Perceived Barriers and Intentions to Receive COVID-19 Vaccines: Psychological Distress as a Moderator

A high vaccination uptake degree is crucial to achieve herd immunity to COVID-19 and restrict the spread of the COVID-19 pandemic. However, little is known about the antecedents that reduce or contribute to shaping the intention to receive COVID-19 vaccines, as well as how psychological distress—a mental health problem—can reinforce or dampen the translation from antecedents into intention to receive COVID-19 vaccines. The objective of this study was to explore the effects of perceived clinical and access barriers, self-efficacy, and attitudes towards COVID-19 vaccines on the intention to receive COVID-19 vaccines. Simultaneously, the moderation effects of psychological distress on this relationship were also examined. Using a sample of 2722 Vietnamese adults and structural equation modeling (SEM), this study illustrated that self-efficacy and attitudes towards COVID-19 vaccines were significantly interrelated with intention to receive COVID-19 vaccines. Self-efficacy, attitudes towards COVID-19 vaccines, and intention to receive COVID-19 vaccines were negatively influenced by perceived access barriers but were positively associated with perceived clinical barriers. Importantly, our study reported that when psychological distress was higher, the link between self-efficacy and intention to receive COVID-19 vaccines will become weaker, but the effect of perceived clinical barriers on intention to receive COVID-19 vaccines will be reinforced. Moreover, self-efficacy and attitudes towards COVID-19 vaccines served as mediators in the linkages between perceived barriers and intention to receive COVID-19 vaccines. Besides providing contributions to the extant COVID-19 vaccine literature, this study provides useful recommendations for practitioners and policymakers to foster adults’ COVID-19 vaccine acceptance.


Introduction
The novel coronavirus disease 2019 (COVID- 19), which originated in Wuhan City, China, has resulted in catastrophic damages worldwide [1][2][3]. More than 200 countries around the world have reported infection cases and deaths. As of 15 January 2023, there were more than 661 million total confirmed cases, with more than 6.7 million total deaths [4,5]. The COVID-19 pandemic has caused massive losses and socioeconomic panic worldwide [6][7][8]. In Vietnam, 11,525,711 infected people and 43,186 deaths were reported while the number of administered vaccine doses accounted for 265,518,865 [9]. However, to date, an effective treatment of COVID-19 disease is still not available; thus, almost all countries are dependent on preventative methods to limit the spread of the COVID-19 pandemic [10,11]. Vaccines have been determined as one of the most efficient preventive measures to control the spread of infectious diseases [12][13][14]. The current extensive efforts devoted to vaccine research and development in response to the COVID-19 pandemic are considered unprecedented regarding speed and scale [15]. Thus far, there have been a number of COVID-19 vaccines approved for use by WHO, including Moderna-USA, Prizer/BioNTech-USA, Janssen-USA, Oxford/AstraZeneca-United Kingdom, Covishield-USA, Bharat Biotech, Sinopharm and Sinovac-Beijing, China. Moreover, hundreds of vaccine candidates and ongoing vaccine trials are being pursued [16]. However, the phenomenon of vaccine hesitancy has been widely documented and the problem of vaccine refusal is present worldwide, despite available vaccines [12,13]. A high vaccination uptake level is necessary to achieve herd immunity during the COVID-19 pandemic [13]. A high level of COVID-19 vaccine refusal would result in failure to achieve herd immunity and thus the containment of the COVID-19 pandemic. Therefore, understanding adults' intention towards COVID-19 vaccination is crucial for the successful implementation of a COVID-19 vaccination campaign [13]. There have been numerous studies that have investigated the issue of COVID-19 vaccine intention and hesitancy in many countries, such as the USA [3], the United Kingdom, Canada [17], India [18] and others [19]. Noticeably, although some studies have been conducted to explore the issues of willingness to get vaccinated against COVID-19 and/or COVID-19 vaccine acceptance in Southeast Asian countries, including Vietnam [7,8,10], attention paid to explore the antecedents of vaccination intention and hesitancy is still scant.
Moreover, although several prior studies have investigated the impacts of negative and positive antecedents on intention/hesitancy to receive COVID-19 vaccines [11,13,14], our understanding of the moderation effects of mental health problems, such as psychological distress, on COVID-19 vaccine acceptance and uptake is still limited [19]. Thus, this research aimed to bridge the literature gap by testing the moderation impacts of psychological distress on the paths from perceived clinical and access barriers, self-efficacy, and attitude towards COVID-19 vaccines to intention to receive COVID-19 vaccines. Simultaneously, the mediating role of self-efficacy and attitudes towards COVID-19 vaccines in the links between perceived barriers and intention to receive COVID-19 vaccines was also tested in our study.

