Introduction

The coronavirus disease 2019 (COVID-19) pandemic has triggered a global public health emergency, demanding a coordinated and robust response from healthcare systems worldwide. This crisis underscored the critical role of integrating primary healthcare (PHC) with other components of the Healthcare Network to effectively address emergency needs, both in terms of prevention and care of individuals affected by severe acute respiratory syndrome coronavirus 2.

In Brazil, PHC represents the central entry point to the Brazilian Health System (in Portuguese: Sistema Único de Saúde–SUS) [1], and its role involves organizing services and implementing actions that promote comprehensive health care for individuals and communities, addressing the impact of social determinants of health on the health-disease-care process [2]. As the first point of contact, PHC should be easily accessible [3] as an equitable, holistic, and comprehensive care so that individuals can use it whenever. In Brazil, PHC has extensive national coverage, allowing for a comprehensive clinical approach that considers the health conditions of the population in social, political, and economic dimensions. These PHC characteristics played a critical role in tackling COVID-19 by enabling responses adapted to local realities [4].

According to Donabedian [5], access to health services can be analyzed from two perspectives: geographical, which examines physical barriers to care, and socio-organizational, which investigates the administrative, social, and structural aspects that hinder access to adequate care. The combination of extensive operational reach and territorial knowledge allows effective responses to health crises.

Brazil was severely affected by the COVID-19 pandemic, ranking second worldwide in terms of the number of deaths in 2020, recording a peak of 4,148 daily deaths in April 2021 [6]. The country recorded 39,181,954 cases and 715,108 deaths by mid-February 2025, with a case fatality rate of 1.8% [7]. There was a 6% increase in cases (194,000 newly reported cases) and a 24% reduction in the number of deaths compared with the previous period, observed between November 11 and December 8, 2024 [7]. In this context, PHC plays an important role in reorganizing care flows, expanding access, and absorbing mild to moderate cases, contributing to the reduction of inequities in care provision [8].

Several countries began vaccine development research as early as 2020, considering the high transmissibility of COVID-19. In Brazil, the immunization process began on January 17, 2021, with the administration of CoronaVac following emergency use authorization by the Brazilian Health Regulatory Agency [9]. The National Plan for Operationalization of COVID-19 Vaccination initially prioritized vulnerable groups, such as people aged 60 years or older, individuals living in long-term care facilities, and healthcare workers [10].

During the pandemic, the vaccines administered in Brazil were incorporated into the Ministry of Health’s National Immunization Program (in Portuguese: Programa Nacional de Imunizações–PNI), which serves as a benchmark for international health organizations [11,12]. The following COVID-19 vaccines were included in this program at various stages of the pandemic: Pfizer/BioNTech (BNT162b2), AstraZeneca/Oxford (ChAdOx1), Janssen/Johnson and Johnson (Ad26.COV2.S), and CoronaVac/Sinovac Life Sciences [13].

Vaccination prevents an increase in severe cases by 74% according to studies and reduces mortality by 82% [14]. Despite advances in immunization, the political context in Brazil during the pandemic influenced vaccine hesitancy [15]. During the pandemic, the President of Brazil repeatedly questioned health measures and promoted disinformation about vaccines, claiming that they contained toxic substances and that natural immunity was more effective [16]. This narrative may have negatively influenced vaccination adherence, because the government’s stance may have triggered insecurity and disinformation in the population. In fact, over 1.5 million Brazilians missed their second vaccine dose by April 2021 owing to unfounded fears, logistical difficulties, and confusion regarding dose intervals [9].

Considering the lack of information on the factors influencing the completion of COVID-19 vaccination schedules among socially vulnerable populations in Brazil, this study aimed to analyze the factors associated with COVID-19 vaccination schedule completion among adults in high-vulnerability neighborhoods in two Brazilian state capitals.

Materials and methods

Study variables

The outcome variable was COVID-19 vaccination schedule completion, categorized as follows:

1. i). Incomplete schedule: the participant reported receiving only the first or second dose or a single-dose vaccine.
2. ii). Complete schedule: the participant reported receiving the primary series (i.e., first and second doses or single-dose vaccine) plus at least one booster dose.

