Table 3 Summary characteristics of adjusted studies for initiation of cigarette smoking among individuals using e-cigarettes regularly (n = 49)

Akre et al. [50]

Longitudinal panel

Individuals who use e-cigarettes: any e-cig use in the past 12 months

Individuals who have not used e-cigs: no e-cig use in the past 12 months

Individuals who never smoked cigarettes at Time 0 reporting smoking cigarettes at Time 1 (12 months)

Aleyan et al. [60]

Prospective cohort

Individuals who never smoked cigarettes at baseline reporting any past 30-day use of e-cigs

Individuals who never smoked cigarettes at baseline reporting no past 30-day use of e-cigs

Individuals who never smoked cigarettes at Wave 1 initiating cigarette smoking at Wave 3:

Past 30-day e-cig use at Wave 1 or Wave 2 and past 30-day cigarette smoking at Wave 3;

Past 30-day e-cig use at Wave 1 or Wave 2 and past 30-day dual use at Wave 3.

Auf et al. [67]

Retrospective cohort

E-cig initiators: Response of “yes” to a question inquiring whether “e-cigarette was the first tried tobacco product” (i.e., individuals who never smoked cigarettes prior to first e-cig use)

Individuals who use e-cigarettes:

Ever users: self-reported any lifetime use of e-cig by individuals who never smoked cigarettes

Current users: self-reported past-30-day use of e-cig by individuals who never smoked cigarettes;

Individuals who have not used e-cigs: Self-reported never use of e-cig (and any other tobacco product) by individuals who never smoked cigarettes

Self-reported initiation of cigarettes via e-cig:

Individuals who ever smoked cigarettes: any lifetime use

Current cigarette users: past-30-day use

Barrington-Trimis et al. [52]

Prospective cohort [NB: Three pooled prospective cohorts]

Individuals who use e-cigarettes:

Individuals reporting either an age of first e-cig use, or an ever use (“even one or two puffs”) were classified as ever use of e-cig

Individuals who use e-cigarette exclusively: Individuals self-reporting past-30-day e-cig use at baseline, and no past-30 day cig use at baseline

Neither product user: Individuals self-reporting no past-30-day use of either e-cig or cigarettes

Individuals who have not used e-cigs: Individuals that reported to have never tried (not “even one or two puffs”) an e-cig

Self-reported cigarette use:

Experimental: Ever use, but not in the past 30 days

Infrequent users: 1–2 days in the past 30 days

Frequent users: 3 or more days in the past 30 days

Exclusive cigarette user: individuals self-reporting past-30-day cigarette use, and no past-30 day e-cig use

Individuals using e-cigs and smoking cigarettes: Individuals self-reporting past-30-day use of both e-cig and cigarettes

Neither product user: Individuals self-reporting no past-30-day use of either e-cig or cigarettes

Barrington-Trimis et al. [52]

Prospective cohort

Ever users: any who have reported an “age of first use” of e-cigs who are individuals who never smoked cigarettes

Never use of e-cigs

Baseline never-smokers reporting smoking at follow up

Barrington-Trimis et al. [90]

Prospective cohort

Individuals who use e-cigarettes: Individuals not smoking cigarettes reporting an age at first use of e-cigs were classified as “ever users”

Individuals who have not used e-cigs: Individuals not smoking cigarettes who had never tried an e-cig (“not even 1 or 2 puffs”)

Reporting at 2015 follow-up the use of cigarettes among individuals not smoking cigarettes at baseline

Berry et al. [68]

Prospective cohort

Individuals who use e-cigarettes in the past: Tobacco-naïve individuals at Wave 1 reported ever use of e-cigs between Waves 1 and 3, which preceded ever use of any other tobacco product

Individuals who have not used e-cigs: Tobacco-naïve individuals at Wave 1 reported never use of e-cigs (or any other non-cigarette tobacco product) between Waves 1 and 3

Self-reported cigarette use at Wave 3:

Individuals who ever smoked cigarettes: any lifetime use (even 1 or 2 puffs)

Current cigarette users: past-30-day use

*Youth reporting any of the following were considered intermediate or high risk: ever alcohol use, ever marijuana use, prescription drug abuse, enjoying frightening things, liking new and exciting experiences, preferring unpredictable friends, willingness to smoke in next yr, curiosity about cigarettes, or susceptibility to cigarette peer pressure from friends. Youths who did not exhibit any risk behaviors, sensation-seeking personality traits, or cigarette susceptibility were considered low risk

Best et al. [34]

Prospective cohort

Individuals not smoking cigarettes who answered, “Tried once or twice,” “More than once a month but less than a week,” or “More than once a week” to question, “Which one of the following is closest to describing your experience of e-cigarettes/vapourisers/shisha pens?”

