Willingness to Pay (WTP) for COVID-19 Vaccine Booster Dose and Its Determinants in Indonesia

Willingness to pay (WTP) for booster doses of coronavirus disease 2019 (COVID-19) vaccines is an under studied research topic. Therefore, the current study aimed to investigate the WTP for the booster doses of COVID-19 vaccines and its predictors in Indonesia using an online survey distributed all over the provinces of this low-middle-income country. The WTP was evaluated using a basic dichotomous contingent valuation approach, and its associated determinants were evaluated using a linear regression model. Out of 2935 responders, 66.2% (1942/2935) were willing to pay for a booster dose of the COVID-19 vaccine. The majority of respondents (63.5%) were willing to pay within a price range of 100,000–500,000 Indonesian rupiah (IDR), i.e., USD 6.71–33.57. Being older than 40 years, having a higher educational level, having a higher income, knowing and understanding that booster doses were important, and having a vaccine status that is certified halal (permissible in Islamic law), were all associated with a higher WTP for the booster dose of COVID-19 vaccines. The study findings imply that the WTP for a booster dose of COVID-19 vaccination in Indonesia is lower compared to acceptance of vaccines provided free of charge. This WTP data can be utilized to develop a pricing scheme for the booster doses of COVID-19 vaccination in the country with potential benefits in other low-income countries. The government may be required to provide subsidies for the herd immunity vaccination process to proceed as anticipated. Furthermore, the public community must be educated on the importance of vaccination as well as the fact that the COVID-19 epidemic is far from being over.


Introduction
The emergence of new variants of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) with higher transmissibility necessitates booster dose vaccination for the global community [1][2][3]. Another factor that highlights the need for booster dose vaccination is the short-lived protective immunity following the primary dose COVID-19 vaccination [4,5], indicating that booster doses of COVID-19 vaccination are critical for proper control of the ongoing pandemic [6]. The concept of booster dose vaccination involves the repeated administration of the vaccine to boost immunity [7]. Based on a systematic review study, COVID-19 vaccines provide significant protection against the infection, but their effectiveness wane over time [8]. Specifically, immunity against COVID-19 infection decreased from 83% after the first month to 22% following five or more months of full primary vaccination [8]. Consequently, booster doses of COVID-19 vaccination appear warranted over the long term [9]. However, the cost of vaccines can be a barrier to self-pay vaccine acceptance [10]; hence, assessing the willingness to pay (WTP) for vaccination is necessary for vaccine policy making and promoting booster vaccination acceptance, particularly in low-middle income countries such as Indonesia.
In the context of healthcare, WTP is a method for determining how much people are willing to spend on healthcare plans, services, and medical interventions [11,12]. Originally derived from the economic literature, WTP was later adopted in various disciplines, particularly to assess judgment in making decisions [13], in this case, the decision to receive a booster dose vaccine or not. Booster dose vaccine WTP research is still scarce and under studied; as far as we know, research on booster dose WTP has only been initiated in China [11,14,15]. Although these studies found that booster dose vaccines are widely accepted in China (nearly 85%), the WTP was generally low, with individuals willing to pay less than 200 Chinese Yuan Renminbi (CNY), i.e., around USD 28.49, and refusing to pay more [11]. Another survey study found that only 14.5% (155/1072) of healthcare workers (HCWs) were willing to pay 100 CNY (equivalent to USD 14.25) for a booster vaccine, while 53% were unwilling to pay [14]. Sociodemographic, individual attitudes, and beliefs are factors that greatly affect booster dose vaccine acceptance and the WTP for booster doses [11,16,17]. Being a HCW, having higher incomes and having a high risk of COVID-19 were previously identified as factors associated with higher WTP for the vaccine [17].
The Indonesian government has previously provided all COVID-19 vaccines free of charge for the residents in this southeast lower-middle income Asian country. However, due to declining immunity and continuous emergence of SARS-CoV-2 variants, it may be necessary to re-vaccinate in the future. Vaccines are likely to be expensive given the limited medical resources available. As a result, determining the public's willingness to pay for the COVID-19 vaccine to achieve proper prevention and control is critical. The purpose of this study was to determine whether Indonesians are willing to pay for a booster dose of the COVID-19 vaccine and the factors associated with it.

