Since its first detection in December 2019 in Wuhan, China, the coronavirus disease 2019 (COVID-19) has been increasingly threatening as a global health challenge with several waves of torrential attacks [1, 2]. To straighten this curve and end the pandemic, organizations, governments, and countries have made a series of great efforts on a global scale [2]. Despite these efforts, the world is still facing an unprecedented health crisis, with 222 countries invaded by the coronavirus, nearly 190 million infected cases, and more than 4 million deaths, making up the mortality rate at 2.16% [3]. On July 6th, 2021, Vietnam recorded 19,933 confirmed cases [4], signaling the rebounding of the virus in this country. In addition, several novel variants of the virus have also been detected in many countries, with faster speed and various modes of transmission. From these evidence, it is undeniable that current surface prevention strategies for COVID-19 seem somewhat ineffective. Therefore, developing strategies to generate community immunity from an individual level is important.

Herd immunization by vaccination against COVID-19 has been reported to be safe, effective, and attracting worldwide attention [5, 6]. A growing body of evidence has supported the protective efficacy of vaccination against COVID-19 [7-14] that vaccinated individuals are more likely to be protected from required hospitalization if infected, compared with non-immune individuals. Mortality from infection in the vaccinated population was also reported to be significantly lower than in the control group. Therefore, the earlier the establishment of individual and herd immunity by COVID-19 vaccination is completed, the higher the protective effects are achieved. The spreading of COVID-19 is believed to successfully end once a high herd immunity threshold at 55-85% has been reached [15] by decreasing the probability of susceptible individuals getting exposed to infectious sources.

However, although health agencies and governments have been making great efforts to raise COVID-19 herd immunity coverage, the proportion of people’s willingness to receive the COVID-19 vaccine varied widely across countries, ranging from very low (< 50%) [16-22], moderate (50-70%) [23-36] to significantly high (> 70%) [37-45].

In addition, the willingness for COVID-19 vaccination was also influenced by many factors. To be detailed, the probability of vaccination orientation was found to be positively related to perceptions of severity or vulnerability to COVID-19 [46, 47], awareness of the benefits of COVID-19 vaccination [20, 26, 46, 47], beliefs on vaccine safety [26, 35, 37, 48], sufficient knowledge regarding the disease [46], high education level [18, 29, 31, 47], history of vaccination in the past [37, 47, 49], elderly [43], undergoing chronic diseases [44, 49], getting married [37] and some others. Furthermore, other features, such as unemployment, low income or no insurance [23], non-medical occupational background [43] were reported to hinder people’s vaccination decisions. In addition, some factors have shown an inconsistent correlation in some studies, such as gender [19, 26, 44].

In Vietnam, little is known about the community’s willingness to receive COVID-19 vaccination and its predictors. A few studies were conducted previously on the intention to vaccinate against COVID-19 among healthcare workers, adult patients with chronic illness, and health science students, with the found proportions at 76.1% [46], 84.0% [50], and 77.7% [51] respectively. However, the surveys in these works were delivered to the target population when the COVID-19 pandemic status in Vietnam was still under well control [52], and the arrival of a potential COVID-19 vaccine shipment had yet to be confirmed. Furthermore, the acceptance of vaccination seemed to vary over time under different conditions, resulting in the unavailability of reliable data regarding the level of intention to get COVID-19 vaccination in the current pandemic outbreak status.

Community vaccination is an undelayable action to find a drastic solution to the threat of COVID-19 in the current context. Therefore, it is important to conduct a large-scale investigation into the knowledge, belief, and acceptance of the affected community for COVID-19 vaccination under the present circumstances. Updated and broadly representative evidence is essential to provide valuable data in the new situation, which is helpful to develop or change strategies and promote the speed and effectiveness of COVID-19 herd immunity. Therefore, this study was conducted to evaluate the acceptance to receive COVID-19 vaccination and its predictors among the Vietnamese people in the regions that were strongly affected by the current COVID-19 relapse, including Ho Chi Minh City and some other areas. To adapt to the lockdown condition during the pandemic, we decided to distribute the survey to participants via the internet.