Individuals not smoking cigarettes who answered, “Never tried, not even once” to question, “Which of the following is closest to describing your experience of e-cigarettes/vapourisers/shisha pens?”

Smoking ≥ 1 puff of a cigarette at 1 yr follow-up

Bold et al. [53]

Longitudinal panel

Individuals not smoking cigarettes who answered any use to, “How many days out of the past 30 days did you use e-cigarettes?” (open-ended response, 0–30)

Individuals not smoking cigarettes who answered “0” days to, “How many days out of the past 30 days did you use e-cigarettes?”

Individuals not smoking cigarettes having ever tried a cigarette, even 1 or 2 puffs in the past 30 days, at follow up

Chaffee et al. [54]

Prospective cohort

Ever user:

Individuals who never smoked cigarettes and lifetime use of e-cigs

Past-30-day users: individuals who never smoked cigarettes and use of e-cigs in the past 30 days

Former user: individuals who never smoked cigarettes and ever use of e-cigs but no use in the past 30 days

Never use of e-cigs

Individuals reporting one of the following outcomes at 1 yr follow-up: 1. Smoking during the past 30 days, 2. having smoked 100 cigarettes at 1 yr follow-up, 3. both outcomes together

Conner et al. [37]

Prospective cohort

Individuals who use e-cigarettes: Individuals who never smoked cigarettes at baseline reporting ever use of e-cigs (“I have tried them once or twice,” “I use them sometimes [more than once a month but less than once a week],” or “I use them often [more than once a week]”);

Individuals who have not used e-cigs: Individuals who never smoked cigarettes reporting never having used e-cigs at baseline

Ever smoker: self-reported ever-use of cigarettes (even once) at follow-up

Conner et al. [61]

Prospective cohort

Individuals with early e-cig use: Baseline (Wave 3) individuals who never smoked cigarettes who reported ever use of e-cigs

Individuals with late e-cig use: Baseline (Wave 3) individuals who never smoked cigarettes and individuals who never used e-cigarettes, who reported never smoking and ever e-cig use at Wave 4

Individuals who never smoked cigarettes reporting never having used e-cigs at baseline

Baseline (Wave 3) individuals who never smoked cigarettes who reported cigarette ever use at follow-up (Wave 4 or Wave 5)

Duan et al. [62]

Prospective cohort

Individuals who never smoked cigarettes at baseline reporting past 30-day e-cig use

Individuals who never smoked cigarettes at baseline reporting no past 30-day e-cig use

Individuals who never smoked cigarettes at baseline who self-reported any past 30-day smoking at follow-up;

Individuals who never smoked cigarettes at baseline who ever smoked cigarettes, even one or two puffs at follow-up

Epstein et al. [49]

Prospective cohort

Individuals who do not smoke cigarettes at baseline who responded “1 and higher occasions” to “on how many occasions have you used electronic cigarettes or e-cigs (‘Vapes’), such as Ruyan or NJOY in the past 12 months?

Individuals who do not smoke cigarettes at baseline who responded “0 occasions” to “on how many occasions have you used electronic cigarettes or e-cigs (‘Vapes’), such as Ruyan or NJOY in the past 12 months?”

Individuals who do not smoke cigarettes at baseline (who never used cigarettes more than “once or twice” and never used 100 cigarettes in their lifetime) at any Wave through age 21, who used cigarettes on 3 or more occasions at age 23

Evans-Polce et al. [41]

Longitudinal panel

Individuals who use e-cigarettes: Individuals who never smoked cigarettes reporting past 30 day e-cig use

Non-e-cig user: Individuals who never smoked cigarettes reporting no past 30 day e-cig use

Individuals who never smoked cigarettes at baseline reporting past 30 day or past 12 month cigarette use at follow-up

Hair et al. [63]

Prospective cohort

Individuals who have ever used e-cigs: Individuals who never smoked cigarettes at baseline who responded “yes” to “have you ever used or tried any e-cigs?”