Study Design and Eligible Criteria
By enlisting 31 collaborators who represented the Indonesia's five large islands, a cross-sectional online survey was distributed to every province in the country. The inclusion criteria for participation were: (1) being an Indonesian citizen ≥ 18 years old, and (2) having internet access. Those without internet access were unable to participate in this study.
The protocol was reviewed and approved by the ethical committee of the Faculty of Medicine at Universitas Syiah Kuala (Approval number: 008/EA/FK/2022). All procedures were carried out in compliance with the principles of the Declaration of Helsinki. Participants were provided with an electronic informed consent form which was mandatory before completing the questionnaire, and their information was kept anonymous.

Questionnaire Structure
The questionnaire consisted of several sections: (1) basic demographic information (age, gender, marital status, the highest attained education, religion, and monthly income); (2) knowledge, awareness, acceptance, perception, the perceived severity, the perceived benefit, and the perceived barriers on getting the booster dose COVID-19 vaccine, and (3) WTP for the booster dose COVID-19 vaccines. The WTP questions were adopted from previous study with modifications [18].
WTP is the highest amount of money a person would likely pay for vaccination [17]. To measure the WTP, a double-bounded dichotomous choice technique was used as commonly used in COVID-19 and non-COVID-19 vaccines previously [19][20][21][22]. The respondents were provided the scenario and question "If the effectiveness of the booster dose of the COVID-19 vaccine is 95% and there is a 5% chance of side effects such as fever or local pain, will you pay 250,000 Indonesian rupiah (IDR) for the vaccine?" Then, they were given a "yes" or "no" choice. Those who choose "yes" the bid was increased to 500,000 IDR, one million IDR, or two million IDR. Meanwhile, those who answer "no" to the first price, the bid was reduced to 125,000 IDR, 60,000 IDR, 30,000 IDR, and 15,000 IDR. Those who were unwilling to pay the 15,000 IDR were given the option of determining the lowest price they are willing to pay. For each respondent, the highest amount of money that they were willing to spend to purchase the vaccine was defined as WTP. The WTP was then converted to USD using an IDR-to-USD exchange rate of USD 1 equivalent to IDR 14,835, i.e., the 31 August 2022 currency exchange rate.

Procedure
The data were collected between 1 and 15 August 2022, utilizing an electronic link distributed via the collaborators' social network. The platforms used in questionnaire distribution were WhatsApp, Telegram, Messenger, Line, Facebook, Instagram, and Twitter. Before completing the questionnaire voluntarily, informed consent was supplied by the respondents. By not gathering any identifiable details of personal information, anonymity and confidentiality were preserved.

Data Analysis
All analyses were carried out using IBM SPSS Statistics for Windows (Armonk, NY, USA: IBM Corp). For continuous variables, descriptive statistics were reported in means and standard deviations (mean ± SD), whereas categorical variables were summarized using frequency and percentage. A linear regression model was utilized to evaluate the factors associated with WTP. This method has been commonly used previously [17,[23][24][25]. Prior to analysis using the model, diagnostic assessments were conducted to ensure that the assumptions of the model were fulfilled (i.e., multicollinearity, heteroscedasticity, and residual normality). To ensure the data met all the assumptions of the linear regression model, the data were transformed using a natural logarithm function. This step was taken because the data violated multicollinearity, heteroscedasticity and residual normality when tested using the variance inflation factor (VIF) [26], Glejser test [27] and Kolmogorov-Smirnov test [28], respectively. The log-transformed WTP data were on a ratio scale. In the initial model, all determinants were included and all determinants with p < 0.05 in the model were entered in the final model. For each variable, one of the categories was used as reference category (R).
To calculate the mean of estimated WTP, Exp(Xβ distribution were WhatsApp, Telegram, Messenger, Line, Facebook, Instagram, and Twitter. Before completing the questionnaire voluntarily, informed consent was supplied by the respondents. By not gathering any identifiable details of personal information, anonymity and confidentiality were preserved.