Attitudes towards COVID-19 Vaccines
Attitudes refer to how a person consider something and tends to act towards it, often in an evaluative way [13,20]. Theory of planned behavior [21] argue that attitude towards behavior is the best predictor of behavioral intentions. Indeed, a number of prior studies, which applied the theory of planned behavior in their hypothesized frameworks, also reported that attitudes towards behaviors is determined as the most influential antecedents of behavioral intentions [21][22][23]. In the research area of COVID-19 vaccine acceptance, some previous studies also revealed that individuals' attitudes towards COVID-19 vaccines was found to be positively related to their intentions to receive COVID-19 vaccines [20]. In the context of Vietnam, it is hypothetical that attitudes towards COVID-19 vaccines play the crucial role in predicting intentions to receive COVID-19 vaccines among Vietnamese adults.

Self-Efficacy
Acknowledging that actions related to health issues, such as receiving COVID-19 vaccines to prevent the unexpected consequences might not be sufficient to motivate individuals carry out these behaviors [11]. Self-efficacy, which reflects the perceived capacity of being vaccinated with COVID-19 vaccines, can play the crucial important role in executing such behaviors. Moreover, some scholars also highlight that self-efficacy can be seen as a similar construct in theory of planned behavior [21,24], namely perceived behavior control, while perceived behavior control was found to be a strong predictor of attitude towards behavior and behavioral control [11,23,25]. Consequently, self-efficacy can be significantly correlated with attitudes towards COVID-19 vaccines and intentions to COVID-19 vaccines among Vietnamese adults. The following hypotheses are therefore formulated. H2: Self-efficacy is significantly related to (a) attitudes towards COVID-19 vaccines and (b) intentions to receive COVID-19 vaccines.

Perceived Barriers
Coe et al. [26] defined perceived barriers as the beliefs related to the efficacy and the costs of the expected actions. They can be divided into perceived clinical barriers and perceived access barriers to vaccination [26]. Empirically, some studies investigated how the perceptions of different barriers to intentions or/and willingness to receive COVID-19 vaccines [12,26]. However, no prior studies considered perceived barriers as two separated constructs and their effects on intentions to receive COVID-19 vaccines. In this study, we suppose that perceived clinical and access barriers can affect significantly self-efficacy, attitudes towards COVID-19 vaccines, and intentions to receive COVID-19 vaccines. In other words, when people perceive barriers related to the safety, side effects, as well as the difficulty level of being vaccinated with COVID-19 vaccines, their intentions to receive COVID-19 vaccines can be influenced. Moreover, Chu and Liu [11] argue that self-efficacy and attitudes towards COVID-19 vaccines can receive the effects of different factors, such as cues to actions, descriptive and injunctive norms, then transfer these impacts on intentions to receive COVID-19 vaccines. Thus, besides direct effects, individuals perceived clinical and access barriers can also indirectly influence their intentions to receive COVID-19 vaccines through self-efficacy and attitudes towards COVID-19 vaccines. Consequently, in the context of Vietnam, some following hypotheses are proposed.

