Smoking remains one of the leading causes of years of life lost and increased morbidity in Denmark and worldwide (1, 2). Globally, a critical political and public health issue is protecting children and adolescents from the health hazards of using tobacco products (3). Individuals initiating a tobacco use in their teenage years are more likely to continue smoking in adulthood, be addicted to nicotine, have issues with smoking cessation, and have higher risks of health adversities in later life (4–6). One significant concern is adolescents' being exposed to smoking in their environment, i.e., among parents, siblings, teachers, or peers (7–9). Specifically, when adolescents see others smoke in their near environment, they are more prone to perceive smoking as socially acceptable and susceptible to smoke themselves. Moreover, second-hand exposure to smoke is considered a health risk in itself (1). Therefore, preventing smoking uptake in adolescence and limiting the exposure of smoking in adolescents' everyday lives may have crucial public health benefits. In Denmark, a continuous goal by stakeholders has been to create a smoke-free future for Danish children (10). For several decades, overall smoking prevalences have decreased, e.g., among Danish schoolchildren, daily smoking had decreased from 18.6% in 1991 to 4.5% in 2014 (11). However, more recent reports indicate changing smoking patterns among the youth population; some numbers show a stagnation in smoking uptake among some parts of the Danish youth (12), while others show decreasing trends in daily but not in occasional smoking (13). In Denmark and internationally, there seems to be a stagnation or even an increase in smoking prevalence among youths from lower socioeconomic backgrounds (11, 14). Additionally, social disparities in smoking have not diminished, quite conversely, research indicates increasing socioeconomic differences in smoking during the last decades (11, 15). Consequently, targeting the socioeconomic differences in smoking is essential in smoking preventive efforts. The social inequalities in tobacco-related harms have also been well-documented internationally (16).

Another increasing public health concern is the use of other tobacco products than conventional cigarettes, including e-cigarettes, smokeless tobacco (e.g., snus), and waterpipe (i.e., alternative tobacco products; ATPs). These products have gained growing interest in a Danish and global context—especially among the youth (17, 18). In fact, there has internationally been an increasing trend in youth use of e-cigarettes, snus, and waterpipe in recent years (18–21). In e.g., USA and Norway, the use of some ATPs has even exceeded the use of conventional cigarettes (22–24). However, the research on socioeconomic differences in the use of ATPs is somewhat inconsistent and limited, with some research indicating a social gradient in the uptake of ATPs among students from different socioeconomic groups (18, 25), while other studies found no socioeconomic differences in the use of ATPs (26–28). Several concerns are linked to the increasing use of ATPs, including health risks and the risk of becoming addicted to nicotine (29, 30). Moreover, adolescents may learn rituals associated with cigarette smoking by using, e.g., e-cigarettes, including the body language, taking smoking breaks, and how to handle a tobacco product (31). Another concern is the concurrent use of multiple tobacco and nicotine products—or that using one tobacco product use may impact decisions to use or try other products. For example, a Norwegian study found that using smokeless tobacco in youth increased the risk of smoking conventional cigarettes in adulthood (32). In this connection, there are concerns that ATPs may be a gateway to conventional cigarette smoking (31, 33), while other studies discuss the common liability hypothesis as a possible explanation for using multiple tobacco products and shifting from one tobacco product to another (34). Given these concerns, tobacco control policies aiming at preventing smoking should focus not only on conventional cigarettes but also on the use of ATPs.

Internationally, a host of interventions have been implemented to reduce smoking uptake among adolescents—many in the school setting—with mixed evidence of their effect (35). One study analyzed data from 49 randomized controlled trials on the effect of smoking preventive interventions (including data from approximately 140,000 schoolchildren) and found a 12% reduction in smoking initiation among children in the intervention groups compared with children in the control groups at more than 1 year follow-up from baseline (35). However, they found no effect at 1 year or less. In a Danish context, one of the first smoking preventive interventions which were found to decrease smoking uptake among adolescents was the X:IT study (36). An evaluation of X:IT indicated that if all intervention components were implemented as intended, X:IT could reduce the proportion of adolescents who smokes up to 25% at 1 year follow up (37). Studies evaluating the effect of preventive interventions on other tobacco products than cigarettes are still very sparse, and research with this aim has been called for in recent years (38).