Ever JUUL user: Individuals who never smoked cigarettes at baseline who responded “yes” to “have you ever used or tried any e-cigs?”, and “yes” to “have you ever used a JUUL vape?”

Ever e-cig (non-JUUL) user: Individuals who never smoked cigarettes at baseline who responded “yes” to “have you ever used or tried any e-cigs?”, and “no” to “have you ever used a JUUL vape?”

Individuals who never smoked cigarettes at baseline who reported never use of e-cigs

Individuals who never smoked cigarettes at baseline (spring 2017) reporting cigarette smoking in 2019.

Individuals who ever smoked cigarettes: ever tried cigarettes, even 1 or 2 puffs;

Current cigarette users: any past-30-day use of cigarettes

Hammond et al. [73]

Prospective cohort

Users at baseline (use of e-cig products in the last 30 days) and cigarette never-smokers (“Never having tried cigarette smoking”)

Never used e-cigs at baseline and never-smokers

Never smoked a whole cigarette at baseline and smoked a whole cigarette at 1-yr follow up

Harlow et al. [78]

Prospective cohort

Individuals who have ever used e-cigs: Individuals who never smoked cigarettes at Wave 2 and e-cig never-users at Wave 1 who reported ever e-cig use (even once or twice) at Waves 2, 3, and 4

Individuals who have ever used e-cigs were divided into the following groups:

Individuals currently using e-cigs: Individuals who have ever used e-cigs who reported current (i.e., past 30-day) e-cig use at a follow-up wave

Former e-cig users: Individuals who have ever used e-cigs who did not use e-cigs in the past 30 days at a follow-up wave

Tobacco flavored e-cig users: Individuals who have ever used e-cigs who initiated e-cig use with tobacco-flavored e-cigs

Nontobacco flavored e-cig users: Individuals who have ever used e-cigs who initiated e-cig use with nontobacco flavored e-cig, such as menthol, mint, clove, spice, fruit, chocolate, alcoholic drinks, candy, or other sweets

Individuals who never smoked cigarettes at baseline who never used e-cigs

Reported having ever smoked a cigarette, even one or two puffs at Waves 3, 4, and 5

Kang et al. [42]

Cross-sectional analytic

Individuals who never smoked cigarettes who reported ever using an e-cig at baseline

Individuals who never smoked cigarettes who reported never using an e-cig at baseline

Individuals who never smoked at baseline reporting ever cigarette ever use (one or two puffs) at end state

Kasza et al. [43]

Longitudinal panel

Individuals who use e-cigarettes: Individuals who never smoked cigarettes reporting ever ENDS use

Individuals who have not used e-cigs: Individuals who never smoked cigarettes reporting no ever use of ENDS

Individuals who never smoked cigarettes at baseline reporting past 30 day cigarette use at any follow-up

Individuals who never smoked cigarettes at baseline reporting ever cigarette use at any follow-up

Keller-Hamilton et al. [64]

Prospective cohort

Individuals who responded “yes” to “have you ever used an e-cig, such as Smoking Everywhere, NJOY, Blu, or Vapor King, even one or two times?”

Never used e-cigs at baseline

Individuals who ever smoked cigarettes: ever tried cigarettes smoking even 1 or 2 puffs;

Current cigarette users: smoked a cigarettes, even one or two puffs in the past 30 days;

Complete case analysis: Individuals who were compliant at all five time points and were not missing data for variables in the propensity score model

Kintz et al. [44]

Prospective cohort

Individuals who use e-cigarettes: Individuals who never smoked cigarettes at baseline who reported an age of first use of e-cigs

Individuals who have not used e-cigs: Individuals who never smoked cigarettes at baseline and who reported having never used an e-cig

Individuals who report an age at first use of cigarettes at follow up

Kong et al. [70]

Prospective cohort [NB: Three pooled prospective cohorts]

Individuals who use e-cigarettes: Individuals who never smoked cigarettes at baseline reporting either an age of first e-cig use or an ever use (“even one or two puffs”) were classified as ever users of e-cig

Individuals who have not used e-cigs: Individuals who never smoked cigarettes at baseline reporting never having tried (not “even one or two puffs”) an e-cig

