The paper by Asfar et al (1) had a noble objective, which was to inform ENDS health risk communications by updating the 2018 evidence review by the US. National Academies of Sciences, Engineering and Medicine (NASEM) (2). The need for improved risk communications about ENDS is reinforced by a recent study which found that only 17.4% of US smokers believe that nicotine vaping is safer than smoking (3). While ENDS use is not safe, the evidence from toxicant exposure studies does show that ENDS use is far safer than smoking cigarettes and may benefit public health by assisting those who smoke to quit smoking (4, 5).
An important limitation of the umbrella review method utilized by the authors is that it does not directly attempt to systematically characterize new research. This is a concern because the marketplace of ENDS products used by consumers has evolved since the 2018 NASEM report (4, 5). Furthermore, the authors have included some meta-analyses of selected reviews for some domains, but these meta-analyses were not in the Prospero pre-registration (6), nor explained in the paper. It’s thus unclear how or why certain reviews were selected for meta-analysis, and also whether the comparators are the same for these reviews. More importantly, these meta-analyses risk single studies contributing multiple times to the same pooled estimate. The authors noted this as a limitation commenting inaccurately that ‘it was impossible to identify articles that were included in multiple reviews’. In our view this serious methodological flaw merits removal of all pooled estimates from their analyses. Additionally in several places, association is conflated with causality (e.g. “ENDS use impedes smoking cessation”) when based on observational data. The classification of evidence is also not transparent, cannot be found in the source the authors cited, and in places does not follow from the evidence presented (e.g. gateway evidence classified as high when based on observational studies).
The review also excludes research reviews supported by ENDS manufacturers. While we recognize and agree with the authors’ concerns about possible bias in industry publishing, we also believe that the exclusion of such research without any analysis of the scientific merits of the research itself precludes a comprehensive assessment of the scientific literature regarding the health risks of ENDS. Also, excluding industry publications necessarily eliminates from consideration evidence that the Center for Tobacco Products may be asked to consider when it is reviewing product applications for product marketing authorizations and modified risk claims.
The paper falls short, as well, in addressing the risk communication implications of the findings since the authors’ recommendations often do not match the evidence of what is known and not known about the risks of using ENDS. A careful analysis of suggested risk messages contained in supplementary material to the paper finds messages that do not appear to be supported by the evidence reviewed in the paper. For example, the suggested risk messages that "nicotine in vapes can harm memory, concentration, and learning in young people," "vaping nicotine can harm learning ability in young people," and "exposure to nicotine during adolescence can interfere with brain development" do not appear to be derived from a comprehensive review of scientific evidence. The evidence of nicotine having adverse effects on brain development or learning in adolescents comes primarily from rodent studies where dosing of nicotine is not necessarily analogous to exposure from ENDS.
For most of the topics reviewed, the umbrella review reveals that the health risks of ENDS remain unsettled at this time. Whilst biomarker exposure data clearly indicate reduced risk compared to tobacco cigarettes (4), we would suggest restraint is needed in communicating absolute risk information to the public (7). Also we would go one step further in noting that whatever the health risks of ENDS may be, they are going to be most observable in those persons using ENDS on a persistent basis for months or years at a minimum. For example, the health risks of cigarette smoking do not reliably emerge until after smokers exceed 10 pack-years or more of exposure (87). Few studies of ENDS health risks have actually focused on the likely higher risk group of persistent ENDS users (4).
We also take issue with the paper’s main conclusion that direct comparison between the harms of cigarettes and ENDS should be avoided (1). In fact, such comparisons are likely unavoidable and necessary since the group most likely to use ENDS on a persistent basis are those who have a history of cigarette use. Moreover, ENDS were originally developed as a cessation aid and evaluations of cessation aids almost always incorporate evidence on the relative harms compared to continuing to smoke. We do recognize that accounting for a person’s smoking history complicates evaluations of the health risks of ENDS, but dismissing such comparisons simply ignores the fact that ENDS are existing or potential cigarette substitutes for many smokers (4, 5). A recent review of biomarker studies found that compared to smoking, using ENDS leads to a substantial reduction in biomarkers of toxicant exposure associated with cigarette smoking, while also acknowledging that the degree of any residual risk from smoking remains unclear because of the lack of comparisons between long-term former smokers, and with those who have never smoked or used ENDS (4).