The effect of school-based smoking and health interventions have been found to differ according to adolescents' socioeconomic backgrounds, although the directions are not unidimensional; some research indicated better intervention effects among students from higher socioeconomic backgrounds (39), while other research found that adolescents from lower socioeconomic backgrounds received the highest benefits of health interventions (40). Consequently, health interventions may both widen or narrow social inequalities in health. In the Danish X:IT study, the process evaluation of the intervention indicated that some aspects of the intervention were more easily adopted by students and parents from higher socioeconomic backgrounds (41). These findings may have important implications for the overall effects of the intervention. To our knowledge, no studies have evaluated socioeconomic differences in the trajectories of ATP use before and after implementing a tobacco preventive intervention.

Considering the limited research evaluating socioeconomic differences in tobacco use after the implementation of a smoking preventive intervention, the current study sought to examine trajectories of cigarette smoking among adolescents participating in a school-based smoking preventive intervention: The X:IT II intervention. Further, as a secondary aim, this study examined trajectories in ATP use across socioeconomic groups during the period. Adolescents were followed over the course of 2 years—from baseline (beginning of grade 7; ~13 years) to second follow-up (end of grade 8; ~15 years).

As presented in Table 1A, 826 participants were eligible for analyses who both answered the baseline measurement as well as the second follow-up measurement. In total, 1,139 (58.0%) were considered non-participants, i.e., they did not respond to the second follow-up measurement. More students from a low and medium OSC, who currently smoke, and who have ever used ATPs did not respond to the second follow-up.

Table 1B outlines a more comprehensive overview of the participants and non-participants in relation to the use of various ATPs; overall, results show that more students who had ever used waterpipe, snus, and e-cigarettes missed responding at second follow-up.

Table 2 displays descriptive information of baseline cases and imputed cases. Overall, no marked differences are seen between baseline cases and imputed cases in relation to gender as well as current smoking and ever use of ATPs at baseline; however, more students currently smoke (23.8 vs. 8.4%) and have ever used ATPs (49.7 vs. 25.4%) at second follow-up in the imputed cases compared with the baseline cases.

Table 3 shows the results of the difference-in-differences analyses of current smoking between socioeconomic groups. Adjusted for gender, the analyses of available cases showed that 0.3% of students from a high OSC currently smoked at baseline, while this was the case for 3.0% of students from a low OSC. At second follow-up, 8.3% of students from a high OSC and 10.9% of students from a low OSC currently smoked cigarettes. The adjusted difference-in-differences analysis of available cases showed an estimate close to zero, and the interaction term between OSC and time was insignificant. This indicates that there were no socioeconomic differences in smoking trajectories between OSC groups. However, the unadjusted and adjusted difference-in-differences analysis of imputed cases showed a somewhat different pattern. Adjusted for gender, 1% of the high OSC students were currently smoking at baseline, while this applied to 4.8% among students from a low OSC. At follow-up, 24.5% of the students from a high OSC and 25.6% from a low OSC were currently smoking.

Results showed that relative more students from a high OSC were currently smoking at second follow up relative to baseline compared with students from a low OSC (see also Figure 2); hence, indicating differential trajectories from baseline to second follow-up in current smoking among students from low and high OSC (p < 0.001).

Table 4 displays the difference-in-differences analyses of ever use of ATPs stratified by OSC of students. In adjusted analyses for gender of available cases, results showed that 7.6% of students from a high OSC and 11.6% from a low OSC had ever used ATPs at baseline. At second follow-up, 8.3% of students from a high OSC and 10.9% from a low OSC had ever used ATPs. The adjusted difference-in-differences analysis of imputed cases showed a similar pattern, although differences between OSC groups were more pronounced; adjusted for gender, 10.0% of students from a high OSC and 13.9% of students from a low OSC had ever used ATPs at baseline. At second follow-up, 46.8% of students from a high OSC and 60.8% of students from a low OSC had ever used ATPs.

