Prevalence of tobacco smoking and associated factors among adults in Ethiopia: a systematic review and meta-analysis

Introduction The public health concern of tobacco smoking is more prevalent in low- and middle-income countries including Ethiopia. Various studies have investigated tobacco smoking in various parts of Ethiopia. However, the findings have been inconsistent and characterized by significant variability. Besides, there is no nationally representative data on the subject, which could deter the design of effective intervention strategies to reduce tobacco-related problems. Therefore, this study aimed to estimate the pooled prevalence of tobacco smoking and associated factors among adults in Ethiopia. Methods The study was conducted based on the Preferred Reporting Items for Systematic Reviews and Meta-Analysis Protocols Guideline. A detailed search was conducted from international databases including PubMed, Cochrane Library, Science Direct, CINAHL, African Journals Online, HINARI, Global Health, and Google Scholar. The extracted data was analyzed using STATA 14 software. A random-effects model was used to estimate the effect size. The Egger regression test and I2 statistics were used to determine potential publication bias and heterogeneity among the reviewed articles, respectively. Results A total of 32 studies with 69,897 study participants were included in this systematic review and meta-analysis. The pooled prevalence of lifetime tobacco smoking among adults in Ethiopia was 16.0% (95% confidence Interval (CI): 13.6–18.39) and there was significant heterogeneity among the included studies (I2 = 99.1%, p < 0.001). Male adults were three times more likely to smoke tobacco as compared with females [OR = 3.22 (95% CI: 2.06–5.03)]. Being an alcohol user [OR = 3.78 (95%CI: 1.27–11.24)] and having tobacco-smoking friends [OR = 7.21 (95%CI: 5.56–9.35)] are potential determinant factors for tobacco smoking. Conclusion The pooled prevalence of lifetime and current tobacco smoking among adults in Ethiopia was high, which calls for urgent intervention. Therefore, prioritization of tobacco control strategies, such as creating awareness about the public health importance of tobacco smoking, can help prevent and mitigate the effects of tobacco smoking. Alcohol control law enforcement should also be strengthened.


Introduction
Tobacco smoking is one of the causes of mortality and morbidity worldwide (1).It is the second significant cause of death globally.Every 6.5 s, one tobacco user dies as a result of a tobacco-related disease (2).According to the World Health Organization (WHO), annually, 8 million deaths are reported worldwide as a result of tobacco smoking (3); 6 million are direct smokers (4,5).It has been confirmed that tobacco smoking can cause various types of cancers (6), chronic obstructive pulmonary disease, and lung disease.For instance, 90% of all lung cancer deaths in the United States were due to tobacco smoking (7).This is associated with the fact that tobacco contains up to 7,000 health-threatening chemicals (8).Beyond mortality and morbidity, the use of tobacco adds a burden to the national economy by increasing global health expenditures by about 6% and other indirect costs (9).According to the WHO report, the cost of smoking alone is estimated to be US$ 1.4 trillion, or 1.8% of the global GDP (10).
Over 80% of the global 1.3 billion smokers are found in lowand middle-income countries, where the burden of tobaccorelated illness and death is significant (1).For instance, the prevalence of tobacco smoking was 60.2% in Bangladesh (11), 30.1% in Cameron (12), 20.2% in India (13), 14.7% in Malaysia (14), 13.7% in Nigeria (15), 12.4% in Yemen (16), and 10.7% in Pakistan (17).In Ethiopia, around 2.9 million adults are cigarette smokers.Of these, one-third of adults are exposed to secondhand smoke in public places (14), which is a significant cause of premature morbidity and mortality.
Numerous studies have been conducted to assess the prevalence of tobacco smoking and associated factors in Ethiopia .However, the study reports are inconsistent and characterized by significant variability [e.g., 2.4% (33) to 45.3% (28)], which could negatively influence the design of effective intervention strategies.Moreover, although a systematic review and meta-analysis is conducted among specific groups of the population (e.g., students) (14), there is no nationwide study assessing the pooled prevalence of tobacco smoking among the general adult population in Ethiopia.Therefore, this systematic review and meta-analysis aimed to estimate the pooled prevalence of tobacco smoking and identify factors contributing to tobacco smoking among adults in Ethiopia.The findings from this metaanalysis would generate evidence that will be important inputs for tobacco smoking prevention and control program planners, policymakers, and health service providers to design and implement evidence-based interventions to reduce the burden of tobacco-related mortality and morbidity in the country and other similar settings.

