Transition from Free to Paid Vaccination: Willingness to Pay for COVID-19 Booster Doses in Vietnam in the Post-Pandemic Era

Introduction

Coronavirus disease 2019 (COVID-19), caused by infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has exerted considerable pressure on public health systems across the world.^1,2 Despite extensive efforts to develop effective therapeutic options, vaccination has remained the cornerstone of COVID-19 control strategies.³ Growing evidence indicates that vaccine-induced immunity declines over time, particularly in the context of emerging variants such as Delta and Omicron.⁴ In response, international and national public health bodies have promoted booster vaccination to maintain individual immunity response, population-level protection and reduce the risk of future outbreaks.^5,6

Unlike routine vaccines for endemic infectious diseases, most COVID-19 vaccines were developed and authorized under emergency use mechanisms during the pandemic, allowing rapid deployment at an unprecedented scale.^7,8 During this period, many countries implemented mass vaccination programs with strong governmental and international financial support, providing vaccines free of charge to achieve high coverage rate.⁹ As COVID-19 transitions from a pandemic emergency to an endemic disease, vaccination strategies are shifting from emergency response to long-term prevention, with booster doses recommended for maintaining immunity, particularly among high-risk populations.¹⁰

In Vietnam, the nationwide vaccination campaign launched in 2021 achieved high coverage, with primary doses provided free of charge.¹¹ By early 2023, more than 265 million doses had been administered, covering nearly the entire adult population.¹² However, in the post-pandemic phase, sustaining fully subsidized vaccination programs poses significant fiscal challenges for low- and middle-income countries (LMICs) such as Vietnam.¹³ Consequently, many countries are considering a transition from fully subsidized vaccination to cost-sharing or user-fee mechanisms, raising important concerns regarding affordability, equity, and long-term sustainability. As a result, individuals may increasingly be expected to share the financial burden of booster vaccination.¹⁴

By 2025, Vietnam’s COVID-19 vaccination program was in a transitional phase, with the Ministry of Health issuing recommendations for booster doses; however, booster vaccines were not yet widely available and no official pricing or cost-sharing policy had been established^15,16 In this context, willingness to pay represents an economic concept used to capture individuals’ valuation of health interventions and their readiness to allocate personal financial resources for preventive services.¹⁷ Previous studies have reported varying levels of willingness to pay (WTP) for COVID-19 vaccination across countries.^18–21 In Indonesia, approximately two-thirds of respondents indicated willingness to pay for vaccination, with stated payment amounts ranging from USD 6.71 to 33.57.²² Evidence from China suggests that approximately half of healthcare workers expressed WTP for a COVID-19 booster dose.²³ In Vietnam, existing evidence remains limited and has largely focused on specific populations, such as pregnant women,² with little data available for the general adult population, particularly in the post-pandemic context.

While the clinical efficacy of booster doses is well-established, the economic feasibility of transitioning from a fully subsidized to a user-fee vaccination model in Vietnam remains uncertain. Understanding public demand and their willingness to pay is crucial for policymakers to design appropriate pricing strategies and subsidy mechanisms. Therefore, this study aims to estimate the WTP for COVID-19 booster vaccines and identify determinants of payment behavior among Vietnamese adults in the post-pandemic context. Findings from such research will provide practical evidence for health authorities in developing equitable financial policies, ensuring fair access to vaccines, in line with the 2025 Law on Disease Prevention which emphasizes equitable access to immunization across the life course, and supporting sustainable pandemic control in the future.²⁴ We hypothesized that a substantial proportion of Vietnamese adults would express a willingness to pay for COVID-19 booster vaccination once free vaccination programs were phased out, and that this willingness would not be uniformly distributed across the population. We further hypothesized that willingness to pay would be associated with socioeconomic and health-related factors, including income level, health status, and prior disease and vaccination experiences.