Psychological Distress
Psychological distress reflects the unpleasant emotional reactions to stress states, which are unmanaged and overwhelmed [27]. Almost all prior studies only considered psychological distress as the detrimental factor of life satisfaction and/or well-being [28,29], while neglecting the moderation effect of psychological distress. In this study, we suppose that psychological distress can serve as the moderator which may facilitate or weaken the impacts of self-efficacy, attitudes towards COVID-19 vaccines, and perceived clinical and access barriers on intentions to receive COVID-19 vaccines. Particularly, the transformations from self-efficacy, attitudes towards COVID-19 vaccines, and perceived clinical and access barriers into intentions to receive COVID-19 vaccines can be strengthened or weakened depending on the high or low degree of psychological distress. In the context of Vietnam, it is hypothetical that psychological distress can significantly moderate the relationship between self-efficacy, attitudes towards COVID-19 vaccines, perceived clinical and access barriers, and intentions to receive COVID-19 vaccines among Vietnamese adults.

Scale and Questionnaire Development
To test the moderation impacts of psychological distress on the association between perceived clinical and access barriers, self-efficacy, attitudes towards COVID-19 vaccines, and intention to receive COVID-19 vaccines, a questionnaire survey was utilized in our study to collect the dataset. The survey and questionnaire is listed in the supplementary material. The Table 1 is presented Demographic characteristics of respondents. All scales used in our study were modified from prior studies. Specifically, the first three items (ICV1, ICV2, and ICV3) of the scale regarding "intention to receive COVID-19 vaccines" were adopted from Chu and Liu [11,30,31], while the last items (ICV4, ICV5, ICV6, ICV7, ICV8) of this scale were modified from Mir et al. [13,32,33]. The five-item scale to assess "attitudes towards receiving COVID-19 vaccines" was modified from Mir et al. [13,[34][35][36]. The three-item scale to assess "self-efficacy" was adopted from Chu and Liu [11]. Finally, the five-item scale to assess "perceived clinical barriers" and the three-item construct for "perceived access barriers" were adopted from Coe et al. [26,37]. The ten-item scale to measure "psychological distress" was adopted from Kessler et al. [27,38]. All items were rated from 1 to 7, representing "strongly disagree" to "strongly agree", respectively. The items of constructs used in the questionnaire was described in Table 2 in detail.
The demographic information of respondents, including gender, age, monthly income, educational level and marital status, was included in the last section of the questionnaire survey. Moreover, because the target respondents were Vietnamese individuals, all items were first transformed into Vietnamese from the English language. Then, two language experts translated them back into English and compared the two versions to ensure the accuracy of the translation process.

Scale and Questionnaire Development
To test the moderation impacts of psychological distress on the association between perceived clinical and access barriers, self-efficacy, attitudes towards COVID-19 vaccines, and intention to receive COVID-19 vaccines, a questionnaire survey was utilized in our study to collect the dataset. The survey and questionnaire is listed in the supplementary material. The Table 1 is presented Demographic characteristics of respondents. All scales used in our study were modified from prior studies. Specifically, the first three items (ICV1, ICV2, and ICV3) of the scale regarding "intention to receive COVID-19 vaccines" were adopted from Chu and Liu [11,30,31], while the last items (ICV4, ICV5, ICV6, ICV7, ICV8) of this scale were modified from Mir et al. [13,32,33]. The five-item scale to assess "attitudes towards receiving COVID-19 vaccines" was modified from Mir et al. [13,[34][35][36]. The three-item scale to assess "self-efficacy" was adopted from Chu and Liu [11]. Finally, the five-item scale to assess "perceived clinical barriers" and the three-item construct for "perceived access barriers" were adopted from Coe et al. [26,37]. The ten-item scale to measure "psychological distress" was adopted from Kessler et al. [27,38]. All items were rated from 1 to 7, representing "strongly disagree" to "strongly agree", respectively. The items of constructs used in the questionnaire was described in Table 2 in detail.
The demographic information of respondents, including gender, age, monthly income, educational level and marital status, was included in the last section of the questionnaire survey. Moreover, because the target respondents were Vietnamese individuals, all items were first transformed into Vietnamese from the English language. Then, two language experts translated them back into English and compared the two versions to ensure the accuracy of the translation process.

