Graphical abstract

Introduction

Measles, a highly infectious disease caused by the measles virus, represents a longstanding public health challenge worldwide. Despite the availability of an effective vaccine, global measles cases and related deaths remain a concern; outbreaks can affect millions, particularly in regions with low vaccination rates [1]. The World Health Organization has prioritised measles for elimination, but achieving this goal has been challenging due to factors such as vaccine hesitancy, accessibility issues, and global mobility [2]. In the wake of the coronavirus disease 2019 (COVID-19) pandemic, the year 2022 underscored the worsening global measles situation, with millions of children missing vaccinations. The resulting increases in cases and deaths highlighted the disease’s persistent threat, especially in areas with low vaccination coverage [1]. In Malaysia, the healthcare system also faces obstacles, including vaccine hesitancy, which is partly fuelled by historical controversies and misinformation. For instance, unfounded fears linking the measles-mumps-rubella (MMR) vaccine to autism have periodically resulted in pockets of unvaccinated children, complicating efforts to achieve herd immunity [3].

Several individual factors have been previously identified as risk factors for measles infection. One outbreak study identified contact history with a measles case as a significant risk factor, with an adjusted odds ratio (aOR) of 1.15 and a 95% confidence interval (CI) of 1.12–3.70 [4]. A separate study in Japan revealed that individuals with a history of international travel had about 10 times higher odds of contracting measles compared to non-travellers (aOR, 10.2; 95% CI, 5.9–17.7) [5]. In Ethiopia, the risk of measles infection was reduced by 83% in individuals who had been vaccinated (aOR, 0.17; 95% CI, 0.05–0.53) [6]. Other individual factors include age, sex, and ethnicity [4,5,7–17]. Beyond biological susceptibility, broader contextual factors also influence measles transmission. Research in Italy demonstrated that areas with higher population densities experienced significantly more measles cases than less populated regions (incidence rate ratio, 1.70; 95% CI, 1.36–2.12) [18]. A study in Nigeria indicated that urban residence was a predictive factor for measles infection (aOR, 1.55; 95% CI, 1.02–2.34) [19]. Healthcare accessibility, vaccination coverage, and environmental factors such as fine particulate matter less than 2.5 μm (PM_2.5), relative humidity, and temperature have also been identified as significant factors in measles transmission [14,20–31].

In Malaysia, persistent measles outbreaks underscore the need to analyse the complex epidemiological landscape of the disease in this setting. A thorough assessment is necessary to overcome the obstacles impeding the country’s progress toward eliminating this condition. Consequently, this study was designed to explore the individual and contextual factors associated with measles infection in Malaysia. By analysing secondary data from 2018 to 2022, we aimed to identify key determinants of measles infections, offering insights supporting the development of targeted public health strategies and interventions.

Materials and Methods

Study Design and Sample

This cross-sectional study utilised secondary data from the e-measles database of the Ministry of Health Malaysia, as well as data from the Department of Statistics Malaysia and the Department of Environment Malaysia. Two levels of variables were investigated: individual factors were analysed at the individual level, while contextual factors were evaluated at the district level. The dataset included all cases registered in the e-measles databases from January 1, 2018, to December 31, 2022. The exclusion criteria encompassed any duplicate entries or participants missing at least 30% of their data. Of the 31,468 individual records available in the e-measles database, 2,946 (9.36%) were excluded from the analysis due to incomplete data, leaving a final sample of 28,522 participant records.

Study Area

This study was conducted in Malaysia, a country in the Southeast Asian region with an approximate land area of 329,847 km². Malaysia consists of 3 federal territories and 13 states, subdivided into a total of 160 districts. The nation is split into Peninsular Malaysia, bordering Thailand, and East Malaysia, located on the island of Borneo. Malaysia has a multiethnic and multicultural society, predominantly made up of Malays, Chinese, Indians, and indigenous groups, each contributing to the nation’s rich cultural heritage and diversity [32]. The country experiences a tropical rainforest climate, with its environmental conditions and precipitation patterns impacted by 2 seasonal monsoons [33].

Dependent Variable

The dependent variable in this study was the measles status of participants, classified as either measles or non-measles. Measles was defined as including cases that were laboratory-confirmed, either by the presence of measles immunoglobulin M antibodies or through the detection of the virus in clinical samples using culture or molecular techniques, as well as any case that was epidemiologically linked to a laboratory-confirmed case. A non-measles case was defined as one with laboratory-confirmed negative results for measles in e-measles databases, including cases classified as discarded, laboratory-confirmed rubella, epidemiologically linked rubella, or vaccine-associated measles.