Up to the time of this study, Vietnam had been suffering the fourth hit of the COVID-19 pandemic and it had been the largest outbreak since the first proclamation of COVID-19 in January 2020. Also, the very first vaccines against COVID-19 began to be officially introduced to the Vietnamese at that time. Although mass vaccination for community immunity has proved its success in preventing the spread of transmissible diseases, refusal from getting vaccinated has posed a challenge to herd immunity, allowing preventable pathogens to persist in the population with inadequate vaccination rates. Being aware of this fact, in the face of the urgent situation of creating herd immunity to combat the pandemic, the Vietnam Government announced encouraging policies to promote residents’ vaccination acceptance. This study was conducted under such circumstances to investigate the public acceptance of COVID-19 vaccination.

The sample of this study has some features in accordance with the country’s demographics, in which the proportion of women was higher (65%) and the young age distribution (34.21±10.09) in this study was consistent with the data of Worldometer about Vietnam's population (50.1% and 32.5 years, respectively) [60].

Noticeably, since internet users were the target population, this study highlighted the widespread usage of social media on the internet as the most important channel for keeping updated with COVID-19 vaccine information, which was also noticed in other studies in Vietnam [46, 51] and Nigeria [36]. There was a significantly lower proportion of television utility to obtain news of the vaccines in this study (46.6%) compared to a previous study also in Vietnam (82.4%) [50]. It might be due to 76.1% of the current study sample aged 20-30. Staying informed by surfing the web has become a daily habit among the young population, which explains the internet as the most preferred information channel in this piece of work. Meanwhile, given the demographics as adults aged 52.9±15.6 years with undergoing chronic illness, it was not much surprising that the most preferred channel of information for Huynh et al.’s [50] subjects was traditional television, which understandably interpreted for this difference. Additionally, given that during the time of sampling, Ho Chi Minh City was the hardest hit place by the outbreak in Vietnam [61, 62], the considerable proportion of Ho Chi Minh citizens in this sample population was understandable. In addition, due to the nature of an online survey, urban citizens were likely to have a higher chance to access the questionnaire via the internet than rural people. Moreover, the majority of individuals residing in urban described in this study were also in agreement with that reported by Urrunaga Pastor et al. [63].

Of note, this study found a significantly higher percentage of acceptance of those accessing COVID-19 vaccine information through academic articles. Obviously, in the development process for any novel vaccine, certain phases need to be performed in an ordered and chronological pathway [64, 65]. Although certain stages might be simplified in the urgent need for a COVID-19 vaccine, results related to this process need to be reported through scientific articles, especially in major journals. In the context of the thirst for reliable information, thus, compared with traditionally given information, scientific results in prestigious journals have particularly high credibility, benefiting audiences in their decision-making to vaccinate. Therefore, this was reasonable that individuals tracking information through academic documents had a higher vaccination acceptance rate.

With 5 items measuring knowledge, most of the participants who submitted correct answers held sufficient knowledge related to the COVID-19 vaccine. The percentage in our study was significantly higher than previous studies with a wide range of individuals giving correct answers of 40.8% to 98.1% [51], 52.2 to 97.6% [46], and 40.0% to 93.2% [50]. These differences can be partially explained by looking at the characteristics of the subjects. While the participants in the reference studies were the elderly [50], with somewhat limited access to information, most of our subjects were young individuals and well-educated with at least a diploma degree, working as healthcare practitioners or in medical-related fields, professionals, or technicians, which all enabled them to have faster and more accurate approaches to information. Moreover, in contrast to the provincial-sited study of Nguyen et al. [51], most of the current participants resided in the most dynamic Ho Chi Minh City, which also partly reflected individuals' forceful access to the torrential flow of information about COVID-19 vaccination campaigns, affecting their level of knowledge. Furthermore, the results of any study are likely to depend on the context and time when the study is performed. Compared to previous studies, this work was conducted when educational information about COVID-19 vaccination was introduced to the community more abundantly and regularly than ever since Ho Chi Minh City had been the most seriously attacked by the 4^th COVID-19 wave, compared to other neighboring provinces. This demonstrated the influence of long exposure and density of information on the community's knowledge.

