E-cigarettes were first marketed in the United States around 2007.¹ Since then, features of e-cigarettes have changed rapidly, including flavors, nicotine concentration, size, shape, and device type. Recent studies have found dramatic increases in the sales of ice or cooling flavored e-cigarettes² and disposable e-cigarettes³ and an increase in the nicotine strength of e-cigarettes sold in the United States.^4,5 The dynamic context of product availability and sales, coupled with changes in federal, state, and local policies, necessitates having adaptable tools for rapid surveillance.

E-cigarette use prevalence has fluctuated markedly among youth and young adults according to estimates from the National Youth Tobacco Survey (NYTS) and the National Health Interview Survey (NHIS). According to weighted NYTS estimates, although the prevalence of e-cigarette use among youth has apparently declined since a peak of 20.0% (5.4 million) in 2019,⁶ 9.4% of middle and high school students (2.6 million) reported that they had used an e-cigarette at least once in the past month in 2022.^7,8 Among high school students who currently used e-cigarettes, 27.7% (approximately 840 000 students) reported frequent (20 or more days in the past month) use in 2018.⁹ This proportion increased to 46.0% (980 000 students) by 2022.⁷ Furthermore, e-cigarette use is more prevalent, and has changed more rapidly, among youth and younger adults compared with older adults. Based on NHIS estimates from January to June 2019, 8.1% (95% CI: 7.1, 9.1) of young adults aged 18–34 years used e-cigarettes, increasing to 12.2% (95% CI: 10.8, 13.6) by January to June 2022.¹⁰ By comparison, e-cigarette use prevalence among adults aged 35–49 years was 4.8% (CI: 4.0, 5.7) during January to June 2019 and 5.9% (CI: 5.1, 6.8) during January to June 2022.¹⁰

E-cigarette use among youth and young adults is a public health concern. Nicotine can harm the adolescent brain, which continues to develop through approximately age 25.¹ Additionally, e-cigarette aerosols contain harmful and potentially harmful constituents, including heavy metals and carcinogens.¹ Flavors are a major driver of youth e-cigarette use; in 2022, 84.9% of high school and middle school students who currently used e-cigarettes reported using flavored products, with fruit, candy, desserts or other sweets, mint and menthol being the most commonly used flavored products.⁷ Additionally, 61.4% of young adults who use e-cigarettes have never smoked cigarettes, suggesting that e-cigarettes may be a starter product for nicotine use among young adults.¹¹

To reduce young people’s access to flavored e-cigarettes and other flavored tobacco products, several states and localities restrict the sale of flavored tobacco products. Policies and strategies vary. Some prohibit the sale of non-tobacco characterizing flavors only in e-cigarettes, or allow for exemptions for menthol flavor, while others comprehensively restrict the sale of all flavors across the full spectrum of tobacco products, including menthol cigarettes, cigars, and e-cigarettes.^12,13 By March 2023, 388 US jurisdictions, including eight states, had enacted some type of restriction on flavored tobacco product sales, covering approximately 35% of the population.¹⁴ Many jurisdictions have also implemented complementary, simultaneous restrictions on nicotine concentrations and tax increases on e-cigarettes.

This context of rapid change in products, behaviors, and policies complicates researchers’ and public health practitioners’ ability to monitor the youth e-cigarette epidemic and evaluate the impact of policies on behavior. Timely data collection efforts leveraging multiple methods are warranted to better understand the real-time impacts of product availability and policy action on youth and young adult behavior, knowledge, attitudes, and perceptions.

The Monitoring E-Cigarette Use Among Youth project’s main aim is to provide rapid data collection and analysis efforts to complement existing national- and state-level surveillance activities and generate robust data for policy evaluation. The project is composed of multiple initiatives,¹⁵ including analysis of retail sales data purchasing^{2–4,16–19} and survey data collection and analysis.¹⁵ As a part of this project, the Tobacco Epidemic Evaluation Network (TEEN+) study longitudinal cohort was established.

Existing surveillance systems that provide annual or biennial cross-sectional data including tobacco use estimates have been the gold standard to assess prevalence and trends, but are not ideally positioned to allow for causal inferences about policy impact. Several notable cohort surveys focus on tobacco use, including the Population Assessment of Tobacco and Health (PATH)²⁰ and the Truth Longitudinal Cohort (TLC),²¹ and provide data to assess within-person product use, knowledge, and perception over time. State-representative estimates, where they exist, are critical in providing evidence given how much tobacco policies vary.

This analysis focuses on the methodological considerations of establishing a longitudinal cohort of youth and young adults, and baseline demographic characteristics from the initial baseline survey (Wave 1). Surveys will be completed approximately every 6 months to allow for timely monitoring of sub-annual trends and changes over time. This analysis will provide an overview of the TEEN+ study longitudinal survey design, sampling, and data collection as part of the multicomponent Monitoring E-Cigarette Use Among Youth project.

To assess overall trends and within-person changes over time, a national cohort was established for the TEEN+ study using probabilistic, address-based sampling. Starting in July 2022, invitations for Wave 1 were mailed and participants completed screening, consent, and survey participation online.

