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 Page 5/22 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 theory VH refers to a delay in acceptance or refusal of vaccination despite availability of vaccination services [8]. In 2019, the World Health Organization (WHO) listed VH as one of the top 10 threats to global health [9]. Today, VH is a global trend, with approximately 90% of countries across the globe reporting this situation [10]. 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 [11]. It could hinder the complete implementation of the immunization program and increase the risk of preventable disease outbreaks and epidemics [12]. 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) [13], we use three second-order factors of complacency, convenience, and confidence to measure VH. In particular, complacency denotes that an individual believes that the COVID19 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 [14]. According to the HAPA model, for health behavior, the individual expects to maximize its positive outcomes and minimize its negative outcomes [15]. Currently, many studies have shown that positive outcome is expected to predict behavior intention [16]. 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) [17], 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 Page 6/22 subjectively believe that the risk and cost of this behavior are high, their willingness to implement it will be greatly reduced [18]. Currently, the side effects, efficacy, rigorousness, and other issues of the COVID19 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 [19]. During this process, an individual will exert perceived selfefficacy 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 [20]. In the context of