Global disease burden and trends of pertussis in individuals under 20 years old: 1990–2021—insights from the global burden of disease study 2021

Introduction

Pertussis, an acute and highly contagious respiratory disease caused by Bordetella pertussis, is characterized by paroxysmal and spasmodic coughing. This causes significant morbidity and severe complications including apnea, pneumonia, pulmonary hypertension, and encephalopathy, posing a substantial threat to child health globally (1,2). Despite substantial reductions in cases and fatalities in many regions owing to advances in healthcare and vaccination, pertussis remains a critical public health challenge, particularly in developing countries and low-vaccination regions (3,4).

Pertussis poses the most severe threat to the youngest infants, particularly those under 3 months of age who are too young to be fully vaccinated or protected by maternal antibodies. This group experiences the highest incidence rates, most severe disease manifestations, and the majority of pertussis-related deaths (5). Historically considered a childhood disease, the epidemiology of pertussis has shifted in recent decades. With waning immunity after both vaccination and natural infection, adolescents and adults have become a significant reservoir for the pathogen (6). Seminal studies from the late 1980s onwards, initially in the United States and France, identified household members—siblings, parents, and grandparents—as the primary sources of infection for vulnerable infants (7,8), a pattern now recognized globally.

Further compounding the issue, factors such as the cyclical nature of pertussis (typically peaking every 2–5 years), waning of vaccine-induced immunity over time, along with hypotheses regarding the evolution of Bordetella pertussis strains, and intensifying global migration patterns are subtly altering the pertussis epidemiology (9). This has contributed to scenarios where regions, even those with high childhood vaccination coverage and previously low incidence rates, experience resurgences or outbreaks. Moreover, increased awareness and the wider availability of sensitive diagnostic tools like polymerase chain reaction (PCR) have improved case detection across all age groups, further revealing the true scale of the disease burden (10). Importantly, pertussis epidemiology was heavily impacted by the coronavirus disease 2019 (COVID-19) pandemic and its associated non-pharmaceutical interventions (NPIs), which dramatically suppressed the transmission of pertussis and other respiratory pathogens globally (11). This evolving epidemiology underscores the necessity for relentless monitoring, research, and reinforcement of control measures.

The Global Burden of Disease (GBD) study revealed that in 2019, over 19 million people worldwide suffered from pertussis. This staggering figure not only underscores the widespread impact of pertussis but also warns us that the actual burden of this disease has been significantly underestimated (12). In response to this dire scenario, our study meticulously examines the GBD 2021 data set, delving into the intricacies of global, regional, and national trends in pertussis burden among individuals under 20 years old, spanning the period from 1990 to 2021. Additionally, we will explore the intricate relationship between socioeconomic factors and pertussis burden, aiming to provide solid theoretical support and scientific evidence for the development of more precise and effective prevention and control strategies for pertussis in this vulnerable population worldwide. We present this article in accordance with the STROBE reporting checklist (available at https://tp.amegroups.com/article/view/10.21037/tp-2025-463/rc).

Methods

Data sources and processing

This study utilizes secondary data from the GBD 2021, adhering to the Guidelines for Accurate and Transparent Health Estimates Reporting (GATHER) guidelines. The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The GBD 2021 dataset provides comprehensive global health estimates spanning 204 countries and territories from 1990 to 2021. It details the burden of 371 diseases and injuries, alongside 88 risk factors, stratified by age, year, location, and sex (13,14). All estimates used in this study are publicly accessible via the GBD Results Tool to ensure transparency and reproducibility (http://ghdx.healthdata.org/gbd-results-tool).

Crucially, GBD estimates are not simple aggregates of reported cases but are sophisticated model-based values. These models synthesize diverse data sources (including vital registration, surveillance systems, surveys, and scientific literature) while accounting for known biases such as underreporting and imperfect diagnostic accuracy using tools like DisMod-MR. Prior to analysis, the data underwent rigorous quality control, including cleansing, standardization, and validation procedures. Detailed descriptions of the GBD 2021 data collection, processing, synthesis, and modeling methodologies are available in the accompanying publications (15).