Material and Methods

Study Setting and Sampling Methods

This study employed a cross-sectional design because it allows the rapid assessment of willingness to pay and associated factors within a defined population during a specific post-pandemic period, and this approach has been widely used in COVID-19 vaccination-related studies.^2,18,25 The study was implemented at Quang Xuong District General Hospital in Thanh Hoa Province, a densely populated province with diverse socioeconomic and cultural characteristics in Vietnam, between October and November 2024. As a district-level healthcare facility, the hospital provides access to adults with varying healthcare experiences and vaccination histories, making it a suitable setting for assessing willingness-to-pay preferences in a real-world healthcare context. This hospital was selected because it serves a large and diverse population in a semi-urban and rural setting, allowing us to capture a broad range of socioeconomic characteristics. In addition, the high patient volume at the hospital ensured that the required sample size could be achieved within the study period. Participants were included in the study if they met the eligibility criteria: (1) were aged 19 years or older, (2) attended the hospital during the study period, (3) agreed to participate in the study and provided written informed consent, and (4) were able to complete the questionnaire without assistance.

The required sample size was calculated using the single-proportion formula: n = Z² × p(1 − p)/d², where Z = 1.96 corresponds to a 95% confidence level, p = 0.826 was the expected proportion of willingness to pay derived from a previous Vietnamese study,² and d = 0.05 was the absolute precision. Based on these parameters, the minimum sample size was estimated at 324 participants. To account for potential non-response and incomplete data, the sample size was increased. A convenience sampling method was used to recruit adult participants attending the hospital during the study period. This approach was applied due to the absence of a predefined sampling frame. Eligible participants who provided written informed consent were invited to a private room for face-to-face interviews to ensure confidentiality. A total of 405 participants were enrolled in the study.

Measurements and Instruments

Data collection was conducted using a structured questionnaire specifically designed for the present study. The structured questionnaire was developed based on the contingent valuation method framework and adapted from previous studies on willingness to pay for COVID-19 vaccination.^2,20 Participants were interviewed in a private setting, where trained investigators explained the study objectives prior to conducting face-to-face interviews. Each interview lasted approximately 14–20 minutes. The questionnaire comprised four main components: (1) sociodemographic characteristics; (2) health-related information; (3) COVID-19–related experiences and information; and (4) WTP for a COVID-19 booster dose. The content of the questionnaire was developed by reviewing previous studies,² after which we pilot tested the instrument and sought expert consultation to ensure its suitability for the research context.

Potential sources of bias included recall bias due to self-reported information, social desirability bias, and interviewer bias during the data collection process. Recall bias may arise when participants do not accurately remember past events, while social desirability bias may occur when respondents provide answers they perceive as more acceptable to the interviewer.

To minimize these biases, interviewers received comprehensive training on standardized interviewing procedures and were not directly involved in participants’ clinical care. Prior to each interview, the study objectives were clearly explained, and participants were given sufficient time to consider their responses. The questionnaire was designed to be simple and easily understandable to reduce misinterpretation. Participants were also assured of confidentiality to encourage honest responses.

Data quality control measures included cross-checking during data entry and random re-verification of 5% of completed questionnaires through follow-up contact with participants to ensure data accuracy.

Predictor Variables

Demographic Characteristics

Age, gender (male/female), ethnicity (Kinh/Other), religion (Buddhism/No religion), educational attainment was categorized as below high school or high school and above, occupation (Worker, farmer/Government or public employee/Business owner or trader/Others), marital status (Single/Married/Others), income (USD), internet use (Yes/No), smoking (Never smoked/Former smoker/Current smoker).

Health Status

Health-related variables comprised self-perceived health status (very good, good, fair, or poor), a history of allergy, and the presence of chronic conditions.

Covid-19 Vaccination Information

COVID-19–related variables comprised vaccination history, number of doses received, experience of post-vaccination side effects, interest in receiving vaccine-related information, and preferred channels for vaccine communication (commune health workers, mass media, digital platforms, or other sources).

