Determination of factors influencing young adults' intention to have COVID-19 vaccine in the Philippines: An integration of Health Belief Model and the Theory of Planned Behavior

Objectives The COVID-19 pandemic continues to increase around the world and businesses and markets across the world significantly decreased. The purpose of this study was to determine the factors that affect the intention to be vaccinated for the COVID-19 vaccine among young Filipino adults by integrating the Health Belief Model and Extended Theory of Planned Behavior. Study design A cross-sectional study design was utilized. Methods Factors such as understanding of the COVID-19 vaccine, self-efficacy, cues to action, perceived barriers, perceived benefits, perceived side effects, perceived behavioral control, attitude, subjective norm, and intention to be vaccinated were analyzed by utilizing Structural Equation Modeling (SEM). Results With 865 young Filipino adults who answered a self-administered survey, it was seen that Understanding of the COVID-19 vaccine has the highest direct significant effect on cues to action, followed by perceived barriers, and perceived benefits. Interestingly, the primary factor was Understanding COVID-19 vaccines which had an indirect significant effect on the intention to get vaccinated. This is because knowing what the vaccine is for, its effects, and the application would lead to the acceptance of the COVID-19 vaccine. Moreover, the impact of being known to have the COVID-19 vaccine would lead to the intention to get vaccinated. Conclusions The findings of this study can be utilized especially by the government in developing strategies for encouraging people to take the COVID-19 vaccine. Finally, the model construct of the study can be applied to explore more factors that can affect the intention to be vaccinated with the COVID-19 vaccine and other vaccines people worldwide.


Introduction
A large part of the world is still in lockdown due to the COVID-19 pandemic and one of the countries that suffered the longest lockdown is the Philippines [1,2]. To which, the COVID-19 vaccines have been developed and are available across the world. However, supplies are not vastly distributed per brand. Considering the Philippines, vaccines are limited based on donations from other countries [3]. The Department of Health [3] has recently updated (March 30, 2022) their records on full vaccine uptake with only 60% of the population, around 66 million out of the 142 Million residents. It was not until the late October of 2021 that the 12-17-year-old population were given authority for vaccination.
However, to date, not all teenagers have been fully vaccinated.
As one of the most effective preventive measures in combating the spread of infectious diseases, vaccines are considered a crucial tool to limit the spread of viruses such as the COVID-19 [4,5]. The process of individual immunity by vaccination to herd immunity can help combat infectious diseases [6,7]. Chu and Liu [6] also stated that despite the effective vaccination, many are still reluctant to be vaccinated. This vaccine hesitancy and resistance can be attributed to many factors: the study of which are critical and urgently needed to increase people's acceptance of the COVID-19 vaccine [7,8].
However, people tend to have different perceptions regarding the COVID-19 vaccines [9]. The source of information about vaccines negatively affects the knowledge of people which often leads to confusion [9]. In contrast, Latkin et al. [10] shows that a high percentage of Americans have no intentions to be vaccinated because of racial and gender differences in vaccine intentions, even low trust to the vaccine [10,11].
Despite the increasing number of cases of the COVID-19 in the world, researchers have only focused on the prediction of the spread of the virus [12], preventive measures [13], impact of the COVID-19 [14], and development of vaccines [15][16][17]. Thus, studying the perception of the COVID-19 vaccine should be explored. Especially in the Philippines, limited to no studies have been conducted regarding the COVID-19 vaccine uptake.
In order to guide health promotion and disease prevention programs, a theoretical model called the Health Belief Model (HBM) could be utilized [18]. The perception towards the current the COVID-19 vaccines could also be assessed using the Theory of Planned Behavior (TPB). Generally, the HBM framework is executed to assess intrapersonal decision-making processes in relevance to a variety of health behaviors such as vaccination and screening [19,20]. Subsequently, TPB is used to accurately predict the intentions and behaviors of an individual, regarding a certain subject matter, by analyzing it through its context, timeframe, motive, and action [22,23]. With this, the application of TPB is evident in different fields of research, including vaccine epidemiology [21,[24][25][26][27][28][29][30][31][32][33]. With that, there were no studies that utilized the integration of HBM and extended TPB towards the intention to be vaccinated with the COVID-19 vaccine particularly among young adults .
The purpose of this study was to determine the factors that affect the intention to be vaccinated for the COVID-19 vaccine among young Filipino adults by integrating the Health Belief Model and Extended Theory of Planned Behavior. This study can be used as a strong theoretical foundation for future studies for other vaccines. Finally, the model construct of the study can be applied to explore more factors that can affect the intention to be vaccinated with the COVID-19 vaccine and other vaccines of people around the world.