Sample
Vietnam is considered as an appropriate nation to recruit the data and examine the perceived barriers on intentions to receive COVID-19 vaccines under the mediation and moderation mechanism of self-efficacy, attitude towards COVID-19 vaccines, and psychological distress because the rationales as follows. First, the success of addressing the COVID-19 spread in three first waves by applying 5K (Khẩu trang-mask-wearing, Khử khuẩn-disinfection, Khoảng cách-social distancing, Không tụ tậpdông người-No garthering, and Khai báo y tế-health declaration) has been proved in this country [3,12,28,39]. Second, at the time of this conducted research, although the self-development of COVID-19 vaccines (Nanocovax) was conducing in Vietnam, almost all Vietnamese people still did not receive COVID-19 vaccines and waited for the sources of COVID-19 vaccines from international supports.  The following formula has been used to estimate a minimal size of sample. N = 100 + X * Y, [29,40]. With X presenting for even per variable (the recommended X is 50) and Y representing for number of independent variables used in the study (Y = 5). Thus, the minimal sample size was 350.
The convenient sampling methodology and an online-based survey were used in our study to collect the dataset from 20 July to 20 October 2021 in Vietnam. In this period, Vietnam was undergoing the fourth serious wave of the COVID-19 pandemic [19,41]. Certain restrictions and social distancing measures had been implemented. Therefore, the use of an online-based survey utilizing Google Forms was more appropriate [6,[29][30][31]. Participants were clearly informed that their participation was completely voluntary, they could withdraw from the survey at any time, and all information was confidential and only used for academic purposes. In total, 9000 online questionnaires were directly distributed through personal emails, and messages on Facebook, Zalo and Viber, to invite participants to take part in the online-based survey. Only 2913 people participated in our survey. However, 191 responses are eliminated due to containing the missing data. After removing the invalid response, finally, 2722 questionnaires were completely filled, and the response rate reached 30.2%. Table 1 provides the demographic information of respondents. While the largest proportion of respondents were female (56%), most of the respondents were aged 18 to 28 years old and accounted for 60.7%, followed by 29-28 (21.0%), 39-48 (13%), 49-58 (3.8%), and over 59 years old (1.6%). Moreover, 57.7% of respondents earned less than 10 million VND each month; 68.6% of them held a bachelor's degree, while 66.3% were single. In total, 890 respondents received at least one dose of the COVID-19 vaccine. Finally, the largest proportion of respondents wished to be vaccinated with AstraZeneca (29.1%), followed by Pfizer (19.5%), Moderna (15.3), Sputnik-V (6.6%), Sinopharm (6.8%), Johnson and Johnson (1.9%) and others (Abdala, Hayat-Vax, Janssen, etc.) (14.7%). Interestingly, 6.1% respondents wished to receive Nanocovax, which is a Vietnamese COVID-19 vaccine candidate developed by Nanogen Pharmaceutical Biotechnology JSC.

Analytical Approach
First, univariate normality was evaluated via using skewness and kurtosis values. Second, Cronbach's alpha, exploratory factor analysis (EFA) and confirmatory factor analysis (CFA) were used to test the reliability and validity of scales. Finally, to estimate both direct and moderation correlations in the conceptual framework, structural equation modeling (SEM) using AMOS 24.0 and the PROCESS macro approach were employed to test the mediation associations [9,42]. Table 2 illustrates the results of the tests of the normality, reliability and validity of the constructs. First, the normality of scales was evaluated via skewness and kurtosis values, and the results showed that the skewness and kurtosis of all items were within the expected values, as the skewness and kurtosis values of all items were less than 3 and 8, respectively [9,43]. Second, the Cronbach's alpha values of all scales were higher than 0.63 (the lowest value of Cronbach's alpha was 0.810), while the corrected itemtotal correlation of each observed variable was greater than 0.3. Therefore, all scales showed internal consistency and reliability [1]. Whole items were adjusted according to the principal axis factoring method and promax rotation (EFA); however, results showed that the factor loading of ICV2, "I actually get vaccinated for COVID-19", was lower than 0.5 (λ ICV2 = 0.424). After extracting this item, the EFA was re-performed [1]. Results revealed that five factors were loaded with a total extracted variance of 72.454%, whereas the Kaiser-Meyer-Olkin (KMO) value reached 0.939, and the factor loadings of all items were higher than 0.5.
Additionally, the average variance extracted (AVE) and composite reliability (CR) of all constructs were evaluated (see Table 3). Results showed that the AVE of all constructs was higher than 0.4, while the CR of all constructs was greater than 0.7. Moreover, the square roots of the AVEs of all constructs in the correlation matrix were higher than the inter-constructed association. Thus, the reliability and validity of all scales were satisfactory [9,33].

