Introduction
Whooping cough, or pertussis, remains a critical global public health issue, particularly in low- and middle-income countries.1 It is caused by the bacterium Bordetella pertussis. The disease primarily affects infants and young children and can lead to severe complications or even death if not managed properly.2 The WHO estimates that pertussis causes approximately 89 000 deaths each year, despite widespread vaccination efforts.3–5 Its persistent burden highlights the need for targeted strategies to reduce morbidity and mortality worldwide. Gender and sex differences play a crucial role in understanding disease dynamics.
Here, ‘sex’ refers to biological differences between males and females, such as genetic or hormonal factors, while ‘gender’ encompasses social and cultural roles that shape health behaviors and access to care. These distinctions can reveal disparities in disease burden and healthcare outcomes.6,7 Research suggests that sex-based biological factors, like varying immune responses, and gender-based social factors, such as unequal access to healthcare, may influence pertussis prevalence and impact.6,7 However, comprehensive studies on these differences are limited, particularly in low-income countries like Sierra Leone. A multicountry equity analysis study examining the coverage of the third dose of the diphtheria-tetanus-pertussis (DTP) vaccine across 14 West African countries revealed an increase in DTP3 coverage compared with skilled birth attendant coverage.8 While these multicountry studies provide critical findings to inform evidence-based interventions, no single-country study on whooping cough prevalence, severity or vaccination exists in Sierra Leone.
Addressing this gap can guide interventions aimed at reducing inequities in the burden of whooping cough in Sierra Leone.
Sierra Leone presents a compelling case for studying whooping cough disparities. The country faces major public health challenges, including high rates of infectious diseases and limited healthcare access. Past civil conflict and economic instability have exacerbated these issues.9 Sierra Leone also has one of the lowest vaccination coverage rates globally, contributing to frequent outbreaks of preventable diseases like pertussis.10 For example, in 2024, the World Bank report revealed a national DPT vaccination coverage of 91% in Sierra Leone, which is below the national coverage of 95% reported in Ghana.11 Additionally, this study found a decline in pertussis prevalence from 81% in 2000 to 43.5% in 2019.
Furthermore, women often face reduced access to care and display lower health-seeking behavior compared with men.12 Examining whooping cough in this context offers unique insights into how sex and gender influence disease patterns and healthcare access in resource-limited settings. This study fills a critical gap by providing reliable estimates of whooping cough prevalence in Sierra Leone, with a focus on sex-based disparities. By analyzing data from the WHO Health Equity Database, it offers a nuanced view of how sex impacts age-standardized pertussis prevalence and sex-based disparities.
Unlike prior research, it integrates sex-disaggregated analysis in a high-burden, low-resource setting. The findings aim to inform policy and intervention strategies for reducing whooping prevalence and sex-based disparities in Sierra Leone and other countries within a similar context.
Methods
Study design and data source
This study utilized a retrospective, descriptive design to analyze the sex-disaggregated prevalence of whooping cough in Sierra Leone from 2000 to 2019. The 5-y intervals (2000, 2005, 2010, 2015, 2019) were chosen based on the availability of consistent and comparable data points in the WHO Health Equity Database during the study period. Data were obtained from the WHO Health Equity Database,13 which provides harmonized and comparable estimates of health outcomes and their associated inequality dimensions across multiple countries.
The WHO Health Equity Database, a global repository integrating data from initiatives such as the Global Burden of Disease study,14 was used as the primary data source, instead of national health records, to ensure standardized and comparable estimates. This database is particularly useful for examining trends in health outcomes over time and assessing disparities across sociodemographic dimensions, such as sex, age and geographic location. For this study, we concentrated on sex-disaggregated data to investigate differences in the prevalence of whooping cough between males and females in Sierra Leone.
The study adhered to principles of secondary data analysis, relying solely on publicly available data, without involving the collection of primary data or interaction with human subjects.15
Outcome measure and dimension of inequality
The primary outcome measure was the prevalence of whooping cough, expressed as the number of cases per 100 000 population. Age standardization was applied to adjust for differences in age distribution across populations and over time, ensuring comparability of prevalence estimates. The analysis focused on sex as the primary dimension of inequality, with prevalence estimates disaggregated by males and females.
Sex-based disparities were quantified using inequality measures such as the absolute difference (D), prevalence ratio (R), Population Attributable Fraction (PAF) and Population Attributable Risk (PAR). These measures—D, R, PAF and PAR—were chosen as they provide both absolute and relative measures of disparity, are widely used in health equity research and are supported by the WHO Health Equity Assessment Toolkit (HEAT) software (https://www.who.int/data/inequality-monitor/assessment_toolkit Access date: April 14, 2025) for standardized computation, ensuring robust assessment of sex-based inequalities.