Methods
This study was approved by the School of Industrial Engineering and Engineering Management Mapua University Research Ethics Committees and followed the National Ethical Guidelines for Health and Health-Related Research 2017 by the Philippine Health Research Ethics board. Prior to the data collection, all participants were required to fill the consent form that described the purpose of the study and the confidential data of the participants. Fig. 1 demonstrates the framework of this study.

Participants
This study investigated the intention to have the COVID-19 vaccine. Purposive sampling of 865 who voluntarily participated in this study was evaluated. An online self-administered survey was utilized in this study. Through social media platforms, the Google Form was distributed among Filipinos. Following the suggestion of Hair [62], frameworks with 8 or more latent should consider collecting 500 or more data to represent the intendent research upon using SEM. Represented in Table 1 are the descriptive statistics of the respondents.

Questionnaire
This study is one of the first studies that integrate these 2 theories and represented in Table 2 are the constructs utilized for this study. There was a total of 11 sections for the online survey questionnaire divided into demographics, understanding the COVID-19 vaccine, selfefficacy, cues to action, perceived barriers, perceived benefits, perceived side effects, perceived behavioral control, acceptance, subjective norm, and intention. A 5-point Likert Scale was utilized to evaluate the constructs of this study [64][65][66][67][68][69][70].

Structural Equation Modeling
SPSS 25 and AMOS23 were utilized to derive the SEM in analyzing the intention to get the COVID-19 vaccine. Hair [62] and Kiraz et al. [68] explained how different exogeneous and endogenous latent variables could be analyzed simultaneously using SEM. Moreover, the strength of SEM highlights the acceptability of the models being utilized in the study [63]. Fig. 2 is the initial SEM model for the intention to have the COVID-19 vaccine uptake. As recommended by Hair [62], insignificant indicators with factor loadings lower than 0.50 may be removed to enhance the model fit. Factor loadings represent the affecting weights for the unobserved latent variables considered in this study [71]. Fig. 3 represents the final SEM model for intention to have the COVID-19 vaccine. After the removal of insignificant indicators, it was seen that understanding on self-efficacy (S) (p-value = 0.101) and perceived behavior control (PBC) (p-value = 0.940) have insignificant relationship with (p-values greater than 0.05) [62]. Moreover, the  I understand the applicability of the differences of the current COVID-19 vaccines. Self-Efficacy S1 I am confident that I will obtain COVID-19 vaccines.