Discussion
Available information in the literature pertaining to which factors contribute to or limit intentions to receive COVID-19 vaccines is scant. In particular, little is known about the moderation impacts of mental health problems, such as psychological distress, on the links between antecedents and the intention to receive a COVID-19 vaccine. Understanding why individuals express the intention to receive a COVID-19 vaccine is important since it can help health officials and governments in increasing citizens' awareness of COVID-19 vaccines, promoting vaccination programs and restricting the spread of COVID-19 [13].
Utilizing a sample of 2722 Vietnamese adults, our study adopted structural equation modeling (SEM) to examine the effects of perceived clinical and access barriers on self-efficacy, attitudes towards COVID-19 vaccines and intention to receive COVID-19 vaccines. Concurrently, the moderation effects of psychological distress on these links were also explored. First, our study found that self-efficacy significantly contributed to shaping intentions to receive COVID-19 vaccines. This finding was in contrast with several prior studies [13], but was in line with other studies that explored the links of self-efficacy with other vaccines, such as HPV vaccines [34] or H1N1 influenza vaccination [35,45]. The significant linkage between self-efficacy and intention to receive COVID-19 vaccines may be attributable to the fact that COVID-19 vaccines were available in Vietnam. Second, being consistent with previous research [36], our study also found that attitudes towards COVID-19 vaccines acted as the most important predictor of the intention to receive COVID-19 vaccines. This finding shows that inspiring individuals with favorable/positive attitudes towards COVID-19 vaccines can be considered an effective measure to foster intentions/actions to receive COVID-19 vaccines [13]. Regarding the effects of perceived barriers, while [4] Coe et al. argue that perceived clinical and access barriers to COVID-19 vaccination were not significantly associated with intention to receive COVID-19 vaccines, our study illustrated that both perceived clinical barriers and perceived access barriers were significantly correlated with self-efficacy, attitudes towards COVID-19 and intentions to receive COVID-19 vaccines. However, perceived access barriers were found to have a negative impact on the formation of intentions to receive COVID-19 vaccines, whereas perceived clinical barriers were positively related to intentions to receive COVID-19 vaccines. These outcomes reflect the fact that although people are concerned about the safety or side effects of COVID-19 vaccines, these particular concerns can increase their COVID-19 vaccine behavior, but when they perceive that access to COVID-19 vaccines is difficult or the cost of receiving a COVID-19 vaccine is high, this can reduce their COVID-19 vaccine behavior. For those who, therefore, intend to obtain a COVID-19 vaccine, increasing convenience and providing easier access to COVID-19 vaccines can be identified as an effective strategy to improve COVID-19 vaccination rates [13].
Most importantly, while almost all previous studies neglected the moderation impacts of mental health problems on the path from antecedents to individuals' receipt of COVID-19 vaccines, our study illustrated that although psychological distress did not increase or reduce the impacts of attitudes towards COVID-19 vaccines and perceived access barriers on the intention to receive COVID-19 vaccines, psychological distress was found to reduce the effect of self-efficacy on intention to receive COVID-19 vaccines. In other words, the translation from self-efficacy into intention to receive COVID-19 vaccines was weakened by psychological distress; although individuals display high levels of confidence regarding COVID-19 vaccines, they hesitate to receive COVID-19 vaccines in case of high psychological distress. Moreover, our study also reveals that the positive impact of perceived clinical barriers on the intention to receive a COVID-19 vaccine can be stronger if the level of psychological distress is high. Thus, this study demonstrated the significant moderating influences of mental health issues on COVID-19 vaccine uptake.
Interestingly, being different from prior studies (e.g., Chu and Liu, 2021; Coe et al., 2021) [11,26], our study not only explored the direct effect of perceived barriers on intention to receive COVID-19 vaccines, but it also examined the mediating role of self-efficacy and attitudes towards COVID-19 vaccines on the links between perceived barriers and intention to receive COVID-19 vaccines. This study found that both self-efficacy and attitudes towards COVID-19 vaccines served as partial mediators in the links between perceived clinical and access barriers and intention to receive COVID-19 vaccines. In other words, perceived clinical and access barriers first significantly affect self-efficacy and attitudes towards COVID-19 vaccines; then, these mediators transfer these impacts to the intention to receive a COVID-19 vaccine. Hence, to increase COVID-19 vaccine receipt and reduce the negative influences of perceived barriers, inspiring favorable attitudes towards COVID-19 vaccines, increasing the availability of COVID-19 vaccines and advancing people's awareness of COVID-19 vaccines are essential [11,13,36].