Individuals who reported a valid age in response to a question asking about the age of ever cigarette use (“even one or two puffs”) at follow-up*

Follow-up for CHS and YASS was approximately 18 months; follow-up for H&H was approximately 12 months

Lee and Fry [71]

Prospective cohort

Individuals who use e-cigarettes: Individuals who never smoked cigarettes at baseline who reported baseline e-cig ever use

Non-e-cig user: Individuals who never smoked cigarettes at baseline who reported never use of e-cigs at baseline

Never use of cigarettes at Wave 1 followed by ever-use of cigarettes by Wave 2

Leventhal et al. [87]

Prospective cohort

Individual using e-cigs at baseline (lifetime and past 6-month use of e-cigs) and never tried cigarettes (“No, not even a few puffs”)

Individuals who never used e-cigs use and never tried cigarettes

Becoming an individual who ever smoked cigarettes was described as responses of “No” at baseline and “Yes” at follow-up periods to question “Have you ever used combustible cigarette” at 6- and 12-month follow-up

Loukas et al. [79]

Longitudinal

Individuals currently using e-cigs (ENDS):

E-cig (ENDS) use on at least one day in the past 30 days

Individuals who have ever used e-cigs (ENDS):

Ever used an e-cig (ENDS) product as intended, even one or two puffs

Individuals not smoking cigarettes who were not “individuals who have ever used e-cigs” and were “non-current ENDS users”

Individuals who currently smoke:

Smoked at least one day in the past 30 days

Individuals who did not currently smoke:

Reported ever smoking, but did not use cigarettes in the past 30-days

Lozano et al. [74]

Prospective cohort

Tried at least once in their lifetime and who are individuals not smoking cigarettes (“No” to question, “Have you ever tried or experimented with cigarette smoking, even one or two puffs?”)

Did not try e-cigs even once in their lifetime and who are individuals not smoking cigarettes

Trial of smoking at follow up: “Yes” to question, “Have you ever tried or experimented with cigarette smoking, even one or two puffs?”

Current cigarette smoking: ≥ 1 cigarette in the past 30 days

Martinelli et al. [84]

Longitudinal panel

Reported ever using an e-cig

Reported never using an e-cig

Individuals who never smoked at baseline reporting ever use of cigarettes at 6-month and 12-month follow-ups

McMillen et al. [72]

Longitudinal panel

Individuals who have ever used e-cigs: Individuals who do not smoke cigarettes responding “yes” to question “Have you ever used an e-cigarette, such as NJOY, Blu, or Smoking Everywhere, even 1 or 2 times?” and responding “not at all” to question “Do you now use e-cigarettes every day, some days, or not at all?”

Past 30-day e-cig users: Individuals who do not smoke cigarettes responding “yes” to question “Have you ever used an e-cigarette, such as NJOY, Blu, or Smoking Everywhere, even 1 or 2 times?” and responding “every day” or “some days” to question “Do you now use e-cigarettes every day, some days, or not at all?”

Individuals who have not used e-cigs: Individuals who have ever used e-cigs: Individuals who do not smoke cigarettes responding “no” to question “Have you ever used an e-cigarette, such as NJOY, Blu, or Smoking Everywhere, even 1 or 2 times?”

Individuals who never smoked cigarettes at baseline reporting having smoked a cigarette in the past 12 months at follow-up

Miech et al. [36]

Longitudinal panel

Individuals who use e-cigarettes; Individuals who never smoked cigarettes who answered “1 or more days” to question, “During the last 30 days, how many days (if any) have you used e-cigs?”

Individuals who have not used e-cigs: Individuals who never smoked cigarettes who answered “0” to question, “During the last 30 days, how many days (if any) have you used e-cigs?”

Reporting cigarette use at follow-up by responding with “smoked once or twice,” or more, to the question “what best describes your cigarette smoking in the last 12 months?”