Data Analysis
All analyses were carried out using IBM SPSS Statistics for Windows (Armonk, NY: IBM Corp). For continuous variables, descriptive statistics were reported in means and standard deviations (mean ± SD), whereas categorical variables were summarized using frequency and percentage. A linear regression model was utilized to evaluate the factors associated with WTP. This method has been commonly used previously [17,[23][24][25]. Prior to analysis using the model, diagnostic assessments were conducted to ensure that the assumptions of the model were fulfilled (i.e., multicollinearity, heteroscedasticity, and residual normality). To ensure the data met all the assumptions of the linear regression model, the data were transformed using a natural logarithm function. This step was taken because the data violated multicollinearity, heteroscedasticity and residual normality when tested using the variance inflation factor (VIF) [26], Glejser test [27] and Kolmogorov-Smirnov test [28], respectively. The log-transformed WTP data were on a ratio scale. In the initial model, all determinants were included and all determinants with p < 0.05 in the model were entered in the final model. For each variable, one of the categories was used as reference category (R).
To calculate the mean of estimated WTP, EXP(Χβ ̂ + σ ̂2/2) was used, in which β ̂ is estimated regression coefficient and σ ̂2 is the mean squared error (MSE) of the model [29,30]. The 95% confidence interval (CI) of the mean of estimated WTP was calculated. Statistical significance was defined as a p value < 0.050.

Sociodemographic and Characteristics of Respondents
Out of 2935 people who filled out the survey, 66.2% (1942/2935) of the respondents were willing to pay for a booster dose of the COVID-19 vaccine and 33.8% (993/2935) were unwilling to pay ( Table 1). The majority of respondents were willing to pay within a price range of 100,000-500.000 IDR, followed by <100,000 IDR (63.5% and 28.4%, respectively).  distribution were WhatsApp, Telegram, Messenger, Line, Facebook, Instagra ter. Before completing the questionnaire voluntarily, informed consent was the respondents. By not gathering any identifiable details of personal infor nymity and confidentiality were preserved.

Data Analysis
All analyses were carried out using IBM SPSS Statistics for Windows (A IBM Corp). For continuous variables, descriptive statistics were reported in standard deviations (mean ± SD), whereas categorical variables were summ frequency and percentage. A linear regression model was utilized to evaluat associated with WTP. This method has been commonly used previously [17, to analysis using the model, diagnostic assessments were conducted to ens assumptions of the model were fulfilled (i.e., multicollinearity, heteroscedast sidual normality). To ensure the data met all the assumptions of the linea model, the data were transformed using a natural logarithm function. This ste because the data violated multicollinearity, heteroscedasticity and residu when tested using the variance inflation factor (VIF) [26], Glejser test [27] an rov-Smirnov test [28], respectively. The log-transformed WTP data were on In the initial model, all determinants were included and all determinants wi the model were entered in the final model. For each variable, one of the ca used as reference category (R).
To calculate the mean of estimated WTP, EXP(Χβ ̂ + σ ̂2/2) was used, in estimated regression coefficient and σ ̂2 is the mean squared error (MSE) o [29,30]. The 95% confidence interval (CI) of the mean of estimated WTP wa Statistical significance was defined as a p value < 0.050.

Sociodemographic and Characteristics of Respondents
Out of 2935 people who filled out the survey, 66.2% (1942/2935) of the were willing to pay for a booster dose of the COVID-19 vaccine and 33.8% (99 unwilling to pay ( Table 1). The majority of respondents were willing to pay w range of 100,000-500.000 IDR, followed by <100,000 IDR (63.5% and 28.4%, re Almost 70% of respondents willing to pay were women, aged 21-30 (49.2%), single (57.2%), and graduated from university (66.1%). Respondents nated by Muslims (78.3%). Almost half of the respondents were employe (49.7%) with a monthly income under 3 million IDR (56.1%) ( Table 2). Table 2. Demographic characteristics of the participants who were willing to pay for COVID-19 vaccine in Indonesia (n = 1942).
/2) was used, in which β hatsApp, Telegram, Messenger, Line, Facebook, Instagram, and Twitting the questionnaire voluntarily, informed consent was supplied by y not gathering any identifiable details of personal information, anoentiality were preserved. ere carried out using IBM SPSS Statistics for Windows (Armonk, NY: ntinuous variables, descriptive statistics were reported in means and s (mean ± SD), whereas categorical variables were summarized using centage. A linear regression model was utilized to evaluate the factors P. This method has been commonly used previously [17,[23][24][25]. Prior he model, diagnostic assessments were conducted to ensure that the model were fulfilled (i.e., multicollinearity, heteroscedasticity, and re-To ensure the data met all the assumptions of the linear regression re transformed using a natural logarithm function. This step was taken violated multicollinearity, heteroscedasticity and residual normality the variance inflation factor (VIF) [ ic characteristics of the participants who were willing to pay for a booster dose Indonesia (n = 1942).
is estimated regression coefficient andσ 2 is the mean squared error (MSE) of the model [29,30]. The 95% confidence interval (CI) of the mean of estimated WTP was calculated. Statistical significance was defined as a p value < 0.050.