In Fig 1, the timeline of COVID-19 vaccination in Brazil is illustrated. At the time of data collection, COVID-19 vaccines were only available at PHC units. The following vaccines were being administered: CoronaVac, Pfizer, Oxford/AstraZeneca, and Janssen, with the latter being a single-dose vaccine.

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Fig 1. Timeline of COVID-19 schedule implementation in Brazil.

https://doi.org/10.1371/journal.pone.0346091.g001

The following potential associated variables were analyzed:

1. i). Sociodemographic characteristics: Age (18–30, 31–50, > 50 years), sex (cisgender man, cisgender woman, transgender people), race/skin color (White, Brown, Black, Indigenous, Asian were grouped in Black/Brown versus others), education level (up to elementary/middle school, high school complete or incomplete, higher education, and graduate), Evangelical religion (no, yes), and monthly family income were categorized using two minimum wages as a cut-off. In 2022 the minimum wage in Brazil was BRL 1,212 (approximately USD 232, based on the 2022 average exchange rate of 1 USD ≈ 5.22 BRL), and density of people per room was calculated by dividing the number of individuals living in the household by the total number of rooms. This variable was categorized using a cut-off point of 0.5: low density (<0.5), indicating better living conditions, and high density (≥0.5), indicating poorer living conditions.
2. ii). Clinical aspects: Participants were asked to self-report diagnosed chronic comorbidities with a predefined list of specific chronic conditions by answering “yes” or “no” to each: obesity, diabetes mellitus, heart disease or high blood pressure, respiratory diseases (e.g., asthma, pulmonary emphysema, tuberculosis), current or past cancer treatment, hematological diseases (including sickle cell anemia), advanced-stage chronic kidney disease, chromosomal conditions associated with immune fragility (e.g., Down syndrome, Turner syndrome), liver diseases (e.g., fatty liver, hepatitis, cirrhosis), autoimmune diseases (e.g., systemic lupus erythematosus, rheumatoid arthritis, immune thrombocytopenia), immunodeficiencies (e.g., HIV infection, leukemia), or were asked to specify other conditions. The participants were then classified as having none, one comorbidity, or two or more comorbidities. Self-perceived risk of contracting COVID-19 (none, low, medium, or high).
3. iii). Access to health services: Forms of access to health services (exclusively through SUS (i.e., public), private, public, and private), household registration at a Family Health Unit (FHU) (yes, no, do not know), last medical consultations (in the last 12 months more than 12 months or never), sought care at the health unit in the last 12 months (yes, and was successful; no or unable to secure an appointment/slot), and reporting of any type of discrimination in the health service (no, yes).

Results

A total of 12,286 participants were registered in the TQT-COVID-19 Study, and 7,193 adults who received at least one dose of any COVID-19 vaccine were included (Fig 2).

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Fig 2. Flowchart of the study population inclusion.

https://doi.org/10.1371/journal.pone.0346091.g002

Among the 7,193 adult participants, most had a complete schedule (79.35%), 2,628 (36.53%) received at least one booster, and 3,080 (42.82%) received two boosters. Among those with incomplete schedules, a small proportion did not complete the primary series (n = 176; 2.45%) (Fig 3).

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Fig 3. Number of doses received and COVID-19 vaccination status among adults participating in the TQT-COVID-19 Study.

(A) Percentage of participants by number of doses received. (B) Distribution of participants by vaccination status. The incomplete schedule includes the primary series (i.e., first dose and/or second dose, or a single-dose vaccine). The complete schedule refers to the primary series plus at least one booster dose.

https://doi.org/10.1371/journal.pone.0346091.g003

Most participants resided in Salvador (76.64%), were cisgender women (70.26%), had Black or Brown race/skin color (84.58%), were aged between 31 and 50 years (43.26%), and had a complete or incomplete high school education (49.59%). Additionally, 35.51% reported being Evangelical, most participants preferred not to respond to income questions, and 27.37% reported an income between BRL 1,212.01 and BRL 2,424.00 (Table 1).

