At the end of 2019, the novel coronavirus disease 2019 (COVID-19) broke out in the form of a pandemic, and the rapid onset of this disease has brought with it considerable challenges for many or all countries in the world (including challenges in political, economic, social, and security sectors) . Today, more and more countries are developing COVID-19 vaccines to prevent the epidemic . For example, the United States of America started vaccinations on December 15, 2020, and as of July 4, 2021, the U.S. had successfully administered 330.60 million doses . By the end of 2020, the 27 countries of the European Union had begun vaccinations. By July 7, 2021, more than 380 million citizens across the European Union had received the COVID-19 vaccine . China began implementing COVID-19 vaccinations at the end of 2020, so that by July 7, 2021, the number of vaccinations had exceeded 1.32 billion doses in that country . Currently, governments around the world still believe that the COVID-19 vaccine will prove an effective means of preventing the epidemic.
Although various countries are actively carrying out COVID-19 vaccinations, it is undeniable that the COVID-19 vaccines developed at this stage usually have an expiration date (maybe the expiration date of the COVID-19 vaccine developed in each country is different, but all vaccines have an expiration date due to limited time for development). In other words, COVID-19 vaccines will expire after a period of time and will no longer protect the public who have been vaccinated. At this time, if countries around the world still cannot identify a better approach to preventing the virus, people will have to vaccinate again in order to protect themselves. Therefore, governments around the world will face the following critical question:
RQ: what factors will affect the continuous vaccination of COVID-19 vaccine?
In response to this issue, we intend to explore the key factors of citizens’ continuous vaccination and analyze the process and effect of these factors on continuous vaccination. At present, there are relatively few academic studies on continuous vaccination for COVID-19. In light of this, the present study explores factors that theoretically affect the public’s continuous vaccination at every stage and constructs a theoretical model to analyze the entire impact process of continuous vaccination based on the Health Action Process Approach (HAPA) Model, the Expectation Confirmation Model (ECM) and the vaccine hesitancy (VH) theory. Then, we empirically evaluate the direct and indirect effects of factors at every stage of the continuous vaccination process.
In this study, we consider the public’s continuous vaccination to be a three-stage response process. In the case of COVID-19, public responses proceed as follows: motivation, -intention, and -behavior. In other words, this process goes from psychological perception to behavioral intention and then from behavioral intention to actual behavior. To explore this response process, we analyze the public’s feelings and experiences related to the COVID-19 pandemic. The theoretical contribution of this research is to construct a theoretical model that can evaluate continuous vaccination for COVID-19 based on the HAPA Model, ECM, and the VH theory. In this model, we explore which factors can produce a significant effect on the public’s continuous vaccination in the case of COVID-19. This model reflects the response process of the public’s continuous vaccination in the case of COVID-19.
Theoretical background and research hypotheses
The HAPA model is a well-established theoretical model in the field of health behaviors in the past 20 years. This model was first proposed by Schwarzer in 1992 based on the social cognition theory, the rational behavior theory, and the theory of will . The HAPA model divides human health behavior into three stages: pre-intention stage, intention stage, and action stage. Specifically, the pre-intention stage means that the individuals are aware of the danger of not taking a certain health behavior and firmly believe that they have the ability to take this behavior, so as to form the behavior intention . This stage includes three variables: action self-efficacy, outcome expectation, and risk perception. The intention stage refers to the individuals belief that they can deal with various obstacles when taking some behavior then promote them to make a behavior plan and try to do so . Maintenance self-efficacy and planning are the main variables at this stage. The action stage indicates actions produced by the individuals under the regulation of self-efficacy, as well as the withdrawal, maintenance, and recovery of this action . Recovery self-efficacy is the main variable at this stage.
In the present study, in order to evaluate the impact process of continuous vaccination of the COVID-19 vaccine, we used the components and variables from the HAPA model and designed a theoretical model that has three stages in sequence: motivation, intention, and behavior. Our theoretical model illustrates the entire response process of individual continuous vaccination. In the motivation stage, we evaluate individuals’ perceptions of the harms of unhealthy behaviors related to COVID-19 and assess whether they are capable of taking on healthy behaviors. This stage includes three variables: risk perception, perceived self-efficacy, and outcome expectancy. In particular, risk perception reflects an individual’s subjective feelings about the possibility and severity of the harm caused by COVID-19. Perceived self-efficacy denotes the evaluation of an individual’s ability to adopt healthy behaviors during the epidemic. Outcome expectancy is an individual’s judgment of the possible consequences of taking on certain healthy behaviors. The intention stage is to measure the subjective intention of the individual to adopt the COVID-19 vaccine, including the vaccination intention, social positive cues, and VH. It is worth noting that social positive cues and VH are the positive and negative factors that affect vaccination, respectively, while vaccination intention is the dependent variable of this stage, which indicates the individual’s intention to pursue vaccination. The third stage of the model is the behavior stage, which is used to reflect the actual behavior of individual vaccination. This stage holds four variables: vaccination behavior, perceived usefulness, satisfaction, and continuous vaccination. The specific content of these four variables will be introduced and explained in the ECM.