Protocol registration
The protocol for this systematic review has been registered in the International Prospective Registry of Systematic Review (PROSPERO) with a specific registration number CRD42023415610.

Searching strategies
This meta-analysis followed the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) guideline (50).Studies were searched through PubMed/Medline, Cochrane Library, Science Direct, CINAHL, African Journals Online, HINARI, Global Health, and Google Scholar.In addition, digital libraries were searched to identify grey literature.

Inclusion and exclusion criteria
Population: this systematic review and meta-analysis includes studies conducted among tobacco-smoking adults in Ethiopia.Exposure: an individual adult who experienced tobacco smoking whereas an individual adult who did not tobacco smoking was considered as a comparison.Outcome: studies assessed tobacco smoking as a primary outcome.All studies conducted with observational studies design (cohort, cross-sectional, and case-control) and institutional and community-based studies were also included in systematic review and meta-analysis.Besides, studies published from 2000 to 15 May 2023 in the English language were also included in the meta-analysis.However, qualitative studies, unretrievable studies, editorial letters, studies with poor methodological quality, and studies that did not report the outcome of interest were excluded from the meta-analysis.

Outcome measurement
The primary outcome of the study was the pooled prevalence of tobacco smoking among adults in Ethiopia, calculated by dividing the number of smokers by the total sample size and multiplying by 100.Additionally, the study aimed to identify the factors linked to tobacco smoking in the form of a log odds ratio.

Data extraction
After searching for relevant studies from the aforementioned electronic databases, they were exported to Endnote X20 (51) and any duplicates were removed.Three authors (CD, BD, and MG) independently extracted all the required data from the included studies by using a standardized data extraction template.The data extraction template consisted of various study details, such as the author's name, region, publication year, study area/region, study population, study setting, study design, sample size, response rate, and tobacco smoking prevalence (current and lifetime prevalence).In the case of any disagreements during data extraction, other authors (AA, MD, and SAD) resolved them.

Quality assessment
After duplicate files were removed, four reviewers (CD, MG, BD, and SAD) screened the relevant studies for inclusion.The Joana Brigg Institute (JBI) critical appraisal checklist for prevalence studies was employed to evaluate the quality of each article (52) and those studies scored more than 50% were included for analysis (53,54).Each study's quality was assessed independently out of 100% by four authors (CD, BD, MG, and SAD).If any discrepancies arose during the quality assessment, the mean score was calculated from the results of all reviewers to resolve the differences.

Statistical analysis
The extracted data was exported into STATA/ SE version-14 statistical software for analysis (55).The level of heterogeneity among the included studies was statistically evaluated using the Higgs I 2 test, with values greater than 75% considered high heterogeneity (56).DerSimonian and Liard's method of random-effects model at a p-value less than 0.05 with a 95% CI was used to estimate the pooled prevalence of tobacco smoking using a forest plot.A p-value of less than 0.05 was considered indicative of the presence of heterogeneity (57).
Subgroup analysis was also conducted by characteristics of the study, such as region (Oromia, Amhara, Tigray, SNNPR (Southern Nation, Nationality, and People Region), all regions, or others), study setting (institutional or community), sample size (large-≥ 600 or small-<600), study population (students or community), and year of publication (before 2015 or 2015 and after).The different factors associated with tobacco smoking were presented using odds ratios (ORs) with 95% CI.Moreover, univariate meta-regression was carried out to identify the source of variations among studies that exhibited high heterogeneity.
Additionally, publication bias was assessed using a funnel plot and Egger's test method with a significance level of p < 0.05 (58).Likewise, Duval and Tweedie's 'trim and fill' analysis was also conducted to adjust the effect of publication bias among the studies included.A sensitivity analysis was also performed to assess the influence of a single study on the pooled prevalence estimates.