Disease definition

In the GBD 2021 study, pertussis is defined as a contagious respiratory illness caused by Bordetella pertussis, operationalized through relevant International Classification of Diseases (ICD) codes. The case definition encompasses ICD-10 codes A37–A37.9, ICD-9 codes 033.0–033.9, and code V03.6. The estimates are derived from a broad range of sources across multiple countries, among which diagnostic practices exhibit significant heterogeneity—spanning from laboratory confirmation to clinical diagnosis based on symptom criteria (13,15). This study focuses on pertussis cases among individuals under 20 years old, spanning the years 1990 to 2021. The incidence, mortality, and disability-adjusted life years (DALYs), along with their corresponding 95% uncertainty intervals (UIs), are derived from GBD 2021 and expressed per 100,000 of the population. 95% UIs for final estimates were derived by randomly sampling 1,000 times from the specific age, sex, location, and year distributions, with boundaries defined by the 2.5th and 97.5th percentiles (16). To gain insights into age-specific trends in disease burden, the population is further segmented into distinct age groups: <1 year, 12–23 months, 2–4, 5–9, 10–14, and 15–19 years.

All estimates in this study are intricately stratified by gender, age, location, encompassing the timeframe from 1990 to 2021. The GBD database encapsulates 21 GBD regions and 204 countries or territories, categorized into quintiles based on the SDI. SDI, a composite metric of development status intimately linked to health outcomes, is calculated as the geometric mean of the total fertility rate under 25 years old (scaled from 0 to 1), mean years of education for individuals aged 15 and above, and a lagged distributed income per capita index (17).

Statistical analysis

To mirror the temporal evolution of pertussis burden, we calculated the estimated annual percentage change (EAPC) in rates and the relative percentage change in cases. The EAPC and its 95% confidence interval (CI) were calculated using a linear regression model: ln (age-standardized rate) = α + β × (calendar year) + ε, where α represents the intercept, β signifies the coefficient for the calendar year (or time variable), and ε denotes the error term. Subsequently, the EAPC was computed using the formula 100 × [exp (β) − 1]. Trends were deemed decreasing if the upper boundary of the 95% CI of EAPC was less than 0, increasing if the lower boundary exceeded 0, and stable in all other cases (18).

Furthermore, smoothing spline models were deployed to scrutinize the correlation between pertussis burdens among individuals under 20 years old and SDI across the 21 regions and 204 countries and territories. Smooth splines were fitted employing the locally weighted scatterplot smoothing (LOESS) model, which dynamically determines the degree, quantity, and positioning of knots based on the data and span parameter (19). Additionally, Spearman’s correlation analysis was undertaken to assess the strength and statistical significance of the associations between incidence, mortality, DALY rates, and SDI values (20). All statistical analyses and data visualizations were executed utilizing R (version 4.3.1), with statistical significance with P value less than 0.05.

Results

Global burden and trend

The global incidence of cases among individuals under 20 years old has witnessed a remarkable decline, decreasing from 31,776,782.9 cases in 1990 to 7,082,164.2 cases in 2021, representing a reduction of 77.7% (95% UI: −81.9% to −72.1%). In parallel, the incidence rate per 100,000 individuals has fallen from 1,406.9 in 1990 to 268.7 in 2021, exhibiting an EAPC of −2.7% (95% CI: −3.4% to −1.9%) (Table 1 and Figure 1). Furthermore, deaths attributed to pertussis have undergone a significant decrease of 80.3%, dwindling from 261,443.2 deaths in 1990 to 51,517.3 in 2021. The mortality rate has also dropped drastically, from 11.6 per 100,000 to 2 per 100,000, with an EAPC of −3.3% (95% CI: −4.0% to −2.6%) (Table 2 and Figure 1). Similarly, DALYs lost due to pertussis have declined by an impressive 80.3%, decreasing from 23,114,984.9 in 1990 to 4,556,396.3 in 2021. This translates into a substantial reduction in the DALY rate, from 1,023.4 per 100,000 to 172.9 per 100,000, with an EAPC of −3.3% (95% CI: −4.0% to −2.6%), underscoring the significant progress made in mitigating the burden of pertussis globally (Table 3 and Figure 1).