Sociodemographic and Characteristics of Respondents
Out of 2935 people who filled out the survey, 66.2% (1942/2935) of the respondents were willing to pay for a booster dose of the COVID-19 vaccine and 33.8% (993/2935) were unwilling to pay ( Table 1). The majority of respondents were willing to pay within a price range of 100,000-500.000 IDR, followed by <100,000 IDR (63.5% and 28.4%, respectively). Almost 70% of respondents willing to pay were women, aged 21-30 years in age (49.2%), single (57.2%), and graduated from university (66.1%). Respondents were dominated by Muslims (78.3%). Almost half of the respondents were employed for wages (49.7%) with a monthly income under 3 million IDR (56.1%) ( Table 2).
Of the respondents, 26.8% had a family member that was seriously ill or died as a result of COVID-19, had been infected with COVID-19 (49.3%), received the first and second dose of the vaccine, and been infected again after vaccination (30.8%). A variable percentage of the study respondents believed that vaccines could stimulate and improve the immune system, lower the hospitalization rate, and protect themselves, their families, and the people around them ( Table 3). The factors that influence the WTP for booster dose of COVID-19 vaccination in Indonesia are also presented in (Table 3).

Factors Associated with WTP for the Booster Dose of the COVID-19 Vaccine
The unadjusted initial linear regression model revealed that 16 of the 35 variables had at least one category with a p value < 0.050. The final linear regression model included all of these explanatory factors. The results of the multivariate model analysis indicated that age, education level, religion, monthly income, having been vaccinated against influenza in the last 5 years, the knowledge that booster dose vaccines can increase immunity, and awareness that booster dose vaccination can protect people who cannot be vaccinated, perception of complications after booster dose vaccine, perceived benefit, and halal status of the vaccine, were all associated with WTP (Table 4).  Among participants who were willing to pay, respondents over 41-50 years had a higher WTP of approximately USD 1.47 than those around 31-40 years, and undergraduate/postgraduate respondents had a higher WTP compared to elementary or high school graduates, of about USD 2.62. Respondents who identified themselves as Catholic and other religions (Hindu/Buddhist/Atheist or agnostic/Confucian) had greater WTPs than Muslims, of around USD 2.26 and USD 2.46, respectively. Those who worked for wages, were students, or retired, had a higher WTP of approximately USD 2.56 and USD 2.62 compared to those who worked as homemakers or were self-employed.
Respondents with a monthly income of more than 10 million IDR were willing to pay around USD 3.02 higher than those with an income of less than 3 million IDR. Respondents who had received an influenza vaccination in the last five years had a greater WTP than those who had not, which was roughly USD 2.32. Those who were not aware that the booster dosage can improve immunity after the second dose and that the booster dose can protect unvaccinated people had greater WTP than those who had such knowledge (approximately USD 1.39 and USD 1.57, respectively). Compared to those who disagreed with the term "I believe that natural immunity is sufficient, and I do not need to be vaccinated", respondents who neither agree nor disagree had a higher WTP of approximately USD 1.44. Those who agreed that "COVID-19 infection is harmless, so I do not have to be vaccinated" had a higher WTP approximately USD 2.50 compared to those who disagreed. Meanwhile, respondents who answered neither agree nor disagree with the statement "Complications may arise after receiving the booster dose" had a higher WTP of around USD 1.64 compared to respondents who answered disagree. Respondents who agreed and neither agreed nor disagreed that getting vaccinated would minimize their risk of contracting infection or infecting others had greater WTP than those who disagreed (about USD 3.41 and USD 3.65, respectively). Respondents who were concerned about the halal status of the COVID-19 booster dose had a higher WTP than those who were not concerned (USD 1.55).