Outcome and Valuation Method

The primary outcomes of interest were participants’ financial willingness to contribute to a COVID-19 booster vaccination and the highest payment amount they indicated. To elicit these outcomes, a contingent valuation approach using a double-bounded dichotomous choice design was employed to capture participants’ stated valuation of a hypothetical COVID-19 booster.^26,27 Participants were first asked whether they would be prepared to pay for a COVID-19 booster dose to maintain immunity if the free vaccination program has ended. Those who agreed were presented with an initial bid (reference price) of 400,000 VND (approximately USD 15.3), which was determined based on the prevailing market prices of COVID-19 booster vaccines and findings from previous studies in similar settings.² Depending on their response (Yes/No), a higher or lower follow-up bid was subsequently offered. This iterative bidding process allowed responses to be categorized into four patterns, reflecting different ranges of WTP. After completing the bidding sequence, participants were asked to state the maximum amount they would be willing to pay for the booster vaccination. The self-reported maximum willingness-to-pay amount was used as the continuous outcome variable in the linear regression analyses examining factors associated with payment amount. This approach enabled estimation of both the proportion of participants willing to pay and the distribution of their stated WTP values. The bidding framework is illustrated in Figure 1. All WTP values were collected in Vietnamese Dong (VND) and converted to US Dollars (USD) using the exchange rate at the time of the survey (1 USD ≈ 26,147 VND).

Figure 1 Sequence of Bid amounts used to elicit stated payment for a COVID-19 booster dose (US$). Abbreviations: Y, yes; N, no.

Statistical Analysis

Data analyses were undertaken using STATA version 16.0 (StataCorp, College Station, Texas, USA) which is a software package suitable for conducting regression analyses commonly used in public health and health economics. The software has also been applied in several studies related to willingness-to-pay.^2,28 Participants’ sociodemographic characteristics and willingness-to-pay outcomes were summarized using descriptive measures, such as frequencies, means, and standard deviations. Differences in willingness to pay and stated payment amounts across sociodemographic, health-related, and COVID-19–related characteristics were examined using chi-square or Fisher’s exact tests for categorical variables, and Mann–Whitney U or Kruskal–Wallis tests for continuous variables, as appropriate.

Factors associated with willingness to pay were explored using multivariable logistic regression analysis. Linear regression was used to examine factors associated with the open-ended maximum willingness-to-pay value reported by participants after completion of the bidding exercise. Variables demonstrating an association at p < 0.20 in bivariate analyses were considered for inclusion in multivariable models. A stepwise selection approach based on likelihood ratio testing was applied to derive parsimonious final models. Statistical significance was defined using a two-sided p-value threshold of 0.05.

Income-related disparities in stated payment for the COVID-19 booster dose were examined using the Lorenz curve, depicting the cumulative share of stated payment amounts across income groups in comparison with the line of equality.

Ethical Considerations

The study protocol was approved by the Ethics Committee of Hanoi Medical University (Approval No. 1216/GCH-MHU-IRB, December 28, 2023). The study was conducted in accordance with the principles of the Declaration of Helsinki. Written informed consent was obtained from all participants prior to enrollment. Participants were informed of the study objectives and their right to withdraw at any time without penalty. All data were anonymized and used solely for research purposes.

Results

Of the 405 participants analyzed, 343 individuals (84.7%) reported readiness to contribute financially toward a COVID-19 booster vaccination. Substantial variation in stated payment willingness was observed across several sociodemographic characteristics (Table 1). Participants who were married and those in higher income groups showed a greater likelihood of willingness to pay, while those unwilling to pay were overrepresented in the lowest income quintile (Q1), comprising approximately half of that subgroup.

Table 1 Sociodemographic and Health Characteristics of Study Participants by Willingness to Pay for a COVID-19 Booster Vaccination

The study population was predominantly of Kinh ethnicity and reported no religious affiliation. Nearly half of participants were farmers or workers, and approximately half had attained at least a high school education. While sex, ethnicity, religion, and smoking status were not significantly related to willingness to pay, income level demonstrated a strong gradient, with increasing willingness observed across income quintiles (p < 0.001). Internet use was also more common among participants willing to pay compared with those unwilling to pay.