Study selection
After conducting the electronic database search, a total of 1,063 studies were identified.Out of these, 802 articles were excluded based on their titles and abstracts.Besides, 20 articles were also excluded due to poor quality and did not report the outcome of the interest.Finally, 32 full-text articles were found eligible for this systematic review (Figure 1).

Publication bias
Publication bias was subjectively evaluated by using a funnel plot.The finding indicated that there is asymmetrical distribution of the studies, which suggests the presence of publication bias (Figure 4).The Egger test statistics also revealed the presence of statistically significant publication bias (p < 0.001).Duval and Tweedie's "trim and fill" analysis indicates significant variation in the newly estimated pooled odds ratio (the adjusted point estimate) [OR = 1.68, (95% CI: 1.36-1.99)]as compared to the initial or observed point estimate [OR = 2.42, (95% CI: 2.11-2.73)](Figure 5).

Sensitivity analysis
The findings of sensitivity analysis suggest no evidence of a single study's effect on the overall pooled prevalence (Figure 6).

Subgroup analysis
To identify the source of heterogeneity among the included research articles, sub-group analysis was carried out based on the region where the studies were conducted, study population, year of publication, study setting, and sample size.Even though heterogeneity was not resolved among the included articles, the pooled prevalence of lifetime tobacco smoking was relatively higher among students than the general population [16.2 (95% CI: 11.94-20.45)].In addition, a high prevalence of tobacco smoking was observed in the studies done in the Oromia region as compared to other regions [24.7 (95%CI: 17.87-31.71)],with significant heterogeneity (I 2 = 96.3;p < 0.001).Similarly, the prevalence of tobacco smoking was high among institutional-based studies [15.9 (95%CI: 13.59-18.39)]as compared with community-based studies [13.1 (95%CI: 9.64-16.60)],with statistically significant heterogeneity (I 2 = 99.4;p < 0.001) (Table 2).In addition, a univariate meta-regression was conducted using the study population, sample size, and study year as factors.However, neither of them was found to be a statistically significant source of heterogeneity (Table 3).
Five studies (22,26,29,36,44) were included to assess if having tobacco-smoking friends is associated with tobacco smoking.All of the included studies had a positive association.The meta-analysis also showed that the odds of tobacco smoking were seven times higher  with extreme heterogeneity (I 2 = 40.1%,p = 0.154) (Figure 9).A total of 4 studies (22,35,39,44) were included to assess if having a tobacco-smoking parent is associated with tobacco smoking.Three of the included studies had a positive association (22,35,44) while a negative association in one study (39).Meta-analysis also showed that there was a non-significant association between parental smoking habits and adult tobacco smoking [OR = 2.16 (95%CI: 0.28-16.83)](Table 4).