Conclusions
Our study provides several strengths and contributions for both theoretical and practical implications. First, although there have been several scholars investigating COVID-19 vaccine hesitancy/intention, this is one of the earliest studies in Southeast Asia, and it helps to broaden our knowledge on the intention to receive COVID-19 vaccines among adults in Asian countries. Second, the present research is the first to explore the moderation effect of psychological distress on adults' COVID-19 vaccination. Third, this study found that perceived barriers were significantly involved in the intention to receive COVID-19 vaccines. Additionally, our study provided statistical evidence of the mediating role of attitudes towards COVID-19 vaccines and self-efficacy in the links between perceived barriers and intentions to receive COVID-19 vaccines. Finally, the findings of this study offer a body of useful recommendations for the government, practitioners and policymakers to foster citizens' COVID-19 vaccine receipt, as well as restrain the spread of the COVID-19 pandemic. Indeed, to increase adults' COVID-19 vaccine receipt, practitioners and policymakers should focus on effective measures to increase favorable attitudes towards COVID-19 vaccines, make COVID-19 vaccines more available and help people to access COVID-19 vaccines more easily.
However, our study was not without limitations. First, convenient sampling via an online-based survey has been used in our study, although the sample was sizable. Further research should use the random sampling approach to increase the representativeness of the sample. Second, during the period of this study, Vietnam had access to the COVID-19 vaccine, but in limited quantities. As a consequence, this study only focused on exploring adults' intention to receive COVID-19 vaccines, and a further study should close the intention-behavior link in terms of COVID-19 vaccines and explain why many individuals are hesitant to be vaccinated, as well as investigating how personal and environmental factors influence this link. Third, this study only focused on some antecedents of intentions to receive COVID-19 vaccines, including perceived clinical and access barriers, self-efficacy, and attitudes towards COVID-19 vaccines, further studies can extend the conceptual framework to explore other factors which affect individuals' intention to receive COVID-19 vaccines. Last, some scales used in our study, which developed by some prior studies, such as Mir et al. [13,32], Chu and Liu [11], and Coe et al. [26]. Although their validity and reliability have been reported through Cronbach's alpha and CFA, further studies can adopt other scales which can reflect the constructs of perceived clinical and access barriers, self-efficacy, attitudes towards COVID-19 vaccines, psychological distress, and intentions to receive COVID-19 vaccines better.

Informed Consent Statement:
The current study was carried out based on the Declaration of Helsinki. Ethical clearance for this research has been reviewed and approved by Department of Research Management, National Economics University, Hanoi, Vietnam. No: CBQT1.2022.07. Moreover, informed consent has been received from all participants who agreed to take part in the survey. Specifically, we informed that their participants are totally voluntary and they can withdraw from the survey at any time they want. Also, the research only serve for academic purposes and non-commerce.

Data Availability Statement:
The datasets collected and analyszed during the research are not publicy available because of the some sensitive information, such as demographic profiles and personal perceptions. Nevethless the dataset and testing results reported from SPSS 28.0, and AMOS 25.0 codes are available from the research team if a reasonable requirement.

Conflicts of Interest:
The authors declare no conflict of interest.