3.2. Sample

Vietnam is considered as an appropriate nation to recruit the data and examine the perceived barriers on intentions to receive COVID-19 vaccines under the mediation and moderation mechanism of self-efficacy, attitude towards COVID-19 vaccines, and psychological distress because the rationales as follows. First, the success of addressing the COVID-19 spread in three first waves by applying 5K (Khẩu trang-mask-wearing, Khử khuẩn-disinfection, Khoảng cách-social distancing, Không tụ tập đông người-No garthering, and Khai báo y tế- health declaration) has been proved in this country [3,12,28,39]. Second, at the time of this conducted research, although the self-development of COVID-19 vaccines (Nanocovax) was conducing in Vietnam, almost all Vietnamese people still did not receive COVID-19 vaccines and waited for the sources of COVID-19 vaccines from international supports.

The following formula has been used to estimate a minimal size of sample.

N = 100 + X ∗ Y, [29,40].

With X presenting for even per variable (the recommended X is 50) and Y representing for number of independent variables used in the study (Y = 5). Thus, the minimal sample size was 350.

The convenient sampling methodology and an online-based survey were used in our study to collect the dataset from 20 July to 20 October 2021 in Vietnam. In this period, Vietnam was undergoing the fourth serious wave of the COVID-19 pandemic [19,41]. Certain restrictions and social distancing measures had been implemented. Therefore, the use of an online-based survey utilizing Google Forms was more appropriate [6,29,30,31]. Participants were clearly informed that their participation was completely voluntary, they could withdraw from the survey at any time, and all information was confidential and only used for academic purposes. In total, 9000 online questionnaires were directly distributed through personal emails, and messages on Facebook, Zalo and Viber, to invite participants to take part in the online-based survey. Only 2913 people participated in our survey. However, 191 responses are eliminated due to containing the missing data. After removing the invalid response, finally, 2722 questionnaires were completely filled, and the response rate reached 30.2%. Table 1 provides the demographic information of respondents. While the largest proportion of respondents were female (56%), most of the respondents were aged 18 to 28 years old and accounted for 60.7%, followed by 29–28 (21.0%), 39–48 (13%), 49–58 (3.8%), and over 59 years old (1.6%). Moreover, 57.7% of respondents earned less than 10 million VND each month; 68.6% of them held a bachelor’s degree, while 66.3% were single. In total, 890 respondents received at least one dose of the COVID-19 vaccine. Finally, the largest proportion of respondents wished to be vaccinated with AstraZeneca (29.1%), followed by Pfizer (19.5%), Moderna (15.3), Sputnik-V (6.6%), Sinopharm (6.8%), Johnson and Johnson (1.9%) and others (Abdala, Hayat-Vax, Janssen, etc.) (14.7%). Interestingly, 6.1% respondents wished to receive Nanocovax, which is a Vietnamese COVID-19 vaccine candidate developed by Nanogen Pharmaceutical Biotechnology JSC.