Owotomo et al. [46]

Prospective cohort

Individuals who never smoked cigarettes at baseline who reported ever use of e-cigs (even one or two times)

Individuals who never smoked cigarettes at baseline who reported never use of e-cigs

Individuals who never smoked cigarettes at baseline who ever smoked cigarettes, even one or two puffs, in the past 12 months

Parnham et al. [85]

Longitudinal

Individuals who answered ‘Only tried once or twice’, ‘In the past but not now’, ‘Less than once a month’, ‘At least once a month but less than once a week’ and ‘At least once a week’ (and yes to ‘ever used e-cigs’ for wave 7) to the questions “have you ever used e-cigs” or “do you ever use e-cigs”

Individuals who answered ‘I have never’ to the questions “have you ever used e-cigs” or “do you ever use e-cigs”

Transition from ever having used an e-cig and never smoked, to currently smoking (past 30 days)

Patanavanich et al. [80]

Longitudinal cohort

Individuals currently using e-cigs: Individuals who used e-cigs during the last 30 days

Individuals who have ever used e-cigs: Individuals who ever tried e-cigs

Experimenters: Ever users who were not current users

Individuals who had never used e-cigs at baseline

Transition from never smoking cigarettes to combustible cigarette smoking and dual product use at the 12-month follow-up

Penzes et al. [55]

Prospective cohort

Individuals not smoking cigarettes and “Yes” to the question “have you ever tried e-cigs?”

Individuals not smoking cigarettes and “No” to question “Have you ever tried e-cigs?”

Individual who never smoked at baseline reporting at least 1 puff of cigarette at 6-months

Primack et al. [56]

Prospective cohort

Individuals using e-cigs at baseline (no frequency/intensity recorded) who are also individuals who never smoked cigarettes (no history of smoking) and non-susceptible to smoking (answered “Definitely no” to the questions “If one of your friends offered you a cigarette, would you try it?” and “Do you think you will smoke a cigarette sometime in the next year?”

No e-cig use at baseline who are also individuals who never smoked cigarettes and non-susceptible to smoking

Individuals who never smoked cigarettes at baseline with ≥ 1 puff of cigarette by follow-up

Primack et al. [88]

Longitudinal panel

E-cig use at baseline who are also never-smokers

No reported e-cig use at baseline among individuals who never smoked cigarettes

Individuals who smoked cigarettes regularly at 1 yr follow-up (no further details)

Primack et al. [89]

Prospective cohort

Individuals using e-cigs at baseline (no frequency/intensity recorded) who are also individuals who never smoked cigarettes (no history of smoking) and non-susceptible to smoking (answered “Definitely no” to the questions “If one of your friends offered you a cigarette, would you try it?” and “Do you think you will smoke a cigarette sometime in the next year?”

No e-cig use at baseline who are also individuals who never smoked cigarettes and non-susceptible to smoking

Individuals who never smoked cigarettes at baseline with ≥ 1 puff of cigarette at 18-month follow-up

Spindle et al. [76]

Longitudinal panel

Individuals who have ever used e-cigs: had used e-cigs at least “only once, even on one occasion” who are individuals who never smoked cigarettes

Individuals currently using e-cigs: indicated use of e-cig in past 30 days who are individuals who never smoked cigarettes

No use of e-cigs with never smoking

Lifetime smoking follow-up: (“How many cigarettes have you smoked in your life?”);

Current smoking at follow up: (“How many cigarettes have you smoked in the past 30 days?”) at 1-yr follow-up

Staff et al. [66]

Prospective cohort

Youth who, at age 14, never smoked cigarettes and reported ever trying an e-cigarette or vaping device

Youth who, at age 14, never smoked cigarettes and reported never trying an e-cigarette or vaping device

Individuals who never smoked cigarettes at age 14 who reported ever smoking by age 17

Stokes et al. [47]

Longitudinal panel

Individuals who use e-cigarettes: Cigarette naïve individuals reporting ever e-cig use, even one or two puffs

Individuals who have not used e-cigs: Cigarette naïve individuals reporting no ever e-cig use, even one or two puffs

Individuals reporting cigarette ever use (one or two puffs) or past 30-day use (any use of a cigarette within 30 days prior to outcome wave) at any follow-up

Sun et al. [81]

Longitudinal panel

Individuals who never smoked cigarettes at baseline reporting ever use of e-cigs (≥ 1) in the initial Wave in each pair of Waves

Individuals who never smoked cigarettes reporting never having used e-cigs at baseline

Individuals who never smoked cigarettes at baseline Wave reporting smoking behaviors (even one puff of cigarette) at the subsequent Wave

Sun et al. [86]