Independent Variables

The independent variables in this study included individual and contextual factors potentially relevant to measles infection in Malaysia. Individual-level factors comprised age, sex, ethnicity, nationality, contact history, travel history, and the number of vaccine doses received. Age was treated as a continuous variable, measured in years. Ethnicity was categorised into Malay, Chinese, Indian, Indigenous Sabah Sarawak, Indigenous Peninsular Malaysia, and other. Nationality was classified as either Malaysian or non-Malaysian. Contact history with a measles case was defined as any known contact with a person with measles within 7 to 21 days prior to the development of a rash [34]. Travel history included any travel to other countries within 7 to 21 days before rash onset. The total vaccine dosage was calculated based on the number of measles vaccine doses the individual had received [17,35].

Eight district-level contextual factors were analysed, including population density, median household income, urbanisation, the number of health and rural clinics, vaccination coverage, PM_2.5 levels, relative humidity, and temperature. Population density was calculated as the ratio of the district’s population to its area in square kilometres. Median household income represented the middle value of the household incomes in a district when arranged in ascending order. The urbanisation status of the district of residence was classified as either rural or urban based on the local council designation, obtained from the Department of Statistics Malaysia [36]. The number of health and rural clinics referred to the total count of these facilities within the district where the participant resided. District vaccination coverage denoted the percentage of the district’s population that had received MMR vaccination. For each person studied, data on relative humidity, temperature, and PM_2.5 levels correspond to the measurements taken in the district 14 days prior to the onset of rash symptoms. This period aligns with previous research indicating that the incubation period for measles ranges from 7 to 21 days, with around 14 days being the most common. Consequently, the environmental conditions, including relative humidity, temperature, and PM_2.5 levels from 14 days before rash appearance, were used to reflect the exposure conditions potentially influencing participants.

Data Sources

The data on measles status and all individual-level factors were obtained from the e-measles database of the Ministry of Health Malaysia. The e-measles system, developed by the Ministry of Health Malaysia and operational since 2007, provides a standardised platform for case investigation and case-based reporting at district, state, and national levels. This system is managed by the Disease Control Division of the Ministry of Health Malaysia.

The data on population density, median household income, urbanisation, and the number of health and rural clinics were sourced from the Department of Statistics Malaysia. Information on district vaccination coverage was obtained from the Health Informatics Centre of the Ministry of Health Malaysia. Environmental data on relative humidity, temperature, and PM_2.5 levels were obtained from the Department of Environment Malaysia.

Statistical Analysis

Descriptive analysis was performed to depict the distribution of individual- and district-level factors in relation to the dependent variables: measles and non-measles. Categorical variables were expressed as frequency and percentage, while continuous variables were summarised using the median and interquartile range (IQR). Initially, a series of univariable analyses were carried out using simple logistic regression. Factors with p-values less than 0.25 were then included in a multivariable analysis employing multilevel logistic regression to account for district-level clustering effects. The multilevel logistic regression model incorporated 2 levels: individual and district.

Four models were constructed for the multilevel logistic regression analysis. Model 1 represented a null model that did not incorporate any individual-level variables or district-level risk factors, establishing a baseline for comparison with the other models. The intraclass correlation coefficient (ICC) was estimated from this model, reflecting the proportion of total variance attributable to the districts. Model 2 included adjustments for individual-level variables. Model 3 accounted for contextual factors at the district level, while model 4 was fitted with both individual-level variables and district-level contextual factors. aORs with 95% CIs were determined, and p-values of less than 0.05 in the multivariable models were considered indicative of significant associations with measles infection.

The Akaike information criterion (AIC) and the area under the receiver operating characteristic curve (AUC) were utilised to evaluate the goodness of fit for the models in this study. A model with a lower AIC and a higher AUC value was considered to display a better fit. The absence of multicollinearity among the independent variables was confirmed by the presence of a variance inflation factor of less than 10 and a tolerance greater than one. Two-way interactions between the significant variables were also examined. Univariable and multilevel modelling was performed using R ver. 4.3.1 (The R Foundation) within RStudio ver. 2023.12.1+402, employing the glm and glmer functions, respectively.