At Wave 1, among households who completed the screening section and consent and were confirmed to have an eligible household member, participation was 81.4% in the national sample and 81.5% in the California oversample. The American Association for Public Opinion Research (AAPOR) Response Rate 3 (RR3) which accounts for cases with unknown eligibility was 12.4% in the National Sample and 9.8% in the California oversample.²⁸ The initial cohort included 10 255 in the national sample, with 820 in California, plus an oversample of 1941 in California, for a total of 2761 Wave 1; this includes 13–17 year-old youth (n = 4211) and 18–24 year-old young adults (n = 6044) in the national sample as well as the California sample (n = 1065 youth and n = 1696 young adults). At Wave 1, the median time for survey completion was 24 min, after excluding any respondents who took the survey in multiple sessions (defined as 2 or more hours on the survey page). Four respondents who were sampled for the California oversample, but who reported having moved to a different state, were considered ineligible and removed from the final sample file. Seven respondents who engaged in suboptimal responding (those who sped through or skipped a majority of questions) were also removed from the final sample file and excluded from weighting.

An examination of unweighted and weighted estimates in the TEEN+ Wave 1 sample suggests representativeness (Table 1). Consistent with other surveys,²⁹ some groups are slightly unbalanced; for example, the proportion of female respondents is slightly higher in the sample than in the general population. However, this is consistent with other nationally representative surveys,²⁹ and was ameliorated with weighting. Table 2 compares the demographics of respondents in the TEEN+ study to US Census benchmarks.

Longitudinal survey methodologies are ideally suited for examinations of within-person changes in tobacco product use behaviors, which are critical to understand given the rapidly changing context of tobacco product availability and related policies. The robust sample size and national and California-representative design of the TEEN+ survey are expected to allow for subgroup analysis by demographic factors and will provide much needed data for evaluating the impact of flavored tobacco product restriction policies. While evidence is emerging on the impact of flavor policies, behavioral data that can be tied directly to policy implementation is a critical component of such evaluations and is less well established in the literature than some of the more near-term policy impacts, such as flavored tobacco product sales.³⁰

The TEEN+ cohort study is similar to PATH and TLC in that it provides for longitudinal analysis of within-person changes; however, the TEEN+ study is unique in several respects. Specifically, TEEN+ uniquely allows for access to respondent geographic location (eg, to determine if the respondent is living in an area with a flavored tobacco sales restriction or not) and more timely data availability. Designed specifically as a policy evaluation, the TEEN+ study includes items to assess potential near- mid- and long-term outcomes of flavored tobacco product policies. In addition, there is an opportunity to add and edit items at each survey wave that reflect the rapidly changing product landscape. Finally, the national sample allows for specific analyses, such as within-person trajectories in areas with and without flavored tobacco restriction policies, that set the TEEN+ study apart.

The TEEN+ study’s California oversample, in particular, offers an opportunity for evaluating the impact of a statewide flavored tobacco ban on youth and young adult tobacco use. In November 2022, California voters overwhelmingly upheld a state law to prohibit sales of all flavored tobacco products.³¹ The law became effective in December 2022, making California the second state in the United States after Massachusetts with such a policy. TEEN+ Wave 1 completed baseline data collection in California before the policy’s effective date, while subsequent waves reflect conditions in California following implementation. Information on tobacco use behaviors and related disparities among California youth and young adults before and after policy implementation is critical both to inform the state’s implementation and enforcement efforts, as well as to add to the emerging evidence base on the impact of flavor restrictions on tobacco use among young people. Further, the California and national samples (excluding participants in Massachusetts, where a statewide comprehensive flavor restriction has been effective since 2020) can be compared as a case and control.

The design of TEEN+ allows for additional, unique analysis opportunities. For example, there is the potential to create an indicator to control for those living in a state/city/county with a local or state restriction on flavored tobacco, including e-cigarettes, and those living in an area without a policy, to allow for comparative analyses of within-person changes in behavior, attitudes, and knowledge in policy versus non-policy areas. Longitudinal weights can be used to aid in the analysis of within-person data and pseudo-cross-sectional weights allow comparisons of patterns of use between waves.

While the TEEN+ study will utilize validated indicators of tobacco use when available, new and innovative items can also be expeditiously added and edited in the TEEN+ study survey, allowing for the assessment of new and emerging products and behavior patterns. For example, recent evidence from retail sales data indicate the emergence of e-cigarettes in “ice,” “cool” and other flavors that do not explicitly use the “menthol” descriptor but provide a cooling sensation.² Using this information, related survey items were rapidly developed for inclusion in the Wave 1 TEEN+ study while adhering to accepted constructs for assessing flavor use and adhering to requirements for institutional review of data collection instruments and protocols.^7,32 Newly-developed measures can be assessed in future validation studies. Individual waves of the survey can also be modified to include modules on a specific topic. The flexibility to add items and pivot measures is a key asset of TEEN+. By keeping pace with the rapidly changing tobacco product marketplace, TEEN+ is expected to yield timely and relevant data to inform public health practitioners and policy makers.

The TEEN+ survey will be subject to limitations inherent to longitudinal survey design, including that it should not be used to assess population-level prevalence or trends. Cross-sectional surveys, such as the National Youth Tobacco Survey, already yield this vitally important information. Further, as is common with longitudinal cohorts,²¹ a degree of attrition is expected, which may reduce the representativeness of subsequent cohorts, and respondents who use tobacco may be more likely to drop out of the cohort than their peers. Efforts will be made to maximize the representativeness of subsequent samples according to demographic characteristics such as race and ethnicity, age, and tobacco product use. Finally, participation in a longitudinal cohort may impact respondents’ likelihood to take future surveys.

The TEEN+ study is a new national, longitudinal survey that provides timely and critical information about youth e-cigarette and other tobacco product use. In addition to being able to generate nationally representative estimates, TEEN+ study data can be used to generate California-representative estimates. This longitudinal survey fills an important gap; sample size is expected to allow for estimates of youth and young adult tobacco product use among demographic subgroups for the investigation of tobacco-related disparities, including potential exploration of the pro-equity impact of flavored tobacco policies among youth and young adults.