Figure 1 Global burden of pertussis among individuals under 20 years old of different age groups in 2021 and trends from 1990 to 2021. (A) Incidence rate in 2021. (B) Incidence rate from 1990 to 2021. (C) Death rate in 2021. (D) Death rate from 1990 to 2021. (E) DALY rate in 2021. (F) DALY rate from 1990 to 2021. DALY, disability-adjusted life year; UI, uncertainty interval.

Regional burden and trend

In 2021, among the five SDI regions, low-SDI regions had the highest incidence, deaths, DALYs, and corresponding rates of pertussis among individuals under 20 years old, while high-SDI regions had the lowest (Table 1). From 1990 to 2021, all five SDI regions experienced significant declines in pertussis incidence, mortality, and DALY rates (Figure 2). Notably, the high-middle SDI region showed the most substantial declines in incidence (EAPC: −4.2%; 95% CI: −5.4% to −2.9%) and DALY rate (EAPC: −7.1%; 95% CI: −7.8% to −6.5%). High-SDI regions had the highest average annual decline in mortality rate at −7.5%, compared to −3.8% in low-SDI regions (Tables 2,3).

Figure 2 Time trends in the burden of pertussis among individuals under 20 years old across five global and SDI regions, 1990–2021. (A) Incidence rate. (B) Death rate. (C) DALY rate. DALY, disability-adjusted life year; SDI, socio-demographic index.

In 2021, South Asia had the highest number of pertussis cases (2,044,642.4; 95% UI: 932,289.2–3,661,968.3). Western Sub-Saharan Africa led in pertussis deaths (16,261.0; 95% UI: 4,803.9–39,768.1) and DALYs (1,438,092.7; 95% UI: 430,802.0–3,504,774.7). Notably, Central Sub-Saharan Africa had the highest incidence, mortality, and DALY rates (Tables 1-3 and Figure S1). From 1990 to 2021, all geographical regions demonstrated a downward trend in pertussis incidence and DALY rates. Among these, high-income Asia Pacific experienced the most pronounced declines, with EAPC of −9.9% (95% CI: −13.6% to −6.0%) in incidence, −12.2% (95% CI: −14.5% to −9.8%) in mortality, and −11.9% (95% CI: −14.3% to −9.4%) in the DALY rate. In contrast, high-income North America showed relatively modest reductions, with an EAPC of −1.0% (95% CI: −2.4% to 0.5%) in incidence, 0.5% (95% CI: −2.6% to 1.7%) in mortality, and −0.8% (95% CI: −2.3% to 0.7%) in the DALY rate. Intriguingly, Australasia stood out as the only region with an increasing trend in mortality due to pertussis, recording an EAPC of 3.9% (95% CI: 0.0–7.9%) (Figure S2). These findings highlight the varying degrees of progress and challenges faced by different regions in combating pertussis.

National trend

In 2021, India had the highest number of pertussis cases among individuals under 20 years old (over 1.59 million), while Nigeria had the highest number of deaths (nearly 8,800). Angola had the highest incidence rate (1,533.9; 95% UI, 776.6–2,699.0) (Tables S1,S2, Figure 3 and Figure S3). Although 90 countries achieved zero pertussis deaths among children, the Central African Republic had the highest mortality and DALY rates globally (19.3; 95% CI: 1.4–65.6 and 1,702.5; 135.6–5,755.6, respectively). Most countries globally made progress in controlling pertussis among children, but eight countries showed increasing incidence, with Iceland having the most significant increase (EAPC: 5.2%; 95% CI: 4.3–6.2%). Mortality trends were mixed, with 18 countries showing increases, Spain having the highest EAPC (22%; 95% CI: 15.9–28.5%). Kyrgyzstan achieved the greatest success in reducing mortality. While most countries reduced DALYs, Ukraine showed an increase, and Nicaragua had the most significant reduction (Tables S1-S3, Figure 3 and Figure S4).

Figure 3 Disease burden of pertussis among individuals under 20 years old by SDI across 21 regions and 204 countries and territories. (A) Incidence rates across 21 regions, 1990–2021. (B) Incidence rates across 204 countries and territories. (C) Death rates across 21 regions, 1990–2021. (D) Death rates across 204 countries and territories. (E) DALY rates across 21 regions, 1990–2021. (F) DALY rates across 204 countries and territories, 1990–2021. DALY, disability-adjusted life year; SDI, socio-demographic index.

