collected quantitative data on the potential mediators of implementation effects, including attitudes toward COVID-19 vaccination (eg, perceived benefits and barriers). Questionnaires which were validated for web-based use and apply Checklist for Reporting Results of Internet E-Surveys items were designed to measure primary and secondary outcomes. Data collection is performed according to standard guidelines. To improve the reliability of data quality, real-time support through instant messages or phone calls regarding data collection were provided during the entire data collection period. The respondents entered a web-based data collection program, self-assessed through web-based questionnaires, which the research team reviews and rechecks for anomalous responses according to a defined monitoring strategy. If an error was found in the data, the research team asked the rater to correct or explain it, and all corrections were made after the research team confirms them. In addition, to facilitate retention, respondents reported any issues with the respondent. In the event of study discontinuation, a report was generated in a concise format. All available data were checked by the research team members. Every respondent was assigned a unique study ID, and the research team trained the researchers to protect the study data to ensure data safety. The data collection form does not contain any personally identifiable information. The password-protected electronic files were stored on a password-protected computer. Data managers and analysts independently conducted inspections for each evaluation.

We sought to detect the main effects of the communication intervention within the 3 groups with at least 90% power using a 2-sided α of .05. We used G*Power (latest version 3.1.9.7; Heinrich-Heine-Universität Düsseldorf) to calculate the sample size for 3 repeated measurements (in-between interaction, small effect size of 0.2). Our target analytic sample size was 450 (150 per study group) participants. To achieve this analytic sample size, we randomized a projected 630 participants (210 per study group) and expected approximately 30% of the initial participants to drop out during the 7-day follow-up period.

Ethics approval for this study was obtained from the Korea National Open University’s institutional review board on December 1, 2021 (IRB number ABN01-202111-21-13), and was registered with the Clinical Research Information Service Registry. The current protocol version is version 1.1, and there is no plan to change it. The risk of negative effects on participant outcomes is minimal. The possibility of SMART-DA not having a positive effect on the participants is present, but the risk of negative participant outcomes because of this is limited. The data will be analyzed at the completion of the trial, and the study findings will be published in major peer-reviewed journals. Upon meeting all the inclusion criteria of the study, all the study participants will provide their informed consent electronically.

This study is conducted to evaluate the effectiveness of a web-based communication intervention in assisting people with underlying diseases in making decisions about whether to receive a COVID-19 vaccine. This RCT will test the effectiveness of the developed communication intervention, patient DA, and user-centered information as implementation strategies to help patients with underlying medical conditions in making decisions regarding COVID-19 vaccination. This communication intervention aimed to shed light on how a DA for patients with underlying diseases with decision-making needs about COVID-19 vaccination should be designed to facilitate an informed decision-making process. Moreover, this intervention study aimed to contribute to a deeper understanding of individuals’ preferences, attitudes, and values regarding COVID-19 vaccination and their impact on decisions.

A set of implications for interventions, communication strategies, and future research can be drawn from this study. First, the outcomes from this study provide valuable new insights into potential DAs for supporting informed decision-making regarding COVID-19 vaccination and discovering barriers to making informed decisions regarding COVID-19 vaccination, especially among patients with underlying medical conditions. Findings about what kind of information is especially misleading can help clinicians focus on these aspects in their consultations [37,38]. Moreover, knowledge about common needs, fears, and perceptions can help tailor information for individuals and develop policies to support them.

Second, this study is a critical step toward achieving the aim of increasing COVID-19 vaccination among patients with underlying diseases. It will comprehensively evaluate SMART-DA and SMART-DA-α. The results will explain the efficacy of the intervention and its potential mediators. If proven efficacious, SMART-DA could fill an important gap as a web-based intervention that allows patients to learn about COVID-19 vaccination and promotes vaccination by stimulating informed decision-making. Moreover, this intervention could be used not only to defeat the battle against COVID-19 but also to prepare for future epidemics. This tool could be potentially distributed on the web before and during the implementation of vaccination programs. As implementation planning is an important part of the process of implementing evidence-based practices [39], this study will enable public health authorities to use these tools as means of support for patients.

This study has several limitations. First, we could not investigate the study participants’ COVID-19 vaccination status as a primary outcome, as their medical records are not accessible. Therefore, we will investigate the participants’ self-reported willingness to receive a COVID-19 vaccine. However, there is a risk of bias in self-reported willingness and actual vaccination behavior. Second, our study participants might have different underlying medical conditions; therefore, the findings will not be generalizable to all patients. Third, the sample of this study is limited to certain underlying conditions; for example, it did not include patients who receive immunosuppressive therapy, who have a high need for COVID-19 vaccination. However, the diseases targeted in this study are major chronic diseases with high prevalence in Korea, for which the Korean government or medical society recommends vaccination against COVID-19. In addition, because the vaccination rate among this population is generally lower than that among the general population, it can be considered a meaningful group to study. Finally, because the recruitment of participants and implementation of the intervention are conducted through the internet, it is limited to those who have no problems using the internet. Therefore, the participation of those with limited internet use, especially older adults, is restricted; therefore, caution is required in interpretation. Finally, as participants are recruited through the internet, we limit participation to individuals aged <70 years, as those aged >70 years tend to have poorer control over internet use in Korea. Nonetheless, we acknowledge that the older adult group is an important group with high rates of underlying conditions and decision-making needs for which the development of alternative in-person modes of DA may be required in future studies.

We initiated an intervention study to improve the situation in which people with underlying conditions requiring COVID-19 vaccination have lower vaccination rates compared with the general population. In this paper, we have outlined the study procedure and data management. This interventional study aimed to evaluate improvements in vaccination intention to control COVID-19. We plan to comprehensively analyze the data and continuously present new research to improve vaccination intentions.