Data collection and calculation methods
Data were accessed from the publicly available WHO Health Equity Database via the HEAT software.16 Prevalence rates were calculated as cases per 100 000 population, age-standardized to the WHO World Standard Population. Inequality measures were computed as follows: D as the absolute prevalence difference between females and males; R as the prevalence ratio of females to males; PAF as the proportion of total prevalence attributable to sex differences, indicating a potential reduction if disparities were eliminated; and PAR as the excess prevalence due to sex disparities. These measures provide valuable public health insights into the magnitude and impact of sex-based disparities.
Data analysis
The WHO HEAT software (version 6.1) was used for analysis. This publicly available tool requires no licensure and is accessible online via the WHO website.16 The HEAT software computes health inequality measures using preloaded data from global health databases, including the WHO equity database. For this study, HEAT was utilized to generate prevalence estimates of whooping cough disaggregated by sex.
The absolute difference was calculated as the difference in prevalence between females and males, providing a measure of disparity magnitude. The sex ratio was computed as the ratio of prevalence among females to males, offering a relative measure of inequality. The PAF measures the proportion of a health outcome linked to a specific risk factor in the population, while the PAR quantified the excess prevalence due to sex disparities.
The 95% CIs for prevalence estimates and inequality measures were computed directly by the WHO HEAT software using standardized methods. The WHO HEAT ensured that analyses were conducted using standardized and validated methods, enhancing the comparability and reliability of findings. Results were interpreted in the context of existing literature on whooping cough and sex-based health disparities.
Inferential analysis and interpretation accuracy
Inferential analysis was constrained by the WHO HEAT software’s capabilities, as it does not perform formal trend tests or direct significance testing beyond CIs. Nevertheless, non-overlapping CIs between female and male prevalence rates in most years indicate statistically significant differences. To ensure accuracy, results were reviewed with input from a biostatistician, confirming the interpretation of CIs as indicative of disparities.
Formal statistical trend analysis was not conducted, which is a limitation of this study.
Age standardization
To enable comparability of prevalence estimates over time, we applied age standardization using the WHO World Standard Population as the reference. This method accounted for variations in age distribution and population growth, with no additional demographic adjustments beyond standardization due to the standardized nature of WHO data. Demographic trends were referenced from national censuses to provide context for population changes.
Handling missing data
The WHO Health Equity Database provided complete data for the selected years and indicators used in this study. No missing data were encountered; therefore, no imputation or additional handling methods were necessary.
Results
Demographic characteristics of Sierra Leone (2000–2019)
Demographic characteristics of Sierra Leone’s population during the study period (2000–2019), as derived from the WHO Health Equity Database and national census data, indicate significant population growth and a youthful age structure. More than 40% of the population was aged <15 y, a group particularly susceptible to whooping cough due to incomplete vaccination or waning immunity. The population grew from approximately 4.9 million in 2000 to 7.9 million by 2019, with rural areas comprising a majority, often experiencing limited access to healthcare.
This demographic context highlights the importance of age-standardized prevalence estimates to account for changes in population structure over time.
Prevalence of whooping cough in Sierra Leone
Figure 1 illustrates a consistent decline in the age-standardized prevalence of whooping cough per 100 000 population in Sierra Leone from 2000 to 2019. The trend reflects a substantial reduction over the two decades, likely driven by improvements in vaccination efforts and public health interventions. This decline suggests progress in controlling a vaccine-preventable disease, although the prevalence in 2019 remains notably high, indicating an ongoing public health challenge.
Potential contributing factors include the introduction of the pentavalent vaccine in 2005 and community-based health programs, although persistent systemic barriers such as rural access issues may hinder the pace of reduction.
Trends in whooping cough prevalence by sex in Sierra Leone, 2000–2019
Table 1 presents the age-standardized prevalence of whooping cough by sex in Sierra Leone from 2000 to 2019. Across all years studied, females consistently exhibited higher prevalence rates compared with males, highlighting a persistent sex-based disparity in disease burden. This pattern holds true across the study period, with the gap between sexes narrowing slightly over time but remaining evident.
The higher prevalence among females suggests potential biological or systemic factors influencing disease susceptibility or reporting, a disparity that warrants further investigation to inform targeted health strategies.
Inequality measures of whooping cough by sex-based dimension in Sierra Leone, 2000–2019
Table 2 provides inequality measures related to the prevalence of whooping cough in Sierra Leone from 2000 to 2019, focusing on sex as the inequality dimension. The D between female and male prevalence shows a decreasing trend over time, indicating a narrowing gap in the disparity. The PAF, remaining constant at -11.7 across all years, suggests that approximately 11.7% of the total whooping cough prevalence could theoretically be reduced if sex disparities were eliminated, reflecting a persistent burden attributable to sex differences.