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Table 1. Sociodemographic, clinical, and health service characteristics, overall and by COVID-19 vaccination schedule completion, among adults participating in the TQT-COVID-19 Study, Brazil, 2022–2023.

https://doi.org/10.1371/journal.pone.0346091.t001

Most respondents resided in households with medium or high density (64.09%) and perceived their risk of contracting COVID-19 as moderate or high (72.65%). Most participants (67.22%) reported no comorbidities. With respect to health service utilization, 68.26% accessed services exclusively through the SUS, and 63.18% had household registration with an FHU. In the 12 months preceding the survey, 78.67% reported having had at least one medical consultation, 79.49% had sought care at a health unit in the last 12 months and were attended to, and 6.18% reported experiencing discrimination within health services (Table 1).

In the multivariate analysis (Table 2), older participants were significantly more likely to complete the COVID-19 vaccination schedule than younger adults. Individuals aged 31–50 years had 49% higher odds of completion (OR=1.49; 95%CI: 1.30–1.71), and those older than 50 years had nearly four times higher odds (OR=3.83; 95%CI: 3.11–4.70) than participants aged 18–30 years. Cisgender women also had 38% higher odds of completing the vaccination than cisgender men (OR=1.38; 95%CI: 1.20–1.57). Higher educational attainment was strongly associated with completion: participants with complete/incomplete high school had 37% (OR=1.37; 95%CI: 1.17–1.60) higher odds, and those with higher education or graduate degrees were more than twice as likely to have completed the vaccination schedule (OR=2.44; 95%CI: 1.99–2.99) compared with those with up to elementary or middle school education. In contrast, identifying with an Evangelical religion was associated with lower odds of completion (OR=0.63; 95%CI: 0.56–0.72). Participants living in more crowded households (≥1 person per room) were less likely to complete the vaccination program (OR=0.69; 95%CI: 0.57–0.82). Comorbidities increased the likelihood of vaccination, particularly in those with two or more conditions (OR=2.14; 95%CI: 1.56–2.92). Finally, greater access to and use of health services were positively associated with vaccination: individuals using both public and private services (OR=1.56; 95%CI: 1.30–1.89), those who had a medical consultation within the past 12 months (OR=1.60; 95%CI: 1.38–1.85) and those who sought care at a primary health care unit in the last 12 months and were able to get it (OR=1.20; 95%CI: 1.03–1.41), were more likely to have completed the vaccination schedule. All variables presented VIF values below 2 (mean VIF = 1.31), indicating the absence of relevant multicollinearity. The multilevel analysis (S1 Table) showed a very low intraclass correlation coefficient (ICC = 1.2%), indicating minimal clustering of the outcome at the UBS level. Additionally, odds ratios and corresponding confidence intervals were nearly identical between the multilevel and standard logistic regression models, with no substantive changes in the interpretation of the results. Therefore, the standard logistic regression model was retained, as the main conclusions remained unchanged.

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Table 2. Multivariate analysis of TQT-COVID-19 Study adult population variables associated with COVID-19 vaccination schedule completion in two Brazilian state capitals, 2022–2023. (N = 6,827).

https://doi.org/10.1371/journal.pone.0346091.t002

Discussion

The findings highlight important sociodemographic and healthcare-related determinants of completing the COVID-19 vaccination schedule. We identified a higher proportion of COVID-19 vaccination schedule completion among those with greater engagement in health services. A higher vaccination completion rate was also associated with older age, female cisgender identity, and higher educational attainment. Conversely, lower completion rates among younger participants, those identifying with the Evangelical religion, and individuals living in crowded households highlight social and structural barriers that may have influenced vaccination uptake.

High vaccination adherence among older individuals aligns with the policies prioritizing this group in the National Plan for the Operationalization of COVID-19 Vaccination. According to national data, in February 2022, 60% of the population aged 60 years or older had received the COVID-19 booster dose, reflecting the effectiveness of the strategy aimed at protecting this vulnerable group [8]. Immunization has become an essential tool for preventing severe infections and mortality associated with vaccine-preventable diseases with aging [20]. Modeling studies indicate that the implementation of COVID-19 vaccination in Brazil had a substantial impact on reducing hospitalizations and deaths among older adults. Between January and August 2021, it is estimated that more than 167,000 COVID-19 hospitalizations among individuals aged 60 years or older were averted, and during the same period, vaccination may have saved more than 58,000 lives of older adults [21]. These data emphasize the effectiveness of vaccines in reducing morbidity and mortality associated with COVID-19.

