The flow diagram of the systematic review according to PRISMA guidelines is shown in Fig. 1. After the first search in the databases and the removal of the duplicate records we found 233 records. Applying the inclusion criteria, the total number of studies that were included in the review was twenty-nine.
Main characteristics of the twenty-nine studies included in the systematic review are shown in Table 1.
The total number of studies consists of 26 cross-sectional studies, 2 prospective cohort studies, and 1 longitudinal study. These studies were published between 2021 and 2023. Among them, 18 investigate hesitancy or acceptance regarding the administration of a booster dose of the COVID-19 vaccine. The remaining 11 are related to hesitancy or acceptance of COVID-19 vaccination more broadly. All of them identify and explore factors that favor the administration of either the vaccine or booster doses and influence the attitudes and beliefs of healthcare professionals or students and the general population, regarding these two population groups. Specifically, out of the total of 29 studies, 15 involved healthcare workers, and 8 involved students of health sciences schools. Additionally, 3 studied the factors influencing the intention of both healthcare professionals and students of related fields, 1 referred to healthcare workers and the general population, and 2 were related to the latter two groups and students.
The majority of studies were conducted in Europe (n = 12) and Asia (n = 9), followed by North America (n = 3), Africa (n = 3), and Australia (n = 1). More specifically, 3 studies were conducted in Greece, 3 in Italy, 2 in Poland, 1 in Belgium, 1 in the Czech Republic, 1 in Wales, 1 in Albania, 2 in Saudi Arabia, 1 each in Saudi Arabia and India, 1 in Jordan, 1 in the United Arab Emirates, 1 in Vietnam, 1 in Thailand, 1 in Singapore, and 1 in Iraq. Additionally, there was 1 study conducted in Sudan, 1 in Ghana, and 1 in Nigeria, as well as 2 in Texas, 1 in New York, and 1 in Australia. It should be noted that from this continent and country distinction, one study (out of the 29) that examined healthcare professionals from a total of 91 countries was excluded.
This review investigated the factors influencing the intention of healthcare professionals and students to accept a booster dose of the COVID-19 vaccine. Additionally, it highlighted the beliefs and attitudes of these two population groups, as well as partially the general population, regarding the broader vaccination coverage of the coronavirus. Key findings of the review and main deterrent factors identified in several of the above studies include long-term adverse effects (Vellappally et al., 2022, Della Polla et al., 2022, Elbadawi et al., 2022, Le, Nguyen and Do, 2022, Al-Metwali et al., 2021, Askarian et al., 2022), and low confidence in the effectiveness (Vellappally et al., 2022, Ryalat et al., 2022, Della Polla et al., 2022, Le, Nguyen and Do, 2022, Al-Metwali et al., 2021, Ford et al., 2023) and safety of the vaccine and booster doses (Asumah et al., 2022, Alobaidi and Hashim, 2022, Le, Nguyen and Do, 2022). Furthermore, the lack of information about vaccination (Vellappally et al., 2022), uncertainty about protection against new virus mutations (Della Polla et al., 2022), the serious complications of COVID-19 (Hosek et al., 2022), the sense of fear (Della Polla et al., 2022, Galanis et al., 2023, Jairoun et al., 2022, Kałucka, Kusideł and Grzegorczyk-Karolak, 2022, Lo Moro et al., 2022), and previous COVID-19 illness (Jorgensen et al., 2023, Paridans et al., 2022), (Kałucka, Kusideł and Grzegorczyk-Karolak, 2022, Kunno et al., 2022) are additional findings of the review.
In some, but only a considerable percentage of studies, concerns about other reactions and clinical consequences (such as thromboses) (Njoga et al., 2022, Peterson et al., 2021) are observed. Doubts about the proper and adequate storage of vaccines (Al-Metwali et al., 2021) and low trust at the government and relevant public health authorities reduced collective responsibility (Alhasan et al., 2021, Bedston et al., 2023, Njoga et al., 2022, Ford et al., 2023). Negative reports, rumors, and misinformation were noted on social media, with characteristic short-term adverse effects such as pain at the injection site, fatigue, fever, chills, and musculoskeletal pain (Jairoun et al., 2022, Bedston et al., 2023, Elbadawi et al., 2022, Njoga et al., 2022, Ford et al., 2023). Finally, in a study conducted in Africa (Njoga et al., 2022), researchers also highlighted the difficulty of access to receive the COVID-19 vaccine, a phenomenon not observed in studies in developed countries. The social and demographic characteristics of the studies included in this review, relating to the willingness to vaccinate against COVID-19 booster doses, vary. Initially, the educational level of healthcare professionals, such as postgraduate students in health science schools (Vellappally et al., 2022, Jairoun et al., 2022, Le, Nguyen and Do, 2022), profession (Kałucka, Kusideł and Grzegorczyk-Karolak, 2022, Koh et al., 2022), and income, which implies leadership positions and responsibility, appear to be highly encouraging factors (Alobaidi and Hashim, 2022). It is noted that a higher and more satisfactory vaccination rate, both for primary vaccinations and booster shots, is observed among doctors, dentists, pharmacists, and medical students, in contrast to nurses and midwives (Jorgensen et al., 2023, Della Polla et al., 2022, Klugar et al., 2021, Fotiadis et al., 2021, Ford et al., 2023). Comorbidities of participants in a specific study (Alobaidi and Hashim, 2022), meaning their serious and chronic illnesses, encouraged them to get vaccinated against COVID-19. The gender of participants in the above studies (Vellappally et al., 2022 ,Jorgensen et al., 2023, Alobaidi and Hashim, 2022, Klugar et al., 2021, Hosek et al., 2022, Lo Moro et al., 2022) is a characteristic variable, with the female population being more hesitant to receive updated vaccines. Additionally, family status (Alobaidi and Hashim, 2022, Kunno et al., 2022) appears to play a role in the intention for vaccination coverage, as according to Alobaidi and Hashim, widows/widowers, divorced individuals, and single/unmarried individuals are more willing to be vaccinated than married individuals.
Other characteristics are described below. Age is a significant variable, as most studies agree that younger students are more reluctant to get vaccinated (Della Polla et al., 2022, Al-Metwali et al., 2021, Hosek et al., 2022, Peterson et al., 2021). The above fact is disapproved by a study (Tomietto et al., 2022), which reveals that Generation X (individuals born between 1965 and 1980) has the highest hesitancy compared to the other generations of Baby Boomers (born between 1946 and 1964), Generation Y (born between 1981 and 1996), and Generation Z (born after 1997). Additionally, geographical residence seems to influence the decision-making process, as urban residents are more positive about receiving vaccination doses compared to those in rural areas (Vellappally et al., 2022, Bedston et al., 2023, Njoga et al., 2022). Finally, in one study (Njoga et al., 2022), it was noted that personal ideology and religious beliefs may contribute to the willingness, or usually unwillingness, of some individuals to get vaccinated against COVID-19.
The Health Belief Model can serve as a useful theoretical framework not only to explore the motives of individuals willing to vaccinate, but even more importantly to investigate the reasons behind refusing vaccination. It is utilized by studies (Alobaidi and Hashim, 2022, Ford et al., 2023, Le, Nguyen and Do, 2022, Al-Metwali et al., 2021) in the results of the present review. The major premise of this model is that existing beliefs can predict future behaviors. Specifically, it includes five major constructs, namely: perceived susceptibility, perceived severity, perceived barriers, perceived benefits, and cues to action (refer to the cues that stimulate a specific behavior) (Al-Metwali et al., 2021).