Regarding domains of the HBM, the findings presented significantly higher mean scores of all domains than the other two studies conducted in Vietnam [50, 51]. Similar trends were observed in parts of cues to action and perceived benefits with higher mean scores in this study (4.14±0.56 and 4.21±0.63, respectively), compared to the referred studies (4.0±0.6 and 3.1±0.7 by Nguyen et al. [51] and 3.96±0.51 and 2.78±0.83 by Huynh et al. [50]). Up to the time of this study, the terrible severity in terms of the pandemic's spreading speed, fatality, and vulnerability was undoubtful. The community, therefore, witnessed the unprecedentedly serious susceptibility and severity caused by the pandemic. On the other hand, as indicated in Table 2, despite doubts about the vaccine's protective efficacy (2.89±1.07), respondents seemed to be more concerned about its safety or side effects (3.49±1.05). These results were in agreement with findings that concern for vaccine side effects was the most significant determinant of a higher likelihood of refusal [66], which suggested that besides educating about vaccine effectiveness against COVID-19, correct information about its side effects should be emphasized to promote the public’s acceptance. Moreover, while the higher the mean score implied the greater the level of hindrance, this study showed a significant perception of barriers (2.87±0.76) than the previous one (2.4±0.5) [50].

It is understandable that at the time of Nguyen et al.'s study [51], the first batch of the COVID-19 vaccine in Vietnam was just assigned to very few prior frontline personnel engaged in the COVID-19 battle but not widely offered for the community [67, 68]. This means that unexpected information about the vaccine and its post-injection effects was not too common in Vietnam, partly illuminating for the negligible level of community concerns then.

In contrast, when the current study survey was disseminated to the target population, the vaccination campaign had already occurred over a certain period. The entire public’s interest seemed to focus on the safety of vaccine first doses in humans and the worrying reported numbers of deaths or serious complications after their emergent use, not only in the world but also in Vietnam [69-71]. Therefore, instead of properly understanding the prevalence of these side effect events, which represented a very small and unsubstantial percentage relative to the vaccinated population, the community had become obsessed with concerns about these misinterpreted figures. In addition to the lack of properly explanatory information for reference provided by healthcare professionals or trustworthy channels, this phenomenon was also influenced by the complexity of the medical specialty, which might require a certain level of knowledge of the audiences. Moreover, seeing reports of adverse reactions or deaths caused by anaphylaxis following COVID-19 vaccination led the community witnesses, mostly without health science knowledge, to worry about its safety, resulting in a higher anxiety score as well.

Regarding the acceptance of the citizens to take the COVID-19 vaccine, this study showed a much higher acceptance rate (87.8%) than other studies conducted in Vietnam or other regions, which reported vaccination intention rates ranging from 27.7% - 83.8% [16, 20, 21, 24, 26, 27, 29, 31, 38, 39, 42, 44, 45, 51, 72]. The cited works and the current ones were similar in the justification for the differences in the HBM domains scores. However, while the vaccine was not introduced to recipients but only through social media or clinical trial results at the time of the previous studies, the participants in our work had real experiences with the COVID-19 vaccine, in which people around them were vaccinated in turn. This could be the most plausible explanation for the aforementioned difference.

Furthermore, the onslaught of the fourth COVID-19 wave at the time of this study partly convinced the participants to choose in favor of vaccination when they weighed between subjective concerns and rare consequences and the unambiguously devastating impact of the pandemic. Additionally, this acceptance rate was lower than a study conducted in March 2020 in the Chinese community, with 91.3% [37]. Wang et al. introduced the survey when China was in an unprecedented state of turmoil due to the COVID-19 pandemic, with the first-ranked number of cases and deaths worldwide [72, 73]. These life-threatening events possibly influenced these people's choice of life pursuits, which was also observed in the present study. Considerably, at that point of the survey, Vietnam only approved the emergency use of 3 vaccines provided by the world's leading vaccine and pharmaceutical manufacturers, Oxford/AstraZeneca COVID-19 vaccine, Pfizer-BioNTech COVID -19 Vaccine, and Moderna COVID-19 vaccine [74]. The beliefs in the safety and effectiveness of these vaccines, combined with the urgent need for community immunization also explained the high vaccination acceptance rate of the participants in this study.