Communicating health risk information about ENDS has to have some context to be meaningful to consumers. A common misconception about tobacco use is that the most dangerous component of the product is nicotine (9-14). However, while nicotine can be addictive, it is the other toxicants in tobacco, especially burned tobacco, that are the true culprits of tobacco-related diseases (2, 4). Thus, when communicating information about the health risks of tobacco products, it makes sense to provide consumers with information about the relative health dangers from burned compared to unburned tobacco products. The example risk messages included in the supplementary materials to the paper appear to be developed with a goal of discouraging anyone from using a vaping product rather than to inform potential users about risks.
Public health authorities can reduce the risk of misinforming or confusing the public by acknowledging when evidence is incomplete or based on statistical association rather than clear evidence of causality, and by updating any statements or recommendations quickly when plausibly causal evidence becomes available (7).

 
References
1. Asfar T, Jebai R, Li W, et al. Risk and safety profile of electronic nicotine delivery systems (ENDS): an umbrella review to inform ENDS health communication strategies. Tob Control Epub ahead of print: [please include Day Month Year]. doi:10.1136/ tobaccocontrol-2022-057495
2. National Academies of Sciences, Engineering, and Medicine; Health and Medicine Division; Board on Population Health and Public Health Practice; Committee on the Review of the Health Effects of Electronic Nicotine Delivery Systems. Public Health Consequences of E-Cigarettes. Eaton DL, Kwan LY, Stratton K, editors. Washington (DC): National Academies Press (US); 2018 Jan 23.
3. Kim S, Shiffman S, Sembower MA. US adult smokers' perceived relative risk on ENDS and its effects on their transitions between cigarettes and ENDS. BMC Public Health. 2022 Sep 19;22(1):1771. doi: 10.1186/s12889-022-14168-8.
4. McNeill, A, Simonavičius, E, Brose, LS, Taylor, E, East, K, Zuikova, E, Calder, R and Robson, D (2022). Nicotine vaping in England: an evidence update including health risks and perceptions, September 2022. A report commissioned by the Office for Health Improvement and Disparities. London: Office for Health Improvement and Disparities.
5. Balfour DJK, Benowitz NL, Colby SM, Hatsukami DK, Lando HA, Leischow SJ, Lerman C, Mermelstein RJ, Niaura R, Perkins KA, Pomerleau OF, Rigotti NA, Swan GE, Warner KE, West R. Balancing Consideration of the Risks and Benefits of E-Cigarettes. Am J Public Health. 2021 Sep;111(9):1661-1672.
6. Rime Jebai, Wei Li, Oluwole Olusanya Joshua, Beck Graefe, Celia Rubio. Systematic Review of Reviews on the Harmful Effects of Electronic Nicotine Delivery Systems: Building Evidence for Health Communication Messaging. PROSPERO 2021 CRD42021241630 Available from: https://www.crd.york.ac.uk/prospero/display_record.php?ID=CRD42021241630
7. United States Surgeon General. Confronting Health Misinformation: The U.S. Surgeon General’ s Advisory on Building a Healthy Information Environment [Internet]. 2021 [cited 2022 Aug 9]. Available from: https://www.hhs.gov/sites/default/files/surgeon-general-misinformation-a...
8. Doll R, Peto R, Boreham J, Sutherland I. Mortality from cancer in relation to smoking: 50 years observations on British doctors. Br J Cancer. 2005 Feb 14;92(3):426-9. doi: 10.1038/sj.bjc.6602359.
