Table 1.

Characteristics and results of included studies.

Authors	Sample Size, Location, and Year	Mean Age, and Gender	Methodology	Results
Bold, K.W., Kong, G., Cavallo, D.A., Camenga, D.R., and Krishnan-Sarin, S. [14]	340 students who reported previous e-cigarette useUnited States2013–2014	15.6 (±1.2) years of age52.6% female	Two waves of data collection (Fall 2013 and Spring 2014) assessing e-cigarette use frequency using the question, “How many days out of the last 30 days did you use e-cigarettes?”	Mean frequency of use increased from 7.4 (±9.6) days at wave 1 to 10.4 (±10.5) days at wave 2.Appeal of e-cigarettes to youth may be the ability to use e-cigarettes anywhere and the relatively low cost.Younger age (B = 10.19, SE = 3.30, ß= 0.24, p = 0.003) and use of traditional cigarettes (B = 4.86, SE = 1.94, ß = 0.23, p = 0.01) were associated with an increase in frequency of e-cigarette use
Goldenson, N., Leventhal, A.M., Stone, M.D., McConnell, R.S., and Barrington-Trimis, J.L. [15]	181 grade 10 studentsUnited States2013–2015	16.1 (±0.4) years of age53.0% male	Past 30-day use of e-cigarettes measured by frequency and intensity across five waves from Fall 2013 school semester to the Fall 2015 school semester.Frequency assessed by capturing number of days out of the past 30 days in which participants used e-cigarettes. Answers were grouped as no use (0 days), infrequent use (1 or 2 days), and frequent use (≥3 days).Intensity was assessed using the two questions: (1) “On the days you vaped, how many times did you usually pick up your e-cigarette to vape?” (responses: 1 time, 2 times, 3–5 times, 6–9 times, 10–14 times, 15–20, or ≥20 times); and (2) Each time you picked up your e-cigarette to vape, how many puffs did you usually take before putting it away?” (responses: 0 puffs, 1 puff, 2 puffs, 3–5 puffs, 6–9 puffs, 10–14 puffs, 15–20 puffs, or ≥20 puffs).	At baseline, 7.2% (n = 235) reported past 30-day vaping.Of an analytical sample of 181 students, 59.7% reported vaping a solution with nicotine in past 30-days, 28.7% vaped low concentration nicotine, 19.3% used medium concentration, and 11.6% used high concentration.After adjusting for covariates, for each 1-level increase in baseline nicotine concentrations, the odds of being a frequent 30-day vaper increased 1.65 times when compared to non-vapers at follow-up (OR: 1.65; 95%CI: 1.09, 2.51)
Lanza, H.I., Barrington-Trimis, J.L., McConnell, R., Cho, J. [16]	3322 high school studentsUnited States2013–2015	16.5 (±0.4) years of age53.5% female	Using data from waves 5–9 (when information regarding e-cigarette use was included in questionnaire) to create a trajectory of nicotine e-cigarette use.Nicotine vaping frequency assessed by the single question: “In the last 30 days, how many total days have you used an electronic cigarette, with nicotine (e-cigs, personal vaporizer, PV (personal vaporizer))?” Responses were coded into 0, 2, 4, 8, 15, 25, or 30 days.	Number of participants reporting any nicotine vaping in the past 30 days ranged from 4.4% in wave 7 to 7.5% in wave 8, before increasing to 22.0% in wave 9.Five trajectories identified:No use (n = 2246; 67.6%)Infrequent use (n = 566; 27.0%)Moderate use (n = 167; 5.0%)Young adult-onset frequent use (n = 213; 6.4%)Adolescent-onset escalating frequent use (n = 131; 3.9%)Highest increase in frequency of e-cigarette use occurred in the group of adolescent onset frequent users. This group was also associated with the cannabis vaping frequent user group. Cannabis and nicotine vaping may have similar underlying risk processes.
Park, E., Livingston, J.A., Wang, W., Kwon, M., Eiden, R.D., and Chang, Y. [17]	801 adolescentsUnited StatesYear not reported	14.14 (±1.37) years of age57% female	Data collected across five waves was used to create trajectories of e-cigarette use.E-cigarette use was assessed by a single question: “On how many occasions (if any) did you use an electronic cigarette or e-cigarette (vape pen) during the last 6 months?”. Response options included the following: Never, 1 or 2 times, 3–5 times, 6–9 times, 10–19 times, 20–39 times, or 40 or more times.	A total of 43.4% of the sample had tried e-cigarettes at least once before.Average age of onset for e-cigarette use was 14.1 (±0.5) years.Membership of trajectory groups:Never use (66.6%)Low and increasing (20.1%)High and increasing (13.3%)Even low frequency of use in early adolescence may be a risk factor for continuing and/or increasing use of e-cigarettes.
Westling, E., Rusby, J.C., Crowley, R., and Light, J.M. [18]	1130 adolescentsUnited States2014–2016	14.4 (±0.5) years of age53% female	E-cigarette use frequency was captured by two questions: 1. “In your whole life, how many different times have you ever smoked an e-cigarette (‘vape pen’) or an e-hookah, even a puff?”; and 2. “In the last 30 days, on how many days would you say you have smoked an e-cigarette (‘vape pen’) or an e-hookah, even a puff?”	Average e-cigarette use frequency was 14 days out of the last 30 days at baseline and increased to 30 days out of the past 30 days at follow up.A total of 1035 students (94.9%) were classified in the trajectory labelled “non or low users of e-cigarettes”.A total of 56 students (5.1%) were members of the “Accelerators” class, which denoted accelerating users of e-cigarettes.

B = unstandardized Beta value; β = standardized Beta value; SE = Standard Error; OR = Odds Ratio; 95%CI = 95% Confidence Intervals; p = probability value.