When looking at the willingness to encourage others to vaccinate, a similarly high proportion (88.5%) of acceptance was observed. Interestingly, despite only 35.5% being definitely willing to get vaccinated for themselves, the number answering “definitely yes” to recommend their family, friends, or others to take COVID-19 vaccination climbed up to 70.8%. Viewing from the personalized aspects, human nature was understandable to hesitate and take a more cautious attitude to decide when facing a dilemma between vaccine efficacy and safety. They preferred to wait for others’ vaccination experience as a prior reference [66]. Therefore, it was possible to partly clarify their stronger preferences for the vaccination of their friends or others.

The present study revealed some significantly relative determinants that describe respondents as “vaccination advocators”. Similar to previous studies [16, 20, 25, 46, 47, 51, 53, 75], we also found positive associations of vaccination probability with perceived benefits and cues to action and a negative one with perceived barriers. These correlations fitted and followed the HBM [54, 55] cited in previous studies [37, 46, 50, 51]. A person would be likely to commence a suggested preventive health action when they were aware that their health was being threatened or progressively cued to action, or the perception of benefits they got was heavier than the barriers [76].

Another positive correlation between the number of sources of information and intention to receive vaccination was also observed. Unsurprisingly, by early accessing more information sources, especially accurate and updated news broadcasted by official providers, audiences' knowledge and positive perception of COVID-19 vaccines were further improved the most effectively, subsequently navigating them to the more intelligent choices of vaccination. This encourages the active running of providing authoritative information, which would take advantage of diverse communitive platforms to keep the community informed and make rational decisions towards their vaccination.

However, there were unpredictable results of negative correlations of vaccine willingness with the employment status change from employed to unemployed, current circulation of COVID-19 in participants’ residents, and the number of chronic illnesses. It was logically anticipated that unemployed people tended to accept vaccination so that they could return to normal life soon. Nonetheless, people experiencing unemployment due to COVID-19, especially in the situation of the city curfew and lockdown policies, were mostly grouped as unprofessional with lower literacy levels, such as public servants or general workers. As a result, the knowledge and perception convincing them to get vaccinated were prone to be less sufficient than those with essential professional or higher education.

Similarly, people with more chronic diseases are often elderly with less accurate access to information and in a more passive way. Furthermore, given some worrying news about the side effects of the vaccines, these people, by their nature of poor health and belonging to the vulnerable population, seemed to face higher hesitancy, which explained why the higher number of chronic diseases they got, the lower possibility of vaccination acceptance. This was also consistent with the result that more consideration of vaccine side effects and health risks made people more hesitant to vaccinate [66]. On the other hand, living in a “red zone” might make people afraid to step out of their houses, which they consider a safer environment to protect them from virus exposure. Thus, excessive fear could also be another reason for their going-out hesitation, including refusing to undergo grouped rapid testing or vaccination. In addition to insufficient awareness of the vaccine benefits, this could mislead them to believe that it would be better to stay at home without contacting anyone than to go out for vaccination and then get the risk of virus exposure and vaccine side effects. This might explain why people in “red zones” delayed their vaccination willingness.

Vaccine undoubtedly receives a high agreement about its crucial role in controlling and eradicating a series of transmissible diseases. The vaccine uptake acceptance rate and its predictors in the present study reflected a general picture of the community’s awareness and action orientation on COVID-19 immunization. The level of participants’ acceptance of COVID-19 vaccination in this study was impressively high, reflecting the efforts of health authorities and the government. The contribution of influential healthcare professionals or authoritative information to vaccination probability implied that in order to achieve significant and rapid results, educational information about COVID-19 vaccines should be widely broadcasted in prominent communication channels such as social media platforms and television by a combination of government and influenceable healthcare providers from medical institutions. It also highlighted the role of citing reliable scientific articles while informing the public to build their confidence in vaccine safety. Moreover, through the study results, during the design of these immunization campaigns, governments and health care managers should pay more attention to people with low acceptance levels, such as chronically ill adults, and rectify confusing information to correct the public’s awareness about immunization and herd immunity benefits. Moreover, the design of educational campaigns for herd immunity against COVID-19 should focus more on offering accurate information about the side effects and safety.