This study aimed to identify the determinants of COVID-19 mortality and temporal trends across São Paulo state’s Regional Health Departments (DRS) [
6]. Using GAMLSS, we found that higher mortality from chronic non-communicable diseases was associated with a lower COVID-19 mortality rate. Conversely, increases in the older adult population, urban populations, and COVID-19 vaccination were associated with higher COVID-19 mortality. Globally, COVID-19 continues to cause significant mortality, exacerbated by emerging variants [
16]. In Brazil, particularly São Paulo state, certain regions were disproportionately affected. From February 2020 to December 2022, Brazil reported 34,542,868 cases and 691,810 deaths, representing incidence and mortality rates of 17,069 and 329.2 per 100,000 inhabitants, respectively [
17]. São Paulo reported even higher rates (13,622.2 and 385.2 per 100,000, respectively) during this period. Among São Paulo’s DRS, DRS I (Grande São Paulo) and DRS IV (Baixada Santista) recorded the highest numbers of cases and deaths. DRS I, encompassing 39 municipalities including the state capital, and DRS IV, with nine municipalities including the major port city of Santos, experienced the greatest impacts [
5,
18]. The first imported COVID-19 case in Brazil was confirmed in São Paulo on February 26, 2020, followed by the first death on March 12, 2020 [
9]. By April 2020, all DRS had reported cases and deaths [
9]. The high mortality in DRS I and IV may be attributed to these regions’ status as economic centers, with large populations and extensive healthcare infrastructure leading to greater demand for services [
18]. The spread of COVID-19 from São Paulo’s capital towards the interior, particularly noticeable in the first year, followed major highways, connecting the capital to coastal and interior regions [
18]. This study’s findings align with existing knowledge regarding COVID-19’s clinical pathophysiology and the influence of various factors on mortality. Higher mortality from chronic non-communicable diseases (NCDs) correlated with lower COVID-19 mortality rates. Individuals with NCDs (hypertension, diabetes, respiratory, and cardiovascular diseases) have been linked to more severe COVID-19 outcomes and higher mortality [
19,
20,
21]. Meta-analyses identified COPD, cardiovascular diseases, hypertension, and diabetes as strong predictors of severe COVID-19, particularly for hospitalized patients [
22,
23]. Other conditions (cancer, asthma, liver disease, HIV/AIDS) were also associated with poor outcomes [
24]. The CDC indicated that individuals with NCDs face a greater risk of severe COVID-19 outcomes or death [
25,
26,
27]. This bidirectional relationship means that pre-existing conditions can worsen due to COVID-19, and these conditions can exacerbate COVID-19 severity, increasing hospitalization duration and mortality [
28,
29]. This ecological study is subject to ecological fallacy; thus, the associations identified cannot be directly generalized to individuals. Our analysis showed that an 8.62 percent relative increase in COVID-19 mortality is associated with a 1 percent increase in the older adult population [
30,
31]. Studies in Brazil and globally showed higher COVID-19 incidence in adults, but higher mortality in older adults, particularly those with comorbidities [
30,
31]. Another Brazilian study reported mortality rates of 8-9percent for those aged 60-79 and 14.8percent for those 80+, significantly higher than the overall average (2-3 percent) [
31]. This highlights the increased risk of death in older adults with comorbidities. A 2.38 percent relative increase in COVID-19 mortality is associated with a 1 percent increase in the urban population [
32]. This aligns with CDC findings showing a 30 percent higher age-adjusted death rate in large metropolitan areas compared to medium-sized urban areas [
31]. This highlights the influence of population density and connectivity on viral spread. The spread of SARS-CoV-2 is linked to human mobility and connectivity, especially in globally connected cities with airports [
16,
33]. The speed of contagion and spatial spread of COVID-19 vary depending on factors such as population density and distribution [
34,
35]. Social distancing measures are crucial in managing outbreaks, especially in densely populated areas [
35,
36]. Higher social exposure correlates with higher incidence and mortality [
37]. Urban centers are implicated in COVID-19 spread [
37], due to increased connectivity and population density. While vaccination is crucial in reducing mortality, our GAMLSS analysis revealed a positive association between vaccination and mortality. However, this counterintuitive finding is likely due to the January 2021 start of Brazil’s nationwide vaccination campaign, meaning that a large portion of our study period (2020-2021) had zero vaccination coverage [
38,
39]. This zero coverage likely confounds our results. Further research focusing on the post-vaccination period is necessary. The uneven distribution of COVID-19 deaths in Brazil, with five states accounting for 81 percent of all deaths, underscores geographic and social disparities [
40]. Brazil’s vaccination campaign began in January 2021, while the UK began in December 2020 [
39]. Manaus, Brazil, quickly became an epicenter after the start of vaccination, with the rapid spread of the Gamma variant overwhelming its health system [
40]. The increased vaccine allocation to Amazonas in response highlights the impact of pre-existing vulnerabilities on the spread of the disease. This increased vaccination in the hardest hit areas likely reduced vaccine hesitancy in these populations [
39]. This study’s limitations include the inherent challenges of ecological fallacy, as aggregated data limits individual-level inferences. Data limitations, with missing information in some municipalities, might also affect the conclusions. These potential issues (underreporting or delayed diagnoses) highlight the need for continuous monitoring and reporting efforts at the local level. Further research using different methodological approaches is needed to fully understand the complex epidemiological dynamics of COVID-19 across Brazil. Studies identifying specific factors associated with negative COVID-19 outcomes are crucial for informing public health policies, strengthening healthcare systems, and improving patient care.