The difference-in-differences estimates for the adjusted difference-in-differences analysis of available cases and imputed data indicated substantial differences in trajectories of ever use of ATPs between OSC groups (p < 0.001). Hence, relative more students from a low OSC had ever used ATPs at second follow up relative to baseline compared with students from a high OSC (see also Figure 3).

This study is among the first to examine differential trajectories of smoking and ATP use between socioeconomic groups before and after implementation of a smoking preventive intervention. Findings provide crucial knowledge for current and future actions against youth tobacco use, as initiatives should focus on benefiting all groups equally—or benefit the groups most in need of prevention efforts.

First, in line with most research (11), this study found that more students from low OSC were currently smoking cigarettes compared with adolescents from high OSC at baseline. However, from baseline to second follow-up, smoking developed somewhat differently among students from diverse socioeconomic backgrounds. Importantly, the gap in current smoking between students from low and high OSC narrowed over time from a difference of almost four percentage points at baseline to about one percentage point at second follow-up. In a recent study, we investigated trajectories of current smoking between OSC groups from baseline to first follow-up, i.e., at the end of grade 7 (52). The study indicated that X:IT II did not create differential trajectories in current smoking among adolescents from diverging socioeconomic backgrounds over the course of one school year. In the present study, students are followed until the end of grade 8 and thus, findings from the current study indicate that tobacco patterns have developed differently at this point in the intervention compared with the first follow-up measurement. Nevertheless, these findings should be viewed positively in the sense that the X:IT II intervention seems to be more beneficial for some groups of adolescents at higher risk of smoking cigarettes, i.e., from a low OSC. Thus, the intervention did not widen the social inequalities in smoking. Considering these findings, the X:IT II seems to be successful in targeting students from lower socioeconomic backgrounds. Similarly, a review by Tinner et al. (40) suggested that adolescents with lower socioeconomic positions benefited most from smoking preventive initiatives. However, there is generally a dearth of literature examining socioeconomic differences in the effect of smoking preventive interventions. The X:IT II intervention builds on a multilevel approach in which several components on individual and environmental levels are initiated to reduce adolescent smoking. This approach is considered ideal for limiting the social inequalities in intervention effects, while interventions solely aiming at the individual level may increase social inequalities (53).

From this study, we do not have knowledge about the development in smoking among adolescents unexposed to the intervention, although knowledge builds on previous findings from the first X:IT intervention with both an exposed and unexposed group. Some of the socioeconomic effects in smoking may have been impacted by an overall societal focus on decreasing smoking among youth, with increasing political actions against smoking (45). In this connection, one Danish study showed that the absolute socioeconomic differences in smoking decreased during the past decade; however, the relative socioeconomic differences in smoking increased during the same period, and smoking remained highest among students in the lowest socioeconomic group during the entire study period from 1991 to 2014 (11). Findings from other westernized countries also suggest rather stable or increasing socioeconomic differences in smoking over time (15, 54). As most political actions were in effect after implementation of this study (45), these initiatives were not expected to impact the study results. Future research may further investigate whether socioeconomic differences in smoking are influenced by the recent increased political and societal attention to smoking–these studies would also be fruitful for investigating whether trajectories in smoking varies over time for adolescents unexposed to intervention components.