Age and gender differences in pertussis burden

In 2021, pertussis posed a considerable threat to individuals under 20 years old worldwide, with infants (aged 0–1 years) experiencing the highest incidence, mortality, and DALY rates, which gradually decreased with age (Figure 1). Gender disparities were apparent, as globally and in key GBD regions heavily impacted by pertussis, females generally exhibited higher incidence, mortality, and DALY rates compared to males among individuals under 20 years old. However, this trend was reversed in Eastern Sub-Saharan Africa, where males had higher DALY rates (Figure S5). Across age groups up to 14 years, females bore a heavier burden of pertussis, whereas in the 15–19 years age bracket, males had slightly elevated mortality rates (Figure 1).

Relationship between SDI and disease burden

Figure 4 illustrates the significant changes in pertussis burden among individuals under 20 years old across the 21 GBD regions from 1990 to 2021, with increasing SDI. Strong negative correlations were observed between SDI and pertussis incidence, mortality, and DALY rates. High-SDI regions, such as Western Europe (SDI =0.85) and high-income North America (SDI =0.86), exhibited lower burdens, whereas low-SDI regions, including Central Sub-Saharan Africa (SDI =0.47), Oceania (SDI =0.47), and South Asia (SDI =0.56), confronted higher burdens. Notably, in certain regions like Central Latin America (SDI =0.64), North Africa and the Middle East (SDI =0.66), and Eastern Europe (SDI =0.80), incidence displayed nonlinear relationships with SDI. Despite regional disparities, the overall trend showed declining pertussis burdens with increasing SDI globally. Angola (SDI =0.45) had the highest incidence, while the Central African Republic (SDI =0.31) had the highest mortality and DALY rates (Figure 4). Correlation analysis revealed no significant association between EAPCs of incidence, mortality, and DALY rates with SDI in 2021 (Figure S6).

Figure 4 Global trends in the disease burden of pertussis among individuals under 20 years old in 204 countries and territories in 2021. (A) Incidence rate. (B) Death rate. (C) DALY rate. DALY, disability-adjusted life year.

Discussion

This study utilizes the extensive GBD 2021 dataset to conduct a comprehensive analysis of the global pertussis burden in individuals under 20 years old, revealing pronounced trends over time and significant regional variations. The results indicate that from 1990 to 2021, the incidence, mortality, and DALYs due to pertussis have shown substantial declines, with global incidence decreasing by 77.7% and mortality by 80.3%. This remarkable progress is primarily attributed to the successful worldwide promotion of pertussis vaccines since the inclusion of the diphtheria-tetanus-pertussis (DTP) vaccine in the World Health Organization (WHO)’s Expanded Program on Immunization in 1974 (21,22). However, pertussis remains a significant global public health challenge. Due to its highly contagious nature and the fact that neither vaccination nor natural infection confer lifelong immunity, Bordetella pertussis continues to circulate, causing periodic resurgences and outbreaks (23).

Despite the overall global decline, our analysis reveals persistent and substantial disparities, necessitating distinct public health strategies for high-income countries (HICs) and low- and middle-income countries (LMICs). The challenges are most acute in low-SDI regions (predominantly LMICs)—such as South Asia, Western Sub-Saharan Africa, and countries including India, Nigeria, and Angola—which continue to bear a disproportionate burden. These areas face multifaceted barriers, including poor living conditions, malnutrition, and, most critically, limited healthcare access and suboptimal vaccination coverage (24,25). In 2022, the global coverage for the first dose of DTP was 89%, while the third dose reached 84%. Alarmingly, 14.3 million children did not receive the first dose, an 11% increase from 2019, with the majority residing in low-income countries (26). This falls far short of the WHO-recommended target of ≥90% coverage with three doses of assured quality pertussis vaccine at all levels (national and subnational) to reliably interrupt community transmission (27). Therefore, the priority in these settings must be to strengthen routine immunization systems, ensure timely administration of all vaccine doses, and improve fundamental healthcare access to reduce mortality.