The ECM was first proposed by Oliver in 1980. It was originally designed to understand consumers’ purchase behaviors . According to this model, whether consumers buy the same product again depends on the consumers’ satisfaction with the purchased product, and this satisfaction is the difference between the expected benefit of the product before the purchase and the actual benefit after the purchase . The ECM is composed of four variables: perceived usefulness, satisfaction, confirmation, and continual use intention. The relationship among the variables is shown as follows. Confirmation affects both satisfaction and perceived usefulness. In addition, perceived usefulness has an impact on satisfaction and continual use intention, respectively, while satisfaction directly affects continual use intention .
In this study, we believe that the ECM can be used to explain the continuous vaccination of the COVID-19 vaccine. The main reason is that the process of individual formation of continuous vaccination is similar to the process in ECM. Specifically, after an individual is vaccinated for the first time, an initial expectation of the COVID-19 vaccine and vaccination will be formed. Secondly, after a period of experience and evaluation, the individual will develop various feelings about the COVID-19 vaccine and vaccination. Finally, an individual with positive feelings will have the vaccination behavior again, while others with negative feelings will stop subsequent vaccinations. In this research, we measure the above process using four variables: vaccination behavior, perceived usefulness, satisfaction, and continuous vaccination. Vaccination behavior means the actual behavior of an individual accepting the COVID-19 vaccine for the first time. Perceived usefulness refers to the individual’s belief that the COVID-19 vaccine will help them after a period of vaccination experience. In addition, satisfaction is an individual’s feeling of satisfaction after vaccination. Continuous vaccination indicates the actual action of an individual who has accepted the COVID-19 vaccine to continue to accept it in the future.
VH refers to a delay in acceptance or refusal of vaccination despite availability of vaccination services . In 2019, the World Health Organization (WHO) listed VH as one of the top 10 threats to global health . Today, VH is a global trend, with approximately 90% of countries across the globe reporting this situation . VH is a complex phenomenon and varies with place, time, and vaccines. According to the VH theory, VH can affect the individual’s acceptance of vaccination, which directly leads to the reduction of vaccination coverage and population immunity . It could hinder the complete implementation of the immunization program and increase the risk of preventable disease outbreaks and epidemics . At present, many studies have shown that complacency, convenience and confidence were the three main factors affecting VH [10, 13].
Since the COVID-19 vaccine was developed as recently as 2020, some questions about its side effects, efficacy, and rigorousness are still being raised by the public, which will lead to VH. We therefore take the important factor of VH into the theoretical model and analyze its impact on vaccination intention. Based on Quinn’s research (2019) , we use three second-order factors of complacency, convenience, and confidence to measure VH. In particular, complacency denotes that an individual believes that the COVID-19 vaccine is unnecessary or worthless. Convenience is the availability of the COVID-19 vaccine, and confidence indicates an individual’s lack of trust in the COVID-19 vaccine or its providers.
Impacts of motivation stage on intention stage
The outcome expectancy is the hypothesis of various possible outcomes of an individual’s health behavior before taking action, including positive and negative outcome expectancy . According to the HAPA model, for health behavior, the individual expects to maximize its positive outcomes and minimize its negative outcomes . Currently, many studies have shown that positive outcome is expected to predict behavior intention . In the context of COVID-19, vaccination intention is an effective health protection behavior intention. In this sense, the positive outcome of this behavior is obvious. Therefore, we propose the following hypothesis:
H1: Outcome expectancy has a positive significant impact on vaccination intention.
Risk is a key factor affecting behavior intention. According to the research of Bhattacherjee and Shrivastava (2018) , when individuals implement a certain behavior, they often fail to implement or terminate the implementation due to perceived risks. In the process of vaccination, the individual’s vaccination intention is a kind of risk behavior intention after a psychological judgment. When individuals subjectively believe that the risk and cost of this behavior are high, their willingness to implement it will be greatly reduced . Currently, the side effects, efficacy, rigorousness, and other issues of the COVID-19 vaccine have not eliminated individuals’ doubts about its risks. Therefore, we propose the following hypothesis:
H2: Risk perception has a negative significant impact on vaccination intention.