S2
Getting vaccinated to prevent COVID-19 is convenient. [14] S3 It will be easy for me to get the vaccines to protect myself from COVID-19.
[ PS4 I think that the COVID-19 vaccine will result to short-term side effects. [76] PS5 [76] (continued on next page) values between two latent variables are measured using the beta (β) coefficient which represents the correlation of the direct effect. The higher the value, the more highly influential the effect is on the overall model [71]. Moreover, presented in Table 3 are the descriptive statistics of the indicators utilized in this study. The composite reliability of the constructs was calculated to determine the overall consistency and validity (Table 4). Hair [62] and Chuenyindee et al. [72] indicated that values greater than or equal to 0.700 for Cronbach's alpha and Composite Reliability for SEM would dictate internal consistency and validity. Moreover, presented in Table 5 is the parameters to measure the model fit for the SEM. Weston and Gore [71] explained how the GFI values represent the absolute fit of the constructs and how it represents a direct assessment of the observed data AGFI is utilized to measure the increase of the GFI for less restricted model [73].
In addition, IFI measures the small number of sample bias. To which, it measures how likely the model would fit even with small number of sample size [73]. On the other hand, CFI measures the improvement of the fit of the model for it to be accepted [71]. Lastly, Hsu [74] explained how TLI measures the covariance within group functions. As stated by Gefen et al. [75], values for TLI, IFI, CFI, GFI, AGFI, with values greater than 0.800 and RMSEA value less than 0.07 [76][77][78] are a good fit. Represented in Table 6 are the direct, indirect, and total effects of the latent variables together with their p-values.
Understanding of the COVID-19 vaccine had the highest significant direct effects on Cues to Action (β: 0.911; p = 0.019) followed by Perceived Barriers (β: 0.909 and p = 0.005). This suggests that comprehensive knowledge about the COVID-19 vaccine triggers an individual to act and accept vaccination. In addition, Understanding negatively affected the Perceived Side Effects (β = − 0.430; p = 0.009). In a study by Teitler-Regev et al. [79], knowledge affects the perceptions towards the vaccine intentions. Similarly, Xie et al. [80] stated that knowledge is a determinant of the effectiveness of a preventive measure. With the said studies, it further proved that knowledge towards a subject matter is a strong determinant of perception and intention. Such cues linked to the COVID-19 consist of direct the COVID-19 experience, health status of family members, and media recommendations [77].
The SEM results suggest that understanding had significant direct effects on Perceived Benefits (β: 0.898; p = 0.002). Wallston et al. [42] posits that perceived benefits provided by Healthcare Information Technology (HIT) are directly influenced by employee factors. This implies that full knowledge about the COVID-19 virus, particularly the biology of the virus, would significantly affect perceived benefits. The relationship between the latent is evident in numerous studies of different health contexts [81][82][83][84]. In health belief, Tola et al. [82] stated that lack of knowledge on a specific disease treatment can cause non-adherent behaviors towards disease treatment. In public health, Hwang et al. [83] stated that the degree of knowledge determines the perception and behavior of an individual.
In addition, Perceived Barriers was found to positively and directly affect attitude (β: 0.484 and p = 0.021), and Cues to Action was revealed to have a significant positive direct effect on PBC (β: 0.460 and p = 0.020). Being well-informed, confident, and the affordability of the vaccine is a necessary factor that greatly affects the attitude of people towards it. This also means that having higher perceived barriers towards the vaccine, the lower the possibility for them to accept it. It is supported by the study of Wong et al. [34] which states that perceived barriers are one of the key factors that influence the acceptance of people to the vaccine. It is also stated that many perceived barriers can cause to a negative attitude and refusal of the vaccine [30,42,53,[85][86][87].
Interestingly, Perceived Side Effects was also found to significantly and negatively affect Subjective Norm (β: 0.338 and p = 0.012). Moreover, there is a negative direct effect of Perceived Benefits towards attitude (β: 0.573 and p = 0.018). Based from the construct, some people believed that the COVID-19 vaccine will cause further health problems which affects their decision on vaccine uptake and increases their worry about the vaccine. This is supported by several studies [58,83], wherein the stronger the information dissemination regarding the side effects of vaccines to the public, the more it results in vaccine hesitancy on most of the population.
Contrary to the negative direct results of Perceived Benefits towards Attitude, Perceived Side Effects had a significant positive direct effect towards Attitude (β = 0.440 and p = 0.019). This finding shows that despite the setbacks of the side effects of the COVID-19 vaccine, people believe that the side effects are not dangerous, short-term, and will not cause further health problems. Supporting this result, Kadali et al. [84] showed findings on the BNT1622b2 mRNA vaccine that majority of their participants are eager to take the vaccine as majority of the participants can continue daily activities while few only experienced short-term side effects.
In relation, Subjective Norm was also found to have a significant positive direct effect on Intention (β: 0.336 and p = 0.014). It shows other people, such as friends and relatives, affect their intention to get vaccinated with the COVID-19 vaccine. Participants believed that Most people who are important to me will get vaccinated for COVID-19. [14]

SN4
If people around me seeks information for COVID-19 vaccine, I will do the same thing. SN5 My family and friends expect me to be vaccinated. Intention I1 I will get the COVID-19 vaccine immediately when it is available. [27] I2 I would get vaccinated if a physician offered me COVID-19 vaccines. [14] I3 I will encourage my family and friends to be vaccinated. [79]

I4
If a COVID-19 vaccine is proven safe and effective, and is available, I will take it. [80] I5 I will most likely get vaccinated if a healthcare provider strongly recommends it. [81] seeking information and expectation of family and friends is vital in the decision making their intention for vaccination. Subsequently, Attitude is found to be a significant factor in affecting Intention. However, it shows a negative direct effect (β: 0.091 and p = 0.025). The results show that Perceived Benefits is the main reason why attitude has a negative direct effect towards Intention. It is supported by Chu and Liu [6] which stated that benefits of being vaccinated is not enough motivation. In contrasts with the study of Barlett [49], the result showed that attitude determines intention and behavior of people towards the vaccine due to different countries and beliefs. Lastly, the findings revealed that PBC positively and directly affects Intention (β: 0.667 and p = 0.009). This relationship implies that  hindrances and difficulties that one might face before, during, and after the vaccination is a determinant of receiving the COVID-19 vaccines. To further elaborate, several studies [60,88,89] provided findings that support the relationship between the said latent. Each of their studies dealt with individual's perceptions towards vaccines against a different type of disease, revealing similar results of PBC as a strong predictor of vaccine intention.