Conversely, the epidemiological landscape and challenges differ markedly in high-SDI regions (typically representing HICs). Some, such as high-income North America, experienced slower declines in burden among individuals under 20 years old, while countries like Australia, Iceland, and Spain reported increases in incidence and mortality. This pattern is not primarily due to low childhood coverage but rather to the consequences of waning immunity in adolescents and adults, who become reservoirs of infection. This shift in transmission dynamics is well-documented, with infants increasingly infected by siblings and parents rather than acting as the primary source, and most infant deaths now occur in those born to unvaccinated mothers (28,29). This underscores the critical need for HICs to go beyond childhood vaccination and implement strategies to maintain immunity across the lifespan. Although achieving high booster coverage in adults remains challenging, vaccination during pregnancy has proven highly effective in protecting young infants (30). Since its introduction in the United Kingdom in 2012, maternal immunization has been adopted successfully in numerous HICs and has significantly reduced pertussis morbidity and mortality in infants under 3 months of age (31).

Recent studies from South Korea provide a concrete example of these dynamics and offer further insights. A school-based outbreak investigation in Busan among a fully vaccinated population estimated a mean serial interval of 9.5 days and provided quantitative evidence of superspreading events, underscoring the high transmissibility of pertussis even in vaccinated groups and the role of individual heterogeneity in driving outbreaks (32). Furthermore, beyond the established role of parents and siblings, a national seroprevalence study suggests that older adults, who often present with mild or atypical symptoms, may act as crucial amplifiers of community transmission in Korea, particularly through frequent contact with grandchildren (33). This silent reservoir facilitates ongoing community transmission, which often remains undetected by routine surveillance systems. The post-pandemic rebound of social interactions may have further accelerated this transmission among older children and adults. These findings from Korea not only corroborate the global challenges but also emphasize the need for enhanced diagnostic capabilities, active case finding across all age groups, and booster vaccination strategies that explicitly include adolescents and older adults, in addition to pregnant women (34).

Surveillance and diagnosis of pertussis have undergone notable shifts that directly impact the understanding of its burden. Prior to the 1990s, pertussis surveillance was practically nonexistent globally, as the disease had become a forgotten pediatric condition. It was only following the clinical trials of acellular vaccines in the 1990s that some countries established surveillance systems to monitor the vaccines’ rollout and effectiveness (35). Diagnostic technologies have also advanced in stages: PCR was introduced in HICs in the late 1990s, followed by real-time PCR around 2008 (36). The recent advent of multiplex PCR panels further increases detection but requires careful validation against reference methods, as done by reference laboratories in the USA and Europe, to ensure accuracy (10). These technological improvements have enhanced diagnostic sensitivity, leading to an increase in reported cases; however, this also means changes in biological diagnostic methods can artificially alter incidence data, which must be noted when interpreting trends or making historical comparisons. For LMICs, advanced tools like real-time PCR or multiplex kits are often unaffordable due to high costs and equipment requirements. Feasible alternatives should be prioritized, such as the practice of freezing nasopharyngeal aspirates and shipping them to reference laboratories for confirmation (37). Additionally, training for physicians and nurses is essential, as pertussis remains an overlooked “old disease” in many regions, hindering timely case identification (35). These adjustments to surveillance and diagnostic approaches are key to accurately capturing pertussis burden and guiding effective control strategies.

In recent years, pertussis incidence has shown alarming upward trends in certain regions, culminating in severe outbreaks (38,39). This resurgence has been particularly notable following the relaxation of COVID-19 NPIs. For instance, the Czech Republic witnessed its largest pertussis outbreak in over 60 years in 2024. By mid-April, the country had registered 7,888 cases. Notably, the 15–19 years age group experienced the highest incidence rate, with 434.7 cases per 100,000 individuals (40). This rise may be attributed to immune decline and the pathogen’s ability to adapt to new environments. Specifically, the introduction of acellular pertussis vaccines (aPV) may have contributed to the emergence of antigenic variants of Bordetella pertussis, including strains that overproduce pertussis toxin (PT) or lack the adhesin protein pertactin (PRN) (41). Notably, many countries, including the USA and Australia, have detected PRN-deficient strains. However, current evidence suggests that aPV effectiveness against severe disease remains high regardless of PRN expression status (42,43). Additionally, the increasing prevalence of pertussis strains resistant to macrolide antibiotics poses a significant challenge for pertussis treatment (44). This resistance underscores the need for ongoing surveillance, research, and the development of new therapeutic strategies to combat pertussis effectively.