Variations in willingness to pay were also observed across health status and COVID-19–related characteristics (Table 2). Participants reporting better self-rated health showed a higher likelihood of willingness to pay compared with those who perceived their health as poor. Although a greater proportion of individuals with a history of allergy was found among participants unwilling to pay, this difference was not statistically significant. The majority of respondents had received three or four doses of a COVID-19 vaccine. Although participants with previous COVID-19 vaccination experience appeared more likely to report willingness to pay, this finding should be interpreted cautiously because only two participants had not previously received a COVID-19 vaccine, limiting the robustness of this comparison. In contrast, participants who reported adverse events following COVID-19 vaccination were significantly less willing to pay than those without such experiences (p = 0.01). Conversely, interest in receiving vaccine-related information was strongly associated with willingness to pay (p < 0.001), with these individuals demonstrating greater readiness to contribute financially.

Table 2 Health Status and COVID-19 Vaccination Information of People

The average WTP for a COVID-19 booster dose was USD 15.7 (SD = 15.8), with a median payment of USD 7.7 (Table 3). Payment amounts varied across sociodemographic and COVID-19–related characteristics. Higher average WTP was observed among male participants, those who had completed at least high school education, and individuals involved in business or self-employment, although some differences were not statistically significant. A distinct income-related gradient was identified, with mean WTP rising progressively across income quintiles, increasing from the lowest (Q1) to the highest (Q5) (p < 0.001). Participants reporting internet use exhibited higher mean WTP than those without internet access. Conversely, mean WTP showed no substantial variation according to the number of vaccine doses received or the presence of post-vaccination adverse events.

Table 3 Willingness to Pay for the COVID-19 Booster Dose Among Participants

Figure 2A illustrates the association between vaccine price levels and WTP. The cumulative curve demonstrates a pronounced inverse pattern, with the share of participants expressing willingness to pay declining steadily as the proposed price increased.

Figure 2 Distribution of willingness to pay for COVID – 19 booster vaccination. (A) Cumulative proportion of participants willingness to pay COVID-19 Booster vaccination. (B) Lorenz curve of willingness to pay for COVID-19 Booster vaccination.

Figure 2B presents the Lorenz curve illustrating the distribution of willingness to pay (WTP) for a COVID-19 booster vaccine among study participants. The curve lies below the line of equality, indicating an unequal distribution of WTP values within the study population. Participants with lower cumulative income rankings contributed a smaller share of the total stated WTP, whereas those with higher income rankings accounted for a larger share. The corresponding Gini coefficient was 0.447, indicating a moderate-to-high degree of inequality in the distribution of willingness-to-pay amounts.

Results from multivariable regression analyses indicated several factors independently related to WTP for a COVID-19 booster vaccination (Table 4). Married participants were more likely to report willingness to pay compared with single individuals (OR = 2.93; 95% CI: 1.20–7.19). Participants belonging to the middle-income group (Q3) also showed significantly higher odds of willingness to pay than those in the lowest income quintile (Q1) (OR = 5.95; 95% CI: 1.56–22.66). In contrast, respondents who rated their health as poor were substantially less likely to express WTP (OR = 0.09; 95% CI: 0.01–0.58). Experiencing adverse events following previous COVID-19 vaccination was likewise associated with reduced WTP (OR = 0.44; 95% CI: 0.23–0.84).

Table 4 Factors Associated with Willingness to Pay for the COVID-19 Booster Vaccination

With respect to the amount of payment, multivariable linear regression analysis revealed that female participants and older age were associated with lower stated payment levels (female sex: Coef = −3.68; 95% CI: −7.08 to −0.27; age: Coef = −0.20; 95% CI: −0.34 to −0.06). Conversely, participants with a history of COVID-19 infection reported higher willingness-to-pay amounts (Coef = 3.81; 95% CI: 0.38–7.23).