Ethics Statement

The research was conducted in accordance with the Declaration of Helsinki guidelines and received ethical clearance from the Human Research Ethics Committee of Universiti Sains Malaysia, under the approval code USM/JEPeM/KK/23010094. Additionally, the Malaysia Medical Research and Ethics Committee granted approval under the code NMRR-23-00290-E4U.

Results

Descriptive Statistics

Table 1 summarises the individual factors within the registered data, consisting of confirmed measles and non-measles cases from 2018 to 2022. Regarding individual-level factors, the median participant age was 0.83 years (IQR, 1.42 years), or equivalently, 10 months (IQR, 17 months). The gender distribution was similar between groups. Among confirmed measles cases, the youngest participant was 1 day old, while the oldest was 64 years old. The majority (89.72%) were of Malay ethnicity, and nearly all (99.94%) were Malaysian. Over 90% of participants had no reported history of contact with measles cases and had not travelled abroad. The median number of measles vaccine doses received was one, with a range of no vaccination to a maximum of 4 doses.

Regarding district-level contextual factors, the median population density was 563.80 persons per km², and the median household income was 6,938.00 Malaysian ringgit (RM). Of the participants, 76.40% resided in urban areas, with a median of 27 health and rural clinics per district. The median vaccination coverage was 98.31%. The median PM_2.5 concentration recorded was 16.90 µg/m³, along with a relative humidity of 80.75% and a median temperature of 27.5 °C. Details regarding these factors are displayed in Table 2.

Multilevel Logistic Regression

The final model, model 4, incorporates both individual and contextual-level factors. This model demonstrated that age, ethnicity, nationality status, history of contact with a measles case, history of travelling abroad, vaccine dosage, the number of health and rural clinics, and the degree of urbanisation were all significantly associated with measles infection (Table 3).

Measures of Association (Fixed Effects)

For individual-level factors, the risk of measles increased by 2% for 1 additional year of age (aOR, 1.02; 95% CI, 1.02–1.03). Ethnicity was also significantly associated with measles infection in Malaysia. Regarding nationality, non-Malaysians faced a risk of measles multiple times higher than that of Malaysians (aOR, 34.53; 95% CI, 8.42–141.51). Individuals with a history of contact with measles cases were 2.36 times more likely to become infected (aOR, 2.36; 95% CI, 2.07–2.69), while those reporting international travel 7 to 21 days prior had 2.3 times the odds of infection of those without such a travel history (aOR, 2.30; 95% CI, 1.13–4.70). An additional dose of the measles vaccine received by an individual was associated with a 24% reduction in the risk of infection (aOR, 0.76; 95% CI, 0.72–0.79) (Table 3).

Among contextual factors, urbanisation was significantly associated with measles infection, with urban residents displaying a higher risk of contracting measles compared to those in rural areas (aOR, 1.56; 95% CI, 1.16–2.10). Additionally, the presence of more health and rural clinics in a district was associated with a lower risk of measles infection among its residents (aOR, 0.98; 95% CI, 0.97–0.99), after adjusting for district clustering effects.

Measures of Variation (Random Effects) and Model Fit Statistics

Based on the multilevel logistic regression analysis, as depicted in Table 3, the null model (model 1) indicated an 18% variation in the odds of measles infection attributable to district-level factors (ICC, 0.18%). Model 4 emerged as the best-fitting model, as evidenced by its lowest log-likelihood (−9,207.73), highest AUC (0.70), lowest deviance (18,098.54) and lowest AIC (18,443.46). Multicollinearity was not observed in model 4.

Discussion

This study examined individual-level and contextual district-level factors in Malaysia that influence the risk of measles infection. The findings revealed that older age was associated with higher odds of contracting measles, contradicting the common perception that measles primarily affects children. Instead, older age groups are also at risk [37]. The identification of age as a risk factor for measles infection is supported by the concept that immunity to measles can vary across age groups. In many regions, individuals born before 1957 and older adults may have been exposed to measles in the pre-vaccination era, thereby acquiring natural immunity [14,38,39]. However, in populations with high vaccination coverage, younger cohorts are typically protected through vaccination [39]. Adults experience a slight increase in the odds of contracting measles, which could indicate a decline in immunity over time [39]. This underscores the importance of continued surveillance and the consideration of booster vaccination strategies for older groups. Measles vaccine-derived immunity is less robust and shorter-lived than natural immunity [38]. Although older adults and the elderly generally possess natural immunity against measles, this study indicates that these individuals are still susceptible to infection. Despite the median age of measles cases being less than 1 year, confirmed cases were reported in individuals up to 64 years old. The estimated decay rates of measles antibodies differ markedly between those with naturally acquired infections and those who have been vaccinated, with a mean duration of immunity of 25 years [40]. A study in Italy revealed that 15% of individuals vaccinated against measles did not exhibit a protective titre of immunoglobulin G antibodies 10 years after vaccination [41]. Similarly, a study among healthcare workers in Korea showed that 82.9% of participants tested negative for measles immunoglobulin G antibodies, with the seronegative status attributed to waning immunity [42]. The findings of this study highlight the importance of maintaining high vaccination coverage across all age groups, particularly young adults, as opposed to only children. Public health strategies may require revision to include booster doses for older age groups, especially in populations with evidence of diminishing immunity or relatively high exposure risk, such as healthcare workers. Additionally, the study highlights the need for ongoing surveillance of measles immunity levels.