Longitudinal panel

Individuals who have ever used e-cigs: Cigarette-naïve individuals at baseline (Wave 3) who self-reported ever use of e-cigs

Individuals currently using e-cigs: Cigarette-naïve individuals at baseline (Wave 3) who self-reported past 30 days e-cigs use

Cigarette-naïve individuals at baseline (Wave 3) who never used e-cigs

Individuals who never smoked cigarettes at baseline (Wave 3) who self-reported ever use of cigarettes in Wave 4 and continued cigarette use at Wave 5 among Wave 4 individuals who reported initiating cigarette smoking;

Continued smoking measure (CSM) was defined as:

CSM-I: Past 12-month cigarette use at wave 4 and past 12-month use at wave 5;

CSM-II: Past 12-month cigarette use at Wave 4 and past 30-day use at wave 5

Treur et al. [57]

Cross-sectional analytic and prospective cohort

Ever-use of e-cigs at least once in their lifetime, with or without nicotine, and who have never smoked, or only tried once or twice

Never use of e-cigs in their lifetime, and who have never smoked, or only tried once or twice

Individuals who never smoked at baseline (Time 0) (“I have never smoked”) reporting smoking behavior at Time 1 with answers “I have smoked once or twice to try,” “I smoke once in a while, but not every day,” “I smoke every day”

Watkins et al. [58]

Prospective cohort

Ever use of at least once in the past 30 days, and never smoked 1 cigarette at baseline

Never use of e-cigs, and never smoked 1 cigarette at baseline

Individuals who never smoked at baseline reporting ever-use of cigarettes (≥ 1 cigarette) or ≥ 1 cigarette use in the past 30 days at 12–24 month follow-up

Wills et al. [77]

Longitudinal panel

Answered “Yes” when asked if having ‘ever used’ an e-cig and giving the use frequency ‘I have smoked e-cigs 1–2 times’, ‘I have smoked e-cigs 3–4 times’, ‘I usually smoke a few e-cigs a year’, ‘I usually smoke a few e-cigs a month’, ‘I usually smoke a few e-cigs each week’, and ‘I usually smoke e-cigs everyday’ who were also individuals who never smoked cigarettes (“Not having smoked a cigarette in their lifetime”)

Answered “I have never smoked an e-cigarette in my life” to the question “which of the following is most true for you about smoking electronic cigarettes?”

Individuals not smoking cigarettes at Time 1 who reported smoking ≥ 1 cigarette at Time 2 by answering “I have smoked cigarettes 1–2, 3–4 times,” “usually smoke every week,” “usually smoke every day” to question, “which of the following is most true for you about smoking?”

Wills et al. [91]

Longitudinal panel

Answered “Yes” when asked if having ‘ever used’ an e-cig and giving the use frequency ‘I have smoked e-cigs 1–2 times’, ‘I have smoked e-cigs 3–4 times’, ‘I usually smoke a few e-cigs a year’, ‘I usually smoke a few e-cigs a month’, ‘I usually smoke a few e-cigs each week’, and ‘I usually smoke e-cigs everyday’ who were also individuals who never smoked cigarettes (“Not having smoked a cigarette in their lifetime”)

Never use of e-cigs (“I have never smoked e-cigs in my life”)

“No” at Time 1 (baseline) and “Yes” at Time 2 to statements: “I have smoked a cigarette once or twice,” or “I have smoked 3–4 times” at 1-yr follow-up

Xu et al. [82]

Longitudinal panel

Baseline (Wave 1) individuals who are tobacco naïve who reported e-cigs use at Wave 2

Baseline (Wave 1) individuals who are tobacco naïve who did not use e-cigs at Wave 2

Individuals who initiated smoking at Wave 3 after e-cig use at Wave 2

Yang et al. [83]

Longitudinal panel

Individuals not smoking cigarettes who used e-cigs in the past 6 months

Individuals not smoking cigarettes who did not use e-cig in the past 6 months

Subsequent use of cigarette smoking by prior use of e-cig

Young-Wolff et al. [59]

Retrospective cohort

At least 1 “e-cig user” entry in electronic health records who are also individuals who never smoked cigarettes (0 entries as “smoker” in electronic health records)

No “e-cig user” entry in electronic health records

Change in smoking status from never use to smoking cigarettes (≥ 1 cigarette) at 12-month follow-up