Discussion

This cross-sectional study provides evidence on WTP for COVID-19 booster vaccination among adults in Thanh Hoa province, Vietnam. Using a contingent valuation approach, we identified several factors associated with willingness to pay, including marital status, income level, self-rated health, prior post-vaccination adverse events. Regarding the amount of payment, gender, age, history of COVID-19 infection were identified as important determinants.

Magnitude of WTP

Our findings are generally consistent with previous studies on willingness to pay for COVID-19 vaccination conducted in different settings. A high proportion of participants (84.7%) reported WTP for a COVID-19 booster dose, with an average stated payment of USD 15.7. This finding reflects a generally favorable attitude toward booster vaccination and a degree of financial readiness among the study population. The observed willingness to pay is comparable to previous findings from Vietnam among women (82.6%),² but higher than findings from China²³ and Indonesia (66.25%).²² However, the mean WTP was lower compared with studies in Jordan (39.63 USD),²⁹ Iran (60.13 USD).²⁵ Even the estimated mean WTP in our study (VND 410,046; approx. US$ 15.7) is slightly lower than findings in other lower-middle-income countries, such as Indonesia (US$ 19.8 reported by Harapan et al)²² and Malaysia (US$ 16.4 by Wong et al).²¹ However, the most critical finding is the substantial discrepancy between the mean and the median WTP (US$ 15.7 vs. US$ 7.7). This right-skewed distribution indicates that while a subset of the population with higher economic capacity is willing to pay a premium, the majority of the general public has a much lower reservation price.

Critically, the median WTP of US$ 7.7 covers only a fraction (approximately one-third) of the market price for Pfizer or Moderna vaccines utilized in private vaccination services (ranging from US$ 20 to US$ 30 per dose). This suggests that a full user-fee policy based on market prices could effectively exclude more than half of the population, undermining herd immunity goals.

Determinants of Willingness to Pay

Consistent with previous international studies, our analysis revealed both similarities and context-specific differences in the determinants associated with willingness to pay for COVID-19 booster vaccination. Our analysis revealed distinct determinants for the decision to pay (likelihood) and the specific amount participants were willing to contribute.

Regarding socio-demographic factors, marital status was a significant predictor of the decision to pay, with married individuals showing a higher likelihood compared to single participants. One possible explanation is that married individuals may perceive a greater responsibility for protecting family members and maintaining household health. However, this mechanism was not directly assessed in the present study. Household income (specifically Group Q3 vs. Q1) remained a strong positive determinant of the likelihood to pay. This finding is consistent with economic theory and previous studies in LMICs,^22,23,25 confirming that financial capacity is a prerequisite for participating in a paid vaccination program. In addition, a systematic review in the Latin America and Caribbean region identified adverse socioeconomic factors, low education levels, and lack of awareness as common obstacles to vaccination uptake, suggesting that individuals in economically disadvantaged and rural communities face greater challenges in accessing vaccines compared with more advantaged groups.³⁰

In terms of the amount of WTP, a gender disparity was observed, with female participants willing to pay significantly lower amounts than males. The observed gender difference in payment amount may reflect household financial decision-making structures and gender-related economic roles in healthcare spending. This interpretation is consistent with findings from other Asian settings; for example, a study in Nepal reported that men are more likely to make decisions regarding major household expenditures, including healthcare-related payments.³¹ However, these factors were not directly measured in the present study and therefore should be considered as possible interpretations rather than confirmed mechanisms. Additionally, differences in individual financial resources may also contribute to the observed gender disparity in willingness-to-pay amounts.

Regarding health status and experiences, participants with a history of COVID-19 infection were willing to pay higher amounts. This aligns with the Health Belief Model, where “perceived susceptibility” drives action.²⁹ Those who have experienced the illness likely have a more tangible perception of its impact, leading to a higher valuation of the vaccine’s protective benefit.