Ethnicity was identified as a significant risk factor for measles infection in this study. Malaysia, a multiethnic nation, is predominantly composed of the Malay ethnic group. The descriptive statistics of the overall sample reflect the demographic composition of Malaysia’s population [43]. Indigenous populations in Peninsular Malaysia were found to face a higher risk of measles compared to the Malay population. This pattern may be attributed to lower vaccination rates among indigenous communities. For instance, during a large-scale measles outbreak among the Bateq tribe in Kuala Koh, Gua Musang, only 61.5% had received the first dose of the MMR vaccine, and just 30% had completed the second dose [44]. The nomadic lifestyle of the Bateq tribe poses a challenge for medical teams to provide consistent health services, such as vaccinations [44]. To reduce the risk of measles among all ethnic groups in Malaysia, it is crucial to improve vaccination coverage and address socioeconomic disparities.

This study highlights a significantly higher risk of measles infection among the non-Malaysian population living in Malaysia, primarily due to barriers in accessing preventive healthcare services. These challenges extend beyond legal limitations to include socioeconomic factors and language barriers [45,46]. Non-Malaysians, particularly undocumented immigrants, encounter difficulties such as legal restrictions and the fear of deportation, which may deter them from seeking healthcare services such as vaccinations [47]. Moreover, the substantial cost of immunization for non-Malaysians, which includes a clinic registration fee of RM 40 and an additional RM 40 per vaccine dose, poses meaningful challenges. In contrast, Malaysian citizens have access to government-subsidised vaccines at a nominal fee of RM 1, highlighting a pronounced disparity in healthcare access [48]. Language barriers profoundly impact the comprehension of vaccine importance and the ability to navigate healthcare systems effectively. Communication issues can result in misconceptions about vaccine significance, schedules, and follow-up protocols, decreasing uptake and completion rates within this demographic. The absence of health promotion materials in various languages means that non-Malaysian populations might not be informed of available vaccination programs or understand the need for vaccinations, resulting in lower immunization rates and heightened measles risk [49,50]. These factors collectively emphasise the critical need for improved healthcare accessibility and communication strategies tailored to the diverse linguistic and socioeconomic needs of non-Malaysian populations. Such improvements are essential to improve health outcomes and protect against diseases such as measles.

In this study, individuals with a history of exposure to measles were found to have a higher risk of contracting the disease compared to those without such exposure. An Indonesian study also highlighted this risk, revealing an OR of 2.31 (95% CI, 1.22–4.27) for individuals exposed to measles. These findings align with previous research from Yemen, Ethiopia, and the United Kingdom [6,12,51]. Given the basic reproduction number (R0) of measles, which ranges from 12 to 18, the likelihood of contracting the disease increases significantly upon exposure to the virus [52]. This elevated risk underscores the importance of effective quarantine measures and contact tracing in controlling the spread of measles. In 2015, Malaysia updated its guidelines for preparedness and response to measles cases and outbreaks. Previously, an outbreak required 2 confirmed cases to trigger a response, but under the updated guideline, a single confirmed case is now sufficient to initiate outbreak protocols [53]. This improved strategy emphasises the importance of prompt and effective public health measures—such as quarantine, contact tracing, and isolation of suspected cases, particularly in settings like schools—to minimise transmission. Furthermore, the provision of specific quarantine leave for parents and caregivers of children with measles, as well as other infectious diseases such as hand, foot, and mouth disease, dengue, chickenpox, diphtheria, malaria, or any medically certified infectious illness, assists in containing the spread of these diseases. Such a policy allows caregivers to take time off work to care for a sick child [54].