The PAR shows a decline over the period, pointing to a reduction in the excess risk associated with sex. Meanwhile, the R, constant at 1.3, indicates that females consistently face a 30% higher relative risk of contracting whooping cough compared with males. These measures collectively highlight the enduring impact of sex-based disparities on disease burden, even as overall prevalence declines.
Inferential analysis and statistical significance
Inferential analysis was limited by the capabilities of the WHO HEAT software, which does not perform formal trend tests or direct significance testing beyond CIs. However, non-overlapping CIs between female and male prevalence rates in most years indicate statistically significant differences in disease burden by sex. To ensure the accuracy of this interpretation, results were reviewed with input from a biostatistician, confirming that non-overlapping CIs are a reasonable indicator of significant disparities.
The absence of formal trend analysis to assess the statistical significance of the prevalence decline over time is a limitation of this study. Future analyses with advanced statistical tools could provide deeper insights into temporal trends and their drivers.
Significance of overall trends
The overall decline in whooping cough prevalence from 2000 to 2019 signifies notable progress in public health efforts in Sierra Leone, likely driven by vaccination programs and enhanced disease surveillance. This trend reflects the potential impact of initiatives such as the Expanded Programme on Immunization, supported by international partners like the WHO and UNICEF. However, the still-elevated prevalence in 2019 highlights the need for sustained interventions to address remaining gaps, particularly in rural and underserved areas.
The persistent sex-based disparity, with females consistently at higher risk, further emphasizes the importance of understanding underlying biological or systemic factors to ensure equitable health outcomes.
Discussion
This study provides critical insights into the prevalence trends of whooping cough in Sierra Leone over the two-decade period from 2000 to 2019, with a specific focus on sex-disaggregated data. Our findings reveal a consistent decline in age-standardized prevalence over time, alongside persistent sex-based disparities, with females experiencing a higher disease burden across all years studied. Inequality measures underscore the disproportionate impact on females, highlighting a significant public health challenge that warrants further investigation.
These trends lay a foundation for understanding the evolving epidemiology of whooping cough in a resource-limited setting and contribute to the global discourse on sex differences in infectious diseases.
The observed decline in whooping cough prevalence in Sierra Leone aligns with global trends, as noted in prior research.3,5 This reduction likely reflects improvements in vaccination coverage and public health interventions targeting preventable diseases.17 In Sierra Leone, national efforts supported by international organizations, such as the WHO and UNICEF, have played a pivotal role. Specifically, the introduction of the pentavalent vaccine (including pertussis) in 2005 through the Expanded Programme on Immunization marked a turning point, with coverage among children aged <5 y reportedly increasing from approximately 40% in the early 2000s to >80% by 2019 in some regions, although rural disparities persist.18 Community-based outreach programs and vaccination campaigns have further strengthened these efforts, contributing to the observed reductions.19 Despite these advances, significant barriers remain, including inadequate healthcare infrastructure, shortages of trained healthcare professionals and limited access to affordable services, particularly in rural areas.20 The persistent high prevalence in 2019 underscores the ongoing risk to children and the urgent need for sustained interventions to address systemic challenges.
Turning to the persistent sex-based disparities, our findings align with global studies indicating that females consistently exhibit higher whooping cough prevalence than males, a pattern largely attributed to biological factors.20 International evidence suggests that biological mechanisms, such as stronger immune responses linked to X chromosome genes and elevated estrogen levels in females, may heighten susceptibility to pertussis.7 While gender-based disparities in healthcare access and socioeconomic factors are pronounced in Sierra Leone, our data do not indicate these as primary drivers of the observed differences. Instead, the consistent inequality measures such as a stable prevalence ratio and PAF point to biological underpinnings, mirroring patterns seen across diverse settings rather than local inequities in care.20 This interpretation is crucial for framing the disparities within a global context, although it does not preclude the need for broader health system improvements. Continued surveillance and research are essential to further elucidate these mechanisms, particularly given historical disruptions to Sierra Leone’s healthcare system, such as the 2014–2016 Ebola outbreak, which strained resources and disease-control efforts.20
Implications for public health outcomes
The sex-based disparities in whooping cough prevalence identified in this study are consistent with global meta-analyses, reinforcing the likelihood of biological rather than local socioeconomic or cultural drivers.20 While addressing gender inequalities in healthcare access remains a critical goal for overall public health in Sierra Leone, our findings do not suggest that such factors underpin the observed disparities in whooping cough. Moreover, as most whooping cough cases are non-fatal, there is no evidence from our study or existing literature to indicate that these sex differences result in increased mortality, long-term complications or vaccine hesitancy. Therefore, our results primarily contribute to the global understanding of biological sex differences in pertussis epidemiology rather than necessitating immediate local interventions targeting gender-specific access or beliefs.