Secondly, this study examined trajectories in ever use of ATPs between OSC groups. At baseline, more students from low OSC compared to high OSC had ever used ATPs. However, conversely to the development in current smoking during the study period, the socioeconomic differences in ever use of ATPs widened over time. There may be several mechanisms to explain these findings. First and foremost, the intervention was designed to prevent cigarette smoking uptake among students (44) and, thus, it was not a primary aim of the intervention to prevent the uptake of other tobacco products. Nonetheless, it has previously been discussed whether smoking preventive interventions may be effective in also reducing the use of ATPs as several characteristics associated with cigarette smoking are associated with ATP use (55). Consequently, many of the same mechanisms or pathways to cigarette smoking may apply to ATP use. However, a main focus on reducing cigarette smoking among adolescents may have some unfortunate side effects, e.g., pushing some groups of youths toward using other substances which may not be seen as just as harmful or damaging to health compared with conventional cigarettes. Our study may reflect this tendency. This may be due to the X:IT II intervention itself, which focused on cigarette smoking, combined with the current societal tendencies where laws on conventional cigarettes have been tightened substantially in recent years (45). The most recent adopted Danish law comprises several initiatives to prevent especially youth smoking and use of other tobacco and nicotine products, e.g., e-cigarettes, although these were not initiated at the time of the study. Moreover, in a recent study among students enrolled in the X:IT II intervention, students reported that knowing smoking is dangerous and not wanting to be addicted to smoking were the top reasons for not smoking cigarettes. These findings indicate a high awareness of the health-damaging effects of cigarette smoking (56). However, we do not have knowledge about students' reasoning toward using or not using ATPs.

The few studies that have evaluated the influence of tobacco prevention interventions on students' use of other tobacco products than conventional cigarettes have shown mixed results (57, 58). For example, an evaluation of an outdoor smoking ban found no impact on students' use of e-cigarettes and waterpipe at follow-up (57). In contrast Hedman et al. (58) found a lower prevalence of snus use among students in the intervention group compared with the control group. Here, the intervention comprised tobacco-free contracts and education in tobacco-related health issues. These findings indicate that interventions with multiple initiatives aiming at ATPs may potentially impact students' decisions about ATP use. The intervention components of X:IT II comprise parental involvement, including smoke-free agreements, as well as smoke-free school time, and smoke-free educational material. Thus, extending current intervention components to increase awareness and knowledge about ATPs may produce positive outcomes. However, none of the identified studies evaluated subgroup effects of the interventions in terms of students' socioeconomic backgrounds. As the use of ATPs is increasingly prevalent among adolescents, there is an urgent need for future research to examine the effect of prevention efforts on ATPs and whether the effect differs between socioeconomic groups.

This study found that socioeconomic disparities in cigarette smoking narrowed over a 2 year-period in which a smoking preventive intervention (X:IT II) was implemented. X:IT II was designed to appeal equally to students from lower and higher socioeconomic backgrounds. Hence, this study indicates promising findings in impacting decisions about smoking among students from lower socioeconomic backgrounds. However, the current study found that social disparities in ever use of ATPs widened over the study period. As this study is among the first to examine differential trajectories of smoking and ATP use between socioeconomic groups before and after the implementation of a smoking preventive intervention, more research in this area is needed.

Data that support the findings of this available from the University of Southern Denmark (SDU). Restrictions to the availability of data that were used under license and are therefore publicly available. However, data are available from the corresponding upon reasonable request and with permission of SDU. Requests to access datasets should be directed to Simone Gad Kjeld, simk@sdu.dk.

Ethical review and approval was not required for the study on human participants in accordance with the local legislation and institutional requirements. Written informed consent from the participants' legal guardian/next of kin was not required to participate in this study in accordance with the national legislation and the institutional requirements. However, parents of students received written information sent out by the schools and were verbally informed at a parents-teacher meeting about the purposes of this study and that participation of their child was voluntary. Further, students were verbally informed by the teachers before answering the questionnaires, and written information about the study was provided in the beginning of the questionnaire, including that students' participation was voluntary, and that their answers would be treated with confidence.

SK, LL, SA, and LB conceptualized and designed this study. LL carried out the main analyses. SK drafted the initial manuscript. LL, SA, and LB critically reviewed and revised the manuscript. All authors read and approved the final manuscript.

This study was funded by the Danish Cancer Society. The Danish Cancer Society developed the intervention. The funding body was not involved in the design of the study, collection, analyses, interpretation of data, nor in the writing of the manuscript, and in the decision to submit the manuscript for publication.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.