This study revealed that individuals with a history of international travel 7 to 21 days prior to the onset of a rash displayed a higher risk of contracting measles compared to those without such a travel history. Research conducted in Amhara indicated that people with a history of travel abroad were about 2.5 times more likely (95% CI, 1.31–7.24) to contract measles [9]. Similar results were reported in China [11]. Travelling internationally to areas where measles is endemic or where outbreaks are occurring can expose individuals to the virus. Travellers can easily become infected in crowded environments such as airports, public transportation, and tourist attractions. Since the incubation period for measles ranges from 7 to 21 days, individuals may appear healthy during their travels and only begin to show symptoms of the disease after returning home. A survey by a French insurance operator found that 69% of Malaysians planned to vacation abroad in 2023 [55]. This poses an increased risk of contracting and subsequently spreading infectious diseases like measles in Malaysia, potentially leading to imported cases. Thus, it is essential to ensure that travellers to endemic areas, especially young children, are fully vaccinated with the MMR vaccine before going abroad. Older children and adults who have not received the 2 recommended doses of the measles vaccine should receive a vaccine dose before travel, as stated in the travel health advisory [56].

This study found that receipt of a second measles vaccine dose decreased the risk of measles infection by 24%. This finding aligns with research from Singapore and the United Kingdom [17,57]. The initial vaccine dose serves to prime the immune system by introducing an attenuated form of the virus, which triggers the production of antibodies and memory cells that target measles. However, the immune response to this vaccine varies among individuals; while the majority develop immunity, a small percentage do not achieve an adequate antibody level for protection. This underscores the importance of administering a second dose to ensure broader immunity within the population [58,59]. Furthermore, recent evidence suggests that immunity may wane over time, particularly among young adults, indicating a potential need for booster doses to strengthen the immune response [40–42]. The adoption of a 2-dose measles vaccination strategy not only maximises individual protection against the disease but also significantly boosts herd immunity. To maintain this immunity, at least 94% of the population must be immunized with a measles-containing vaccine [60]. Herd immunity is crucial for safeguarding vulnerable groups who cannot receive vaccinations, such as infants, individuals with certain health conditions, and those with allergies to vaccine components. The high coverage produced by the 2-dose vaccination schedule serves to protect against measles outbreaks.

Our research indicates that individuals living in urban areas faced a higher risk of contracting measles compared to their rural counterparts. This pattern is consistent with findings from studies conducted in Nigeria, which also reported higher rates of measles infection among urban residents [19,61]. The higher population density and close quarters typical of urban environments facilitate the rapid transmission of measles, particularly in crowded settings such as schools and public transportation systems. Furthermore, cities often serve as international travel hubs, featuring airports that receive visitors from all over the world. This continuous arrival of people from regions where measles may be more prevalent can introduce the virus into urban communities.

The COVID-19 pandemic has heightened the risk of measles outbreaks in urban areas by disrupting routine vaccination programs, including those targeting measles. As healthcare resources were reallocated to combat COVID-19, missed opportunities for measles vaccinations increased, leading to a larger pool of susceptible individuals and raising the potential for outbreaks. The recent pandemic highlights the vulnerability of public health systems and the critical importance of sustained immunization efforts to prevent vaccine-preventable diseases such as measles, especially in high-risk urban settings [62]. Consequently, the implementation of nationwide supplementary immunization activities may represent a vital step toward achieving herd immunity across Malaysia, particularly in the context of the COVID-19 pandemic.

Residents of districts with fewer health and rural clinics were found to face a higher risk of contracting measles. The number of these clinics within a district reflects the accessibility and effectiveness of healthcare delivery in Malaysia. The presence of more clinics improves diagnostic capabilities, enabling the swift isolation and treatment of measles cases. This quick response helps to mitigate both the severity of the disease and its spread within communities. Such prompt management meaningfully reduces the impact of measles, highlighting the critical role of accessible healthcare facilities in the control of infectious diseases [63]. Furthermore, these clinics play an essential role in improving vaccination coverage by facilitating immunization programs and improving vaccine accessibility, thus promoting equitable healthcare. The Ministry of Health Malaysia leverages health and rural clinics to implement various strategies—such as the child home-based book, the family doctor concept, home visits for defaulter tracing, and appointment reminders—to increase vaccine uptake, especially in remote regions [64].