9. O’Brien EK, Nguyen AB, Persoskie A, Hoffman AC. U.S. adults’ addiction and harm beliefs about nicotine and low nicotine cigarettes. Prev Med. 2017;96:94-100.
10. Steinberg MB, Bover-Manderski MT, Wackowski OA, Singh B, Strasser AA, Delnevo CD. Nicotine Risk Misperception Among US Physicians. J Gen Intern Med. 2021, 36(12):3888-3890.
11. Elton-Marshall T, Driezen P, Fong GT, et al. Adult perceptions of the relative harm of tobacco products and subsequent tobacco product use: Longitudinal findings from waves 1 and 2 of the population assessment of tobacco and health (PATH) study. Addict Behav. doi:10.1016/j.addbeh.2020.106337.
12. Parker MA, Villanti AC, Quisenberry AJ, Stanton CA, et al. Tobacco Product Harm Perceptions and New Use. Pediatrics. 2018 Dec;142(6):e20181505. doi: 10.1542/peds.2018-1505.
13. Yong HH, Gravely S, Borland R, Gartner C, et al. Perceptions of the Harmfulness of Nicotine Replacement Therapy and Nicotine Vaping Products as Compared to Cigarettes Influence Their Use as an Aid for Smoking Cessation? Findings from the ITC Four Country Smoking and Vaping Surveys. Nicotine Tob Res. 2022 Aug 6;24(9):1413-1421. doi: 10.1093/ntr/ntac087.
14. National Cancer Institute. Health Information National Trends Survey. HINTS 5 cycle 3, 2019. Available at: https://hints.cancer.gov/view-questions-topics/question-details.aspx?PK_...

We thank Cummings and colleagues for their interest in and comments on our umbrella review published recently in Tobacco Control.[1] The authors criticize us for not including the latest studies. Yet, for an umbrella review, those studies need to be in a published review to be included, as we indicated in our methods and limitations. Generally, given the lengthy review and publication processes, any review will not be inclusive of all studies in a field that has as high a publication breadth and intensity as tobacco regulatory science. In addition, the authors mentioned that our meta-analysis was not available in PROSPERO pre-registration. This is because the review registration was completed in the very early stages of the review. We have updated this information in PROSPERO now to include the meta-analysis. The issue of overlap was addressed in our limitations, as we had to screen over 3,000 studies included in our selected reviews. However, given the importance of this issue for the meta-analysis, we performed a new meta-analysis that included the individual studies in each domain instead of using the odds ratio reported by the review to eliminate the effect of including the same study more than one time on our results. We confirm that the results of the new meta-analysis, which includes each study only once, are similar to the original meta-analysis (Supplement A: https://www.publichealth.med.miami.edu/_assets/pdf/meta-analysis.pdf).
The authors also accuse us of not being transparent about our adopted classification of evidence strategy, while a careful check of the reference we provided shows it (Morton et al. Page 131 (Box 4-2).[2] This classification is also adopted by the National Academies of Sciences.[3] Our assessment of the gateway effect as high evidence is consistent with this classification (National Academies of Sciences, Engineering, and Medicine 2018; Page 5, Box S-2: High evidence (including conclusive and substantial) (Supplement B: https://www.publichealth.med.miami.edu/_assets/pdf/level-of-evidence.pdf).[3] Generally, we object to the authors’ characterization that observational studies cannot imply causality. In fact, carefully designed observational studies led to most of what we know about major risks to health, such as smoking, hypertension, diabetes, and high cholesterol levels.[4-7]
We excluded research supported by the tobacco industry, given the ample evidence of the industry's fraudulent scientific behavior, which prompts objective scientists to question the extent to which industry-sponsored authors report methods and results accurately.[8, 9] We note that our stance regarding industry-supported publications is also consistent with the policy of Tobacco Control. Contrary to the commentary’s critique, the message about nicotine’s effect on the developing brain is supported by evidence from human and animal studies and endorsed by the CDC as well as major credible public health bodies.[10-13]
The authors state that comparing ENDS to cigarette smoking is needed, given their potential to help addicted smokers quit. Alas, an accurate comparison of these products is currently not feasible. Unlike cigarettes, which were suspected of causing lung cancer as far back as the late 19th century and for which we have more than a half-century of robust evidence of the health effects, we have much more modest literature on ENDS health effects, spanning less than two decades.[14] Also, unlike today’s combustible cigarette, a rather standardized tobacco use method with a standardized pattern of use and standardized assessment tools, ENDS are not standardized. The heterogeneity of ENDS products, their use patterns, and still nascent long-term ENDS exposure assessment tools make accurate comparisons impossible. In fact, even the same product, manufactured by the same maker, can have variability in its liquid content and ingredient proportions.[15] While the commentary criticized our review based on the acknowledged lack of long-term data about ENDS effects on health, they make an unsubstantiated claim that the group most likely to use ENDS on a persistent basis are smokers.