Another prioritized population was individuals with comorbidities and noncommunicable diseases (NCDs) owing to their increased vulnerability to COVID-19, which increases disease severity, hospital stay, and mortality risk [22]. Individuals with NCDs tend to use health services more frequently because of their continuous need for medical care. Consequently, the increased use of services may explain the higher vaccination rates among these patients [23]. The pandemic may have further exacerbated this situation by reducing access to elective consultations for individuals with NCDs [24,25].

Studies indicate that women show higher vaccination adherence than men; for instance, a study conducted in Brazil revealed that women had less vaccine hesitancy than men, despite men having a higher risk of clinical worsening of the disease [26]. Vasconcelos et al. highlighted that factors such as risk perception and self-care positively influence this discrepancy in vaccine adherence between the sexes [27]. In this regard, women express higher concern for disease prevention and risk perception, which contributes to higher vaccination rates [28].

Educational level is also a relevant factor for vaccination adherence. Silva et al. documented that individuals with higher educational levels tend to have lower vaccine hesitancy, owing to greater access to trustworthy scientific information and less exposure to disinformation [29]. Likewise, Edwards et al. demonstrated that individuals with higher education are more likely to trust science and reject conspiracy theories about vaccination [30].

The relationship between household crowding and socioeconomic status is well documented in scientific literature, with evidence suggesting that larger families often face considerable economic challenges. These challenges are often associated with precarious housing conditions and limited access to essential resources [31]. Monteiro and Veras reported that housing is a high-cost commodity with selective access that disproportionately excludes economically disadvantaged groups [32]. Such findings underscore the critical need for public policies aimed at improving housing and the broader socioeconomic conditions of larger families. Such policies are essential for fostering social inclusion and ensuring equitable access to the basic services necessary for a dignified life. Crawshaw et al. found that hesitancy was more evident in regions with greater social inequality [33]. In Brazil, the vaccine uptake intention is notably lower in regions with increased social inequality [34].

Vaccine hesitancy is a complex concept, and religion played an important role in vaccination completion among the study participants. Studies have shown that religious beliefs and disinformation are determining factors for vaccine denial [15]. The dissemination of false news was another critical factor in reducing vaccination adherence, as reported by Gonçalves et al. [15,33].

This study had limitations that should be acknowledged. First, its cross-sectional design does not allow the establishment of causal relationships between the factors analyzed and the completion of the COVID-19 vaccination schedule. Second, the type of vaccine received, which could have influenced schedule completion owing to misinformation or preferences, was not adequately collected, preventing a more detailed analysis of this aspect. In addition, income, an important socioeconomic indicator, had a high proportion of missing data and was therefore excluded from the bivariate and multivariate analyses. However, other variables that reflect socioeconomic conditions, such as educational level, household crowding, and access to private health services, were included and consistently indicated that individuals with greater socioeconomic disadvantages were less likely to complete the vaccination schedule. Furthermore, external validity may be limited, as the study was based on a non-probabilistic sample recruited through health services. Consequently, caution is warranted when generalizing the results beyond similar service-based populations. Despite these limitations, the study has notable strengths, including its large sample size, ability to assess the relationship between healthcare engagement and vaccination completion, use of a standardized questionnaire, and employment of trained interviewers.

Conclusions

This study demonstrated that sociodemographic and healthcare-related factors play a pivotal role in supporting adherence to COVID-19 vaccination, contributing to higher coverage rates and reducing inequalities in vaccine access. Individuals in continuous contact with the healthcare system exhibited higher rates of vaccination schedule completion. Proactive outreach by Brazil’s PHC teams was fundamental in facilitating vaccination access. Moreover, age, sex, and educational attainment significantly influenced adherence, highlighting the need for targeted policies for vaccine-hesitant groups.

The implementation of educational initiatives, in conjunction with active outreach and home visits by CHWs, could further improve vaccination uptake. Strengthening PHC, along with clear and evidence-based communication, is essential for addressing vaccine hesitancy and advancing health equity. In conclusion, PHC should continue to be reinforced as a strategic pillar of the national immunization policy to ensure broad, equitable, and effective access to vaccination, both for COVID-19 and future public health emergencies. Investments in PHC, health education, and strategies to combat disinformation are crucial for increasing vaccination coverage and decreasing inequities in access to vaccines.

Supporting information