Strengthening healthcare infrastructure, including the establishment of additional clinics, can improve health practices and education among the population. This may lead to increased public awareness of measles and a subsequent rise in vaccination rates. Health education initiatives carried out in these facilities can help disseminate information about measles and the importance of timely vaccination. Studies conducted in India and Pakistan have demonstrated the positive impact of intensive information and education campaigns on vaccination rates [65]. Thus, an increase in the number of health and rural clinics was associated with improved healthcare accessibility and more effective health education dissemination in Malaysia. Enhanced healthcare infrastructure not only makes measles vaccination more accessible but also ensures the widespread distribution of information regarding the importance of immunization.

Environmental factors such as relative humidity and temperature were not found to be significant factors in measles infection in Malaysia. This is largely because Malaysia is situated on the equator and experiences a tropical rainforest climate, which results in consistent temperatures ranging from 23 °C to 32 °C and high humidity throughout the year [33].

A notable strength of this study is its comprehensive approach to analysing both individual and district-level contextual factors through multilevel logistic regression analyses. This methodological strategy illuminated how various demographic, socioeconomic, and environmental factors contribute to the risk of measles, providing a solid foundation for targeted public health interventions. Additionally, the study’s utilisation of a large, diverse dataset spanning several years enriched the robustness of its findings, offering a detailed snapshot of the measles landscape in Malaysia. Such a detailed analysis may be invaluable for policymakers and health practitioners aiming to design and implement effective measles control strategies tailored to the specific needs and challenges of certain communities. However, the study has certain limitations, which are inherent in its design and the nature of its data sources. The reliance on secondary data and the cross-sectional study design limit the ability to establish causality between the identified risk factors and measles infection.

Future research should aim to address several key areas. Longitudinal studies could provide deeper insights into the temporal relationships between risk factors and measles infection, helping to establish causality and clarify the dynamics of measles transmission over time. Additionally, further research into social determinants of health, such as access to healthcare and vaccine hesitancy, could illuminate underlying factors contributing to disparities in measles risk and vaccination coverage. Investigating these aspects could offer more targeted recommendations for improving vaccination strategies and healthcare access, particularly among vulnerable populations.

Conclusion

This study highlights the complexity of measles transmission and underscores the necessity for multifaceted public health strategies to address this persistent challenge. Targeted vaccination campaigns are key, especially in urban areas and among populations with lower vaccination coverage, such as non-Malaysians and certain ethnic groups. To progress toward the elimination of measles, Malaysia must implement comprehensive public health measures, improve surveillance, promote vaccination, and engage in international collaboration to effectively mitigate these multidimensional risk factors.

HIGHLIGHTS

• This study comprehensively examines individual and contextual factors contributing to measles infection in Malaysia, utilising multilevel logistic regression analysis.

• The results reveal that measles infection is significantly associated with individual factors including age, ethnicity, travel history, and vaccination status, alongside the contextual factors of urbanisation and healthcare facility density.

• These findings highlight the complexity of measles transmission and underscore the need for targeted public health strategies addressing both individual susceptibilities and environmental influences.

• This research offers valuable insights into the development of effective interventions and policies for the prevention and control of measles in Malaysia.

Article information

Ethics Approval

This study received approval from the Human Research Ethics Committee of University Sains Malaysia, under the approval code USM/JEPeM/KK/23010094, as well as from the Malaysia Medical Research and Ethics Committee, with the approval code NMRR-23-00290-E4U. The research was performed in accordance with the principles of the Declaration of Helsinki. Due to the retrospective nature of the study, the requirement for informed consent was waived.

Conflicts of Interest

The authors have no conflicts of interest to declare.

Funding

None.

Availability of Data

The datasets are not publicly available but can be obtained from the corresponding author upon reasonable request.

Authors’ Contributions

Conceptualization: MRHMD, NAY, WNA; Data curation: MRHMD, JAMS, WAHWI; Formal analysis: MRHMD, NAY, WNA; Methodology: MRHMD, NAY, WNA; Project administration: MRHMD, NAY; Visualization: MRHMD, NAY, WNA; Writing–original draft: MRHMD; Writing–review & editing: all authors. All authors read and approved the final manuscript.

Acknowledgements

We extend our gratitude to the Director-General of Health at the Ministry of Health, Malaysia, for granting permission for the publication of this study. We also thank everyone who contributed to this research, whether directly or indirectly.

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