The HAPA model indicates that if individuals expect a positive outcome of health behaviors, they will be more likely to take on these behaviors . During this process, an individual will exert perceived self-efficacy by evaluating the positive outcome of behavior. At present, several recent studies have indicated that an individual’s perceived self-efficacy can positively influence behavior intention . In the context of COVID-19, vaccination is an effective way to protect health. Therefore, individuals’ perceived self-efficacy on the positive outcome of this behavior can stimulate their vaccination intention. Therefore, we propose the following hypothesis:
H3: Perceived self-efficacy has a positive significant impact on vaccination intention.
Impacts of intention stage on behavior stage
Previous studies have demonstrated that social positive cues have a significant positive impact on an individual’s behavioral intentions [21, 22]. In the context of COVID-19, the doctor’s recommendations and vaccination reports are social positive cues to vaccine adoption. In accordance with the conclusions of existing research, we argue that social positive cues can motivate the public to vaccinate and to promote behavioral intention conducive to vaccination. Therefore, we propose the following hypothesis:
H4: Social positive cues have a positive significant impact on vaccination intention.
VH theory believes that VH is a comprehensive manifestation of individual gender, age, culture, religious beliefs, and other factors . In recent years, with the rise of various anti-vaccine behaviors, VH has also produced a non-negligible influence on vaccination intention. At present, studies have shown that VH has a negative significant effect on behavior intention [13, 24]. In the context of COVID-19, Sallam et al. (2021)  indicated that VH was related to the COVID-19 vaccination. The similar conclusion was also confirmed by the research of Kwok et al. (2021) . Therefore, based on the above argument, we propose the following hypothesis:
H5: VH has a negative significant impact on vaccination intention.
The HAPA model proves that intention has a positive significant impact on behavior . In fact, the effect from intention to behavior is also a specific process from individual decision-making to implementation, and there is an inevitable connection between the two phases . Moreover, the positive impact of intention on behavior can be supported by other classic theories and models, such as the Theory of Planned Behavior . In terms of vaccination, we also believe that vaccination intention can positively affect vaccination behavior. Therefore, we propose the following hypothesis:
H6: Vaccination intention has a positive significant impact on vaccination behavior.
Impacts in the behavior stage
In the ECM, confirmation can have a direct impact on perceived usefulness and satisfaction . Moreover, many previous studies have also verified that confirmation can positively affect perceived usefulness  and satisfaction . The actual behavior can achieve the comparison of individual expectations and experiences, and it is a process by which individual expectations are confirmed in the actual process . We believe that this argument can also be applied to COVID-19. The public compares their own experience of vaccination with their own expectations after vaccination behavior. If their expectations are confirmed, they will feel that the vaccination is useful and be satisfied with vaccination. Therefore, we propose the following hypotheses:
H7: Vaccination behavior has a positive significant impact on perceived usefulness.
H8: Vaccination behavior has a positive significant impact on satisfaction.
In the ECM, when individuals adopt a certain behavior, they will continuously evaluate their perception of their own behavior then guide their emotions and subsequent behaviors . Bhattacherjee, Perols, and Sanford (2008)  believed that perceived usefulness was regarded as a degree of utilitarianism to improve work performance, which was a long-term belief that combined expectations. According to this point of view, given that individuals have the tendency to subconsciously pursue behaviors or strive for external rewards, perceived usefulness is the decisive factor influencing individual emotions and subsequent behaviors. At present, most studies have demonstrated that perceived usefulness has a direct impact on individual satisfaction  and continuous behaviors . As far as vaccination is concerned, if the public has a high evaluation of the perceived usefulness of vaccination, then their satisfaction with vaccination will increase and they are likely to continue vaccination. Therefore, we propose the following hypotheses:
H9: Perceived usefulness has a positive significant impact on satisfaction.
H10: Perceived usefulness has a positive significant impact on continuous vaccination.
The ECM believes that satisfaction is an important variable for predicting continued use intentions and behaviors . Many scholars, working in various fields, have proven that satisfaction has a positive effect on continued use intentions and behaviors [28, 34]. In addition, from the perspective of rational people, individuals are more willing to continue to use products or services that are useful to them and that satisfy them . We believe that this evidence can also be applied in the context of COVID-19. The more satisfied the public is with vaccination, the more they tend to continue vaccination. Therefore, we propose the following hypothesis:
H11: Satisfaction has a positive significant impact on continuous vaccination.
In view of the above hypotheses, we construct the theoretical model of this study in Fig. 1.