Discussion
The major finding of the current study was the demonstration of much lower intention to receive the booster doses of COVID-19 vaccination if payment is needed. A recent study among the general public in Indonesia showed that the willingness to receive the booster doses of COVID-19 vaccines, if provided free of charge, was 95% [31]. The current study with the same target population showed that the need to pay for the vaccine was linked with a discernable decline of willingness to get the booster doses. Specifically, the WTP for a booster dose was only 66% in the current study sample. This result was expected given the previous evidence that affordability of vaccines is an important driving factor of vaccination convenience. The importance of vaccination convenience is manifested by its inclusion in various models conceived to understand and measure the predictors of vaccination hesitancy (e.g., 3C, 5C and 7C models) [32][33][34]. Therefore, paying for vaccines in general, including the booster doses of COVID-19 vaccines, can be a major hurdle to the efforts aimed to promote vaccination. Consistent with this observation, several previous studies showed a similar lower rate of vaccine acceptance if payment was needed in the context of COVID-19 [18,35], HPV [36], and influenza vaccination [37].
The relevance of the current study is that, first, the sustainability of vaccine procurement can be compromised over time particularly in low-income countries [38]. Second, there is growing evidence showing the value of booster doses of COVID-19 vaccines stemming from the continuous emergence of SARS-CoV-2 variants with immune escape potential, and the declining immunity following the uptake of the primary vaccine doses [5,[39][40][41][42]. Third, the continuous need to receive the booster doses for proper control of COVID-19 might lead to vaccination fatigue, which highlights the need for separate studies to evaluate the general public attitude towards repeated vaccination [43]. The phenomenon of vaccination fatigue is associated with large infectious disease outbreaks, and it entails inaction towards vaccine information and recommendation [43]. Consequently, this can lead to high perceived burden of such information with burnout and hesitancy to receive the booster doses, let alone paying for these doses. Therefore, the intention to get the booster doses of COVID-19 vaccines might be compromised, particularly if payment is needed. The current study was conducted in Indonesia which is a low-income country in southeast Asia. Previous studies showed that the rates of COVID-19 vaccine acceptance in Indonesia were among the highest in the world [44][45][46]. However, the economic burden of the pandemic in a low-income setting can force governments to require payment for vaccination [47], with subsequent risk of hesitancy to receive the booster doses of COVID-19 vaccines.
In this study, WTP for the booster dose of COVID-19 vaccines in Indonesia tended to be relatively low at 66.2% (1942/2935), yet it is not definite that if this booster dose vaccine is provided free of charge, the public will refuse it, as evidenced by our previous study on the acceptance of booster COVID-19 vaccination in Indonesia [31]. Variability in WTP for the primary COVID-19 vaccination has been reported in different studies globally [17,[48][49][50][51][52][53]; however, our study can be considered among the first studies to assess WTP for the booster dose of the vaccine. A recent study that was conducted among HCWs in China showed that the WTP for the booster dose was reported among only 47% of the participants [14].
In this study, the majority of Indonesians (63.5%) were willing to pay 100,000-500,000 IDR (equivalent to USD 6.71-33.57) for the booster dose vaccination. In the previous study that assessed WTP for the primary COVID-19 vaccination in Indonesia [17], 1065/1359 Indonesians (78.3%) were showed WTP for the vaccine, and most respondents were willing to pay USD 30.94 (mean: USD 57.20) for the primary COVID-19 vaccination [17].
Many factors influence a person's WTP level for the COVID-19 vaccine, as previously reported [17,[48][49][50][51][52][53]. In the current study, WTP was associated with age, education level, religion, monthly income, previous uptake of influenza vaccines in the last 5 years, the knowledge that booster dose vaccines can increase immunity, and awareness that booster dose vaccination can protect people who cannot be vaccinated, perception of complications after booster dose vaccine, perceived benefit, and halal status of the vaccine.
Individuals aged between 41-50, highly educated and earning more than 10 million IDR were willing to spend more for booster doses of COVID-19 vaccines. As mentioned earlier, WTP is influenced by factors such as age, education level, and monthly income, according to numerous studies [14,17,50,54]. More knowledge of the danger of COVID-19 can be ascribed to age and education, with lower levels of complacency linked to higher acceptance of vaccination [16]. According to Cerda et al. [50], individuals with lower levels of education showed less willingness to get vaccinated. This was linked to low health literacy and being less aware of threats associated with the disease [55,56]. Furthermore, in this study, individuals older than 40 years showed higher WTP, which can be linked to higher perceived threats due to previous evidence showing that older individuals are at higher risk of developing severe disease with higher risk of mortality [57,58]. Thus, it is understandable to observe higher willing to receive the vaccine among individuals who are aware that COVID-19 can