In 2019, the World Health Organization declared the reluctance to vaccinate despite the availability of vaccination services as one of the top 10 threats to global health [1]. Since COVID-19 emerged in late 2019, COVID-19 vaccination specifically has been deemed a critical public health priority [2]. Vaccination is associated with reduced hospitalization and death rates following COVID-19 infection [3-5], as well as reductions in transmission and severity of infection [5,6]. Yet, it has proven challenging to achieve the desired rates of vaccination in the United States generally [7] and within specific subpopulations such as rural dwellers [8], political conservatives [9], and people with previous diagnoses of COVID-19 [10].

Combating uneven and inadequate uptake of COVID-19 vaccines requires an understanding of the behavioral determinants (ie, barriers and facilitators of the behavior) leading people to vaccinate or not [11]. Behavioral design frameworks, such as the COM-B (capability, opportunity, motivation, behavior) model of behavior change which suggests that capability, opportunity, and motivation are essential for behavior change, provide a categorization scheme for behavioral determinants that then allows interventionists to select techniques that target the specific barriers preventing people from completing a desired behavior [12-16]. While there are some behavioral determinants that may be addressable through directives from credible sources, as traditional public health campaigns may be described, these campaigns are unlikely to be broadly successful or sufficient in isolation to promote widespread behavior change. This is because they do not adequately account for the breadth of people’s barriers to performing the behavior or adequately enhance the benefits of the behavior [17,18]. Counteracting a crisis like the COVID-19 pandemic necessitates understanding how people make decisions, what gets in the way, and what gets people to take action [19]. Qualitative research approaches that yield richer insights might contribute to a better understanding of these nuances [20-23].

Barriers to COVID-19 vaccination in the general population include, among others, beliefs of low susceptibility to infection, perceived low severity of COVID-19, beliefs of low vaccine effectiveness, concerns about side effects, desires for more information, and not having explicit endorsement from a health care provider [24,25]. Not having received a vaccine in the past year is also associated with lower vaccination rates [26]. Barriers differ for population subgroups based on characteristics such as geographical location, work responsibilities, income level, educational attainment, and insurance status [25], several of which are social determinants of health. To effectively influence vaccination behavior within a population subgroup, researchers should seek to understand the specific barriers prevalent in that group.

One such group is military personnel and veterans. A specific study of the behavioral determinants of COVID-19 vaccination in military personnel is critical, as vaccination against infectious diseases supports operational readiness [27]. Moreover, given military veterans’ high rates of mental health issues and suicide, there has been some effort to consider veteran status as a social determinant of health [28]. A better understanding of the health decisions of military personnel therefore has the added potential of supporting improved health equity.

In early 2021, medical experts and government leaders considered the lack of COVID-19 vaccination among military personnel a potential threat to national security [29,30]. Unlike influenza, tetanus, diphtheria, pertussis, smallpox, and other vaccines, which are required aspects of force readiness, the COVID-19 vaccine was initially optional for military personnel before becoming mandatory [31] and finally becoming optional again on January 10, 2023 [32]. Having a choice architecture that makes COVID-19 vaccination optional for military personnel [33] could inadvertently undermine military readiness if vaccination uptake does not reach an acceptable threshold. Therefore, it is important to understand the reasons why military personnel might not vaccinate to appropriately intervene on those barriers.

In the United States, military personnel are socialized to a specific set of values, including honor, bravery, and personal sacrifice [34]. Each branch of the US military then has characteristics that may influence the specific behavioral determinants of its troops toward vaccination. For example, within the United States Marine Corps, values such as the need for control and independence are reinforced, sometimes to the detriment of health-promoting behaviors [35,36]. Marines report a lower likelihood of vaccine receipt than other branches of service [37,38]. These differences may reflect true differences between the branches or, more likely, may reflect underlying differences in the characteristics of individuals choosing to serve in different branches.

Junior Marines (E1-E4) experiencing a high degree of external control and loss of agency due to service [39] might feel an enhanced need to establish autonomy when given the chance, such as by declining an optional vaccine. Research has found that military personnel ranked E1-E4 were the most likely to reject an avian influenza vaccine [37]. While the Marine Corps has embraced an approach of informed health care choice for its members, there is a rocky history of medical paternalism (eg, anthrax vaccination in the 1991 Gulf War, but see [40]) that may negatively influence modern Marines’ perceptions of vaccination campaigns.

Given their overall health and fitness and the risks that come with their jobs, Marines may also have a low perceived personal risk from the COVID-19 infection and a tolerance for risk that others may find uncomfortably high. Although data were limited at the time, it was projected in early 2021 that those who contracted COVID-19 had a 0.5%-5% chance of developing a critical illness requiring intensive care or resulting in death [41]. Those odds might have seemed acceptable to Fleet Antiterrorism Security Team (FAST) Marines, who are trained to provide security forces to guard high-value naval installations, most notably those containing nuclear vessels and weapons. FAST Marines find themselves in high-risk situations as part of their regular training, for example with live fire, extreme exposure to the elements, and sleep and nutrition deprivation. FAST Marines’ daily work likely undermines their perceived severity of contracting COVID-19. In contrast, the perceived severity of contracting anthrax among 22 Department of Defense (DOD) personnel in 2015 resulted in near 100% adherence to a complex emergency postexposure prophylaxis following accidental exposure to live Bacillus anthracis spores [42].

Americans’ willingness to get the COVID-19 vaccine declined from 72% in May 2020 to 51% in September 2020 [43] and rebounded to 60% in November 2020 [44]. Surveys from early 2021 found that fewer than half of military troops and military spouses planned to receive the vaccine [45]. At the time of the study, FAST Marines in Yorktown, Virginia, were in a privileged position: they were 1 of 12 bases nationwide to be offered the COVID-19 vaccine before widespread availability [46]. Yet an informal poll of Marines in 1 FAST company showed that only 10% planned to get the COVID-19 vaccine, much lower than other estimates at the time [43,44]. The FAST Marines’ company command introduced key friction to those FAST Marines who planned to decline the COVID-19 vaccine: they had to write formal papers outlining up to 5 reasons why. This observational study makes use of those written data to understand FAST Marines’ barriers to COVID-19 vaccination.

Frequencies of coded responses can be found in Table 2. FAST Marines’ reasons for planning to decline the COVID-19 vaccine were categorized into the following 7 barriers: difficulty understanding health information (105/235, 44.7%), perceived low susceptibility of COVID-19 and its severity (33/235, 14%), concern over vaccine safety and potential side effects (29/235, 12.3%), status quo bias may impact preventive pandemic behaviors (28/235, 11.9%), distrust in health care (12/235, 5.1%), social norms may influence attitudes and beliefs about vaccines (12/235, 5.1%), and believing in COVID-19 conspiracy theories (5/235, 2.1%). The last 4.7% (11/235) of responses were categorized as other or unclear.

A total of 45% (105/235) of all responses (each FAST Marine gave up to 5), and 3 times as many as the second largest barrier, had to do with difficulty understanding medical information. In fact, 96% (45/47) of FAST Marines—all except 2 FAST Marines—indicated difficulty understanding medical information, ranging from the inability to understand basic medical information to the inability to grasp nuances in the medical literature.