Due to the gradual decline in immune protection over time post-vaccination, the susceptible population broadens. Coupled with advancements in pertussis diagnostic techniques, a notable surge in pertussis cases has been observed among older children and adolescents across multiple countries (45,46). This demographic plays a pivotal role in transmitting pertussis to infants. Despite this, the global burden of pertussis on infants remains severe, particularly for those under four months old, who exhibit the highest incidence rates and most severe disease manifestations, often necessitating invasive life support (47). Furthermore, while maternal antibody levels transmitted from unvaccinated mothers are generally inadequate to form an effective protective barrier, maternal pertussis vaccination is highly effective. Most studies show approximately 90% effectiveness in preventing pertussis in infants during the first 2–3 months of life. The WHO recommended and actual vaccination initiation age in most countries is between 2 and 3 months, leaving infants in their first few months at heightened risk of severe pertussis and death due to an extended unprotected period (2) WHO data indicates that in 2014, the total number of pertussis cases among children under 5 years old globally amounted to 24.1 million, with 160,000 deaths. Notably, 21% of these cases and 53% of these deaths were concentrated in infants under one year old (48), aligning with current research findings. Addressing this challenge involves implementing booster immunization programs for adolescents, expanding pertussis vaccination programs for pregnant women (49,50), and ensuring timely infant vaccination.

It is noteworthy that our study also observed slight gender-based differences in the pertussis burden among individuals under 20 years old. Although this finding concurs with previously reported results (51), further research and discussion are required to elucidate the specific mechanisms underlying these gender differences. Possible explanations could encompass biological disparities, social behavioral factors, and access to healthcare services (52).

Despite the robustness of our study in leveraging the extensive GBD 2021 dataset to analyze the global burden of pertussis among individuals under 20 years old, several limitations should be acknowledged. Firstly, we relied on modeled estimates from GBD 2021, which incorporate both laboratory-confirmed and clinically diagnosed cases. However, underreporting and diagnostic bias remain significant limitations, especially in older adults and those with mild symptoms, as demonstrated in South Korean studies (32,33). Therefore, the true incidence of pertussis is likely higher than reported, particularly in regions with limited diagnostic capacity. Additionally, the study period includes 2020 and 2021, which were profoundly affected by the COVID-19 pandemic and associated NPIs. These NPIs drastically reduced the transmission of many respiratory pathogens, including pertussis, leading to historically low reported and likely actual incidence during this period (53). The GBD 2021 estimates for pertussis burden in 2021 reflect this artificial suppression of transmission and should not be interpreted as representing the underlying, pre-pandemic epidemiology or a sustainable trend. The observed declining trends in our results for 2020–2021 are therefore heavily influenced by these temporary pandemic effects and do not reflect the typical disease dynamics. The subsequent global resurgence of pertussis observed post-2021, with cases often exceeding pre-pandemic levels in many regions, underscores this limitation and highlights the challenge of interpreting trends during this atypical period.

Conclusions

In summary, despite a general global decline in pertussis incidence and mortality among individuals under 20 years old, significant regional and national variations persist. Our findings highlight that the strategies to address these disparities must be context-specific. In LMICs, the priority is to strengthen routine immunization systems to achieve ≥95% coverage with the primary series and timely boosters. In HICs, the focus should shift to sustaining immunity across the lifespan through adolescent, adult, and maternal booster programs. Additionally, heightened attention to diagnostic capacity, pathogen evolution, and antimicrobial resistance is essential. Future strategies should also incorporate insights from recent outbreak and transmission studies, such as those from South Korea, to develop more effective and tailored public health responses.

Acknowledgments

We highly appreciate the efforts of GBD 2021 collaborators.

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://tp.amegroups.com/article/view/10.21037/tp-2025-463/rc

Peer Review File: Available at https://tp.amegroups.com/article/view/10.21037/tp-2025-463/prf

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://tp.amegroups.com/article/view/10.21037/tp-2025-463/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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