be a serious disease [54,59], with subsequent higher willingness to pay for the vaccine even if it is slightly more expensive. A higher WTP tendency increase was also previously linked with higher monthly income [17,50]. Previous studies have shown that income is positively correlated with COVID-19 vaccine WTP, which is related to an individual's ability to pay [17,18,50]. Therefore, if payment for the booster dose is required, it is necessary to consider providing these doses at an affordable price. In the context of the current study results, if the vaccines provided are relatively expensive, the WTP would decline to a large extent, with individuals having an income of 3 million being unwilling to be vaccinated, which is translated into more than half of the Indonesian population. This will have an impact on the government's goal of vaccinating 70% of the population in order to achieve herd immunity [60].
In this study, we also found that people who reported uptake of influenza vaccines in the last 5 years showed a higher WTP for the booster COVID-19 vaccination. This result can be linked with higher likelihood of positive perceived benefits associated with a previous history of vaccination. Getting the COVID-19 vaccine is a safer and more reliable way to build protection rather than having natural immunity a result of the disease. COVID-19 vaccination helps to protect individuals by creating an antibody response without having to experience a potentially severe illness or post-COVID conditions [61]. In addition, COVID-19 vaccination lowers the rate of hospitalization care and the risk of death [62][63][64]. However, studies show that the level of protection of vaccination against infection decreases over time (after 6 months) [5,65]. In addition, new variants of the SARS-CoV-2 virus continue to emerge with vaccine resistance capabilities [42,66], so a subsequent booster dose is needed to retain immunity [5]. Interestingly, the knowledge that booster dose vaccines can increase immunity, and awareness that booster dose vaccination can protect people who cannot be vaccinated, increases WTP in those without this knowledge compared to those with it.
In addition to the factors indicated above, perception of complications after booster dose vaccine was known to be associated with WTP for booster doses. Individuals who believed that booster doses of vaccines are less safe or who were concerned about the possibility of adverse event occurrence following vaccination had a low WTP, whereas those who believed that vaccination was safe were more willing to pay a higher price. Vaccination hesitancy related to vaccine security was experienced not only by common citizens, but also by health students and those who work in the health sector, as has been widely reported in previous studies, both in developing countries such as Sudan [67], Uganda [68], India [69], Turkey [70], and in developed countries as well, such as the United States [71,72], Poland [73], Slovenia [74] and China [75]. The main reason for their skepticism was that they had heard negative information concerning the COVID-19 vaccine [67,68].
Indonesian society, which is predominantly Islamic, pays great attention to the "halalness" of a product following Sharia Law. Muslims are obligated to follow Sharia Law, which is authoritative in Islam [76]. Many individuals were still skeptical about vaccines with the fear that such vaccines may contain substances prohibited in Islam. Substances used in vaccine manufacturing may be of animal origin, including swine or derivatives, dead animals, or blood, all of which are haram or forbidden for Muslims [77,78]. In Muslim-populated countries, halal certification administrators award the halal certificate to applicants based on the Holy Quran. Meanwhile, many COVID-19 vaccines do not have these certificates. As a result, they rely on ulama fatwas (opinions or interpretations on an issue related to Islamic law by the Indonesian Ulama Council (MUI)). A study on the Islamic sharia perspective on COVID-19 vaccines detailed the legal provisions of various forms of vaccinations used in Indonesia [79]. The MUI declared the Sinovac and Anhui vaccines halal since they were manufactured without the usage of porcine derivatives [77,80]. MUI asserts AstraZeneca is haram-permittable; it is haram since it employs porcine trypsin in the early stages of manufacturing, but it is permissible to use (or Mubah) due to the urgency of controlling COVID-19 [81]. In the meantime, several additional vaccinations, including Moderna, J&J, Sputnik V, and CanSino, were not halal/haram certified [79].
This research has certain limitations as follows. First, in order to observe the impact of halal status on acceptance and WTP for the booster dose of COVID-19 vaccines, it would have been preferable if the religion included was exclusive of Islam only, as it is highly forbidden in Islam to use something that contains forbidden ingredients. Second, because this study used an internet-based platform, individuals who did not use gadgets or have internet access were unable to participate, and this might cause selection bias. Third, the comparisons made with the previous studies on WTP for the booster dose of COVID-19 vaccines should be done with caution considering the variability in survey instruments used for this aim. An important caveat of the WTP assessment, according to Tung et al., is that it is hypothetical [11]. Regardless of the individual's response to the survey instrument, the individual had not paid the actual price by the end of the study. What people claim they will do, and what they actually do, may be inconsistent [11,82].