Discussion
Tobacco smoking continues to be a global public health problem and its burden increases over time, particularly in low-and middle-income countries.This systematic review and meta-analysis was conducted to estimate the pooled prevalence of tobacco smoking among adults in Ethiopia.The lifetime and current pooled prevalence of tobacco smoking among adults were 16.0 and 13.7%, respectively, with high heterogeneity.The figures in our report are greater than the findings from a metaanalysis conducted in Mainland China (10.8%) (59) and East Africa (9.0%) (60), which could be due to the lack of full enforcement and implementation of tobacco smoking in Ethiopia.It is documented that the lack of effective implementations of cigarette sale prevention policies contributes to the high proportion of tobacco smokers (61).
On the other hand, the prevalence in our study is lower than that reported in Saudi Arabia (28.1%) (62), Kenya (42.8%) (63), Poland (30.8%) (64), China (31.8%) (65), Madagascar (28.5%) (66), and Bangladesh (23.2%) (67).This difference could be due to variations in the study population and setting.For instance, most of the aforementioned studies were conducted among students, who were likely to be exposed to smoking as a result of their age and peer pressure.Evidence from our subgroup analysis also showed that the pooled prevalence of tobacco smoking was significantly higher among students than studies conducted among the general population, which is in agreement with previous reports from systematic review and meta-analyses (68).
Gender was a significant predictor of tobacco smoking.The odds of tobacco smoking were two times higher among males than females.This finding was in line with a report from systematic reviews done in Iran (69), Sub-Saharan Africa (70), Saudi Arabia (62), and a meta-analysis conducted among students in Ethiopia (68).This might be associated with better social acceptance of substance use, such as khat, tobacco, and alcohol, among males than females.On the other hand, family The odds of tobacco smoking were three times higher among alcohol users than non-users.Our finding was in agreement with a previous report from the Nigerian community (72).This might be due to the fact that alcohol use often takes place in social settings where smoking is prevalent, especially in public places.Another possible reason for this finding might be due to peer influence and social norms that can enhance tobacco smoking behavior.Other studies also indicated that adults who drink alcohol are highly likely to smoke tobacco (73,74).This implies that addiction invites further addictions.
The meta-analysis showed that the odds of tobacco smoking were seven times higher among adults who have smoking friends compared to those who did not have smoking friends.This finding was consistent with studies conducted in Nepal (75), Madagascar (76), Cameroon (77), and Jordan (78).This implies the need for continuous social influence programs that can help create social norms that discourage young people from starting to smoke.

Strengths and limitations of the study
This review included most regions of the country and a diverse target population (students and other community members including healthcare workers and patients).The study followed preferred reporting items for systematic review and meta-analysis guideline.However, studies included in this systematic review and meta-analysis  were cross-sectional, which limits the causality of predictors on tobacco smoking.Besides, this study included articles published in the English language only, which could induce publication bias.

Conclusion
The pooled prevalence of tobacco smoking was high in Ethiopia.We found that one in six adults was engaged in tobacco smoking, which calls for urgent intervention.Being male and alcohol users, and having tobacco-smoking parents and friends were found to be factors positively associated with tobacco smoking.Therefore, urgent comprehensive public health intervention should be taken to reduce the burden of the problem.Moreover, the federal Ministry of Health, community leaders, and other concerned bodies should take strong action to protect the health of adults.In addition, alcohol control law enforcement should be strengthened."Trim and fill" analysis funnel plot.

FIGURE 6
Sensitivity analysis of the pooled prevalence of lifetime cigarette smoking among adults in Ethiopia, 2023.Forest plot of odds ratio for the association between male and tobacco smoking among adults in Ethiopia, 2023.Forest plot of odds ratio describing having tobacco smoking friends and tobacco smoking among adults in Ethiopia, 2023.Forest plot of odds ratio for the association between alcohol use and tobacco smoking among adults in Ethiopia, 2023.

FIGURE 1 PRISMA
FIGURE 1PRISMA flow diagram of the included studies for the systematic review and meta-analysis of the prevalence of tobacco smoking and associated factors among adults in Ethiopia, 2023.

FIGURE 2 Forest
FIGURE 2Forest plot showing the pooled prevalence of lifetime tobacco smoking among adults in Ethiopia, 2023.

FIGURE 3 Forest
FIGURE 3Forest plot of the pooled prevalence of current tobacco smoking among adults in Ethiopia, 2023.

FIGURE 4 Funnel
FIGURE 4Funnel plot of the pooled lifetime prevalence of tobacco smoking among adults in Ethiopia, 2023.

TABLE 1 A
descriptive summary of studies included estimating the pooled prevalence of tobacco smoking and associated factors among adults in Ethiopia, 2023.

TABLE 2
Subgroup analysis of the pooled prevalence of tobacco smoking among adults in Ethiopia, 2023.

TABLE 3
Univariate meta-regression analysis to identify factors associated with the heterogeneity of the prevalence of tobacco smoking in Ethiopia, 2023.