Nevertheless, practical implications for Sierra Leone’s public health landscape emerge from this research. To address the broader burden of whooping cough, efforts should focus on overcoming systemic barriers through context-specific strategies. These include deploying mobile vaccination units to remote rural areas, strengthening partnerships with local leaders to combat vaccine hesitancy and enhancing disease surveillance to monitor sex-disaggregated trends in real time.
Such initiatives can build on existing frameworks like the Expanded Programme on Immunization to ensure equitable access to preventive care. Future research should also explore whether subtle differences in healthcare access or outcomes by sex exist, even if not evident in our data, to inform comprehensive health equity strategies. By prioritizing these actionable steps alongside continued global collaboration, Sierra Leone can further reduce the burden of vaccine-preventable diseases while contributing to the broader understanding of pertussis epidemiology.
Limitations
This study has several limitations that should be considered when interpreting these findings. First, while age-standardized prevalence estimates ensure comparability across time periods, the absence of detailed data on the underlying causes of whooping cough limits our ability to identify specific drivers of the observed trends, such as vaccination coverage gaps or pathogen dynamics. Second, reliance on national-level estimates may obscure critical geographic variations within Sierra Leone, particularly between urban and rural areas, where healthcare access and disease burden likely differ significantly.
Third, the study did not account for other dimensions of inequality, such as socioeconomic status or educational attainment, which may influence whooping cough prevalence and interact with sex-based disparities. Fourth, while robust sex-disaggregated data are provided, intersectional factors such as the combined effects of sex with age, income or location were not explored, potentially limiting a nuanced understanding of health disparities. Fifth, the analysis is confined to Sierra Leone without comparative data from other countries with similar socioeconomic or epidemiological profiles, which could contextualize whether observed sex disparities are unique or part of broader regional patterns.
Sixth, the lack of formal inferential statistical analysis, such as trend tests or significance testing beyond CIs, restricts conclusions about the statistical robustness of observed differences and trends over time. Seventh, the study does not provide insights into the geographic distribution of cases, the potential influence of cultural factors on healthcare-seeking behavior or specific vaccination coverage rates across regions and demographic groups within Sierra Leone. Finally, reliance on secondary data from the WHO Health Equity Database may introduce biases or gaps not fully accounted for, such as potential under-reporting in resource-limited settings.
These limitations highlight the need for cautious interpretation and more granular data in future research.
Future directions
Building on the findings of this study, future research should prioritize identifying the specific causes of whooping cough in Sierra Leone, including pathogen transmission dynamics, vaccination coverage gaps and environmental factors, to inform targeted interventions. Longitudinal studies tracking changes in prevalence and inequalities over time could provide critical insights into the effectiveness of public health interventions, such as the Expanded Programme on Immunization, in reducing the whooping cough burden. Additionally, exploring the intersectionality of sex with other social determinants of health such as age, socioeconomic status and geographic location could uncover nuanced disparities and guide equitable health policies.
Comparative analyses with other West African or low-resource countries facing similar epidemiological challenges could contextualize whether sex-based disparities in Sierra Leone are unique or reflective of regional patterns. Moreover, research should investigate subnational variations in disease burden, particularly between urban and rural areas, and assess the role of cultural beliefs and gender norms in shaping healthcare-seeking behavior. Implementing and evaluating community-based programs tailored to Sierra Leone’s context, such as mobile vaccination units targeting remote regions, partnerships with local leaders to address vaccine hesitancy and gender-sensitive health education campaigns, could help to reduce disparities and improve access to preventive care.
Finally, integrating real-time surveillance systems to monitor sex-disaggregated data on whooping cough incidence and vaccination rates could support evidence-based decision-making for public health strategies in resource-limited settings like Sierra Leone.
Conclusions
This study revealed the persistent burden of whooping cough in Sierra Leone, with females consistently showing a higher prevalence across the study period from 2000 to 2019. Despite a notable decline in overall prevalence over two decades, significant sex-based disparities remain, reflecting a critical public health challenge. Consistent with global evidence, our findings suggest these disparities are likely driven by biological factors, such as differences in immune responses, rather than local socioeconomic or cultural determinants specific to Sierra Leone.
By offering robust sex-disaggregated estimates and detailed inequality measures, this research contributes to the global understanding of sex differences in pertussis epidemiology. While enhancing vaccine coverage through initiatives like Sierra Leone’s Expanded Programme on Immunization and improving healthcare access remain vital for overall public health, our data do not support the need for targeted interventions based solely on gender-specific access or beliefs at this time. Instead, efforts should focus on addressing broader systemic barriers, such as rural healthcare access, through context-specific strategies like mobile health units and community outreach.
Future research should delve deeper into both biological mechanisms and potential social influences on disease burden to refine our understanding of these disparities. Continued surveillance and targeted studies are essential to inform effective, evidence-based public health strategies and to monitor progress towards reducing the burden of vaccine-preventable diseases in Sierra Leone.