Discussion
The novelty of the current the COVID-19 vaccines draws different negative perceptions. Karlsson et al. [55] even stated that the perceived risk towards the COVID-19 vaccines outweighs the perceived risk of the disease itself. Therefore, it is vital to trace the root cause of these negative perceptions for health authorities can provide a solution. It is not just on the COVID-19 virus as a threat to public health but also on the suppositions. With its unfamiliarity and newness, there is still a lack of research on the COVID-19 vaccines, hindering the government and the public health authorities to be knowledgeable on the different perceptions.
It could be seen from the study that integrating both TPB and HBM holistically measured the intention to have the vaccine uptake. Usual studies have considered individual framework to compare results of vaccine hesitancy. Hossain et al. [90] used TPB, HBM, and the 5C antecedents separately to compare the highest variance among the     [92]. Lastly, Richie et al. [93] considered studies that utilized HBM and TPB separately using meta-analysis. They explained how most studies have considered the individual latent separately, with no consistency on the effectiveness of the models. Thus, this study was able to determine holistically the intention to have the COVID-19 vaccine uptake due to the integration of both HBM and TPB.

Research implications
Out study indicated that understanding the COVID-19 vaccine was seen to be the factor that would drive the people's intention to uptake the vaccine. Based from the results of the study, knowing what the vaccine is for, its effects, and the application would lead to the acceptance of the COVID-19 vaccine. Therefore, this key finding could help government officials in promoting the COVID-19 vaccine uptake [94]. Moreover, the World Health Organization and the Central for Disease Control and Prevention should highlight the importance, benefits, and side-effects of up-taking the COVID-19 vaccine. This would pave a way for better understanding and knowledge towards accepting the COVID-19 vaccine and would lead to the intention to be vaccinated. The findings of this study can be utilized to form strategies in order to attain herd immunity. Attaining herd immunity would make the country go one step further in returning to what was known as the normal state.

Limitations
This study was conducted during the COVID-19 lockdown. With that, this study considered several limitations. First, this study was conducted by distributing questionnaires online. In line with this, majority of the respondents were within the range of 15-24 years old (96.8%). The study tried to cover several age groups, however, since social media platforms were utilized, majority are of the young age group. Second, with the progressive development of the COVID-19 vaccines, it is notable that the findings of this study are limited to the current situation of the pandemic. Additionally, there might be a change in the perceptions of an individual as the vaccines are continuously being improved and as the population of individuals who receive vaccines grow. Lastly, the study only considered perception to get vaccinated. It is also recommended to consider consumer preference on the type of vaccine using conjoint analysis approach.

Conclusions
The COVID-19 pandemic continuous to increase around the world, therefore, the need for COVID-19 vaccination should be explored. The purpose of this study was to determine the factors that affect the intention to be vaccinated of COVID-19 vaccine among Filipino young adults by integrating the Health Belief Model and Extended Theory of Planned Behavior. With 865 young Filipino adults who answered a selfadministered survey, it was seen that Understanding of the COVID-19 vaccine has the highest direct significant effect on cues to action, followed by perceived barriers, and perceived benefits. Interestingly, the primary factor was Understanding COVID-19 vaccines which had an indirect significant effect on the intention to get vaccinated. This is because knowing what the vaccine is for, its effects, and the application would lead to the acceptance of the COVID-19 vaccine. Moreover, the impact of being known to have the COVID-19 vaccine would lead to the intention to get vaccinated.
This study is one of the first studies that integrated HBM and ETPB for analyzing the intention to be vaccinated of COVID-19 vaccine. The findings of this study can be utilized in developing strategies for encouraging people to take the COVID-19 vaccine. In addition, the model construct of the study can be applied to explore more factors that can affect the intention to be vaccinated with COVID-19 vaccine of the people all around the world.

Funding
This research was funded by Mapúa University Directed Research for Innovation and Value Enhancement (DRIVE).

Institutional review board statement
This study was approved by Mapua University Research Ethics Committees and followed the National Ethical Guidelines for Health and Health-Related Research 2017 by the Philippine Health Research Ethics board.

Informed consent statement
Informed consent was obtained from all subjects involved in the study.