Conversely, experiencing adverse events following previous doses negatively affected the likelihood of paying. This suggests that fear of side effects remains a significant barrier,²⁵ where the perceived “physical cost” (discomfort/pain) outweighs the potential health benefits, especially when a financial cost is added. Interestingly, those reporting poor self-rated health were less likely to pay. While one might expect vulnerable groups to seek more protection, this negative association could stem from “competing financial needs” where limited resources are prioritized for treating existing conditions or a fear that their fragile health cannot withstand vaccine side effects.

Lower levels of stated payment were more frequently observed among female respondents and older age groups. Similar age-related patterns have been reported in previous studies from China,³² suggesting that these differences may reflect variations in perceived risk and preventive health behaviors. Among older adults, limited financial flexibility, heightened concerns about adverse effects, and existing barriers in accessing healthcare services may further reduce their willingness to allocate personal resources for booster vaccination.

Policy Implications

Given that the median WTP (VND 201,307) falls below the cost of imported vaccines, a complete withdrawal of government subsidies is not recommended. Instead, a co-payment mechanism (where the government subsidizes part of the cost) or a tiered pricing strategy could be effective. Targeted subsidies should be prioritized for low-income groups and women to prevent inequitable access. Additionally, risk communication strategies should focus on the long-term protection of boosters to sustain the high perceived value of vaccination among the public.

Several limitations merit consideration. First, the cross-sectional design precludes the assessment of causal relationships between stated payment willingness and its associated factors.^33,34 Second, the use of convenience sampling and recruitment from a single hospital setting may limit the generalizability of the findings, as participants attending healthcare facilities may have greater health awareness and higher willingness to pay than the general population.^35–38 Therefore, the findings should be interpreted primarily as evidence from a local healthcare setting rather than nationally representative estimates, although they may provide useful insights for future discussions on COVID-19 booster vaccination financing strategies in Vietnam. Furthermore, although participants in the middle-income group (Q3) showed significantly higher willingness to pay than those in the lowest income quintile, a consistent gradient across all income groups was not observed. Therefore, income-related findings should be interpreted cautiously. Third, willingness to pay was assessed using a hypothetical valuation scenario; consequently, stated payment amounts may not fully reflect actual payment behavior under real-world conditions.^25,39,40 Finally, despite efforts to minimize bias, recall bias and social desirability bias may have influenced participants’ responses.^41,42 Several directions for future research emerge from the findings and limitations of the present study. First, longitudinal study designs are needed to track changes in willingness to pay over time as Vietnam’s vaccination financing policies evolve, thereby allowing stronger causal inferences. Second, probability-based sampling across diverse regions and provinces should be employed to improve representativeness and enhance the generalizability of findings to the broader Vietnamese population. In addition, qualitative research should be incorporated to complement and further explain the quantitative findings. Finally, cost-effectiveness analyses integrating willingness-to-pay data with epidemiological models could provide policymakers with more comprehensive evidence to inform the design of optimal co-payment thresholds and tiered pricing strategies during the post-pandemic endemic phase.

Conclusion

In summary, this study was based on the hypothesis that, as COVID-19 transitions to an endemic phase and free vaccination programs are gradually phased out, a substantial proportion of Vietnamese adults would express a willingness to pay for booster vaccine doses. The findings generally support this hypothesis, as a substantial proportion of participants expressed willingness to pay for a COVID-19 booster dose, and both willingness to pay and stated payment amounts were associated with several socioeconomic and health-related factors, including income level, health status, and prior vaccination experiences.

These findings provide local empirical evidence that may inform discussions on sustainable and equitable COVID-19 booster vaccination financing approaches, particularly in settings with constrained public health resources. The results also suggest that targeted financial support for vulnerable populations and strengthened vaccine-related communication warrant further consideration to promote equitable access to booster vaccination. However, given the use of a hypothetical willingness-to-pay scenario and a convenience sample from a single study site, the findings should be interpreted cautiously and should not be considered representative of actual market behavior or the broader Vietnamese population. Further multi-site studies with more representative samples are needed to inform broader vaccination financing policy decisions.