To navigate the complexity of ENDS, we adopted a consumer rights stand that recognizes that every consumer needs to be aware of the potential risks and benefits of the products they are using. So, although the evidence is inconclusive about the real-world effects of ENDS in helping smokers quit, and the fact that the FDA has not yet approved any of them as a cessation device, we have created some messages to support this based on evidence from randomized clinical trials.[16] We agree that it will take more time before robust scientific evidence about the long-term effects of ENDS accumulates, but this lack of knowledge should not be an excuse for failing to alert users of potential adverse health consequences of ENDS use. The public has a right to know whether any novel product that was being used by a significant proportion of the population contains known toxicants, despite lacking robust evidence of long-term effects on health. Why should ENDS be any different?
 
References

1. Asfar, T., et al., Risk and safety profile of electronic nicotine delivery systems (ENDS): an umbrella review to inform ENDS health communication strategies. Tobacco Control, 2022: p. tobaccocontrol-2022-057495.
2. Morton, S., et al., Finding what works in health care: standards for systematic reviews. 2011.
3. National Academies of Sciences, E., Medicine., Public health consequences of e-cigarettes. 2018.
4. Mahmood, S.S., et al., The Framingham Heart Study and the epidemiology of cardiovascular disease: a historical perspective. The lancet, 2014. 383(9921): p. 999-1008.
5. Doll, R., et al., Mortality in relation to smoking: 50 years' observations on male British doctors. Bmj, 2004. 328(7455): p. 1519.
6. Kannel, W.B. and D.L. McGee, Diabetes and cardiovascular disease: the Framingham study. Jama, 1979. 241(19): p. 2035-2038.
7. Castelli, W.P., et al., Incidence of coronary heart disease and lipoprotein cholesterol levels: the Framingham Study. Jama, 1986. 256(20): p. 2835-2838.
8. Kessler, G., Amended Final Opinion. USA v. Philip Morris, 2006.
9. Pisinger, C., N. Godtfredsen, and A.M. Bender, A conflict of interest is strongly associated with tobacco industry-favourable results, indicating no harm of e-cigarettes. Prev Med, 2019. 119: p. 124-131.
10. Centers for Disease Control and Prevention. It’s not like you can buy a new brain. 2019 [cited 2022 November 29th]; Available from: https://www.cdc.gov/tobacco/basic_information/e-cigarettes/Quick-Facts-o...
11. Goriounova, N.A. and H.D. Mansvelder, Short-and long-term consequences of nicotine exposure during adolescence for prefrontal cortex neuronal network function. Cold Spring Harbor perspectives in medicine, 2012. 2(12): p. a012120.
12. England, L.J., et al., Nicotine and the developing human: a neglected element in the electronic cigarette debate. American journal of preventive medicine, 2015. 49(2): p. 286-293.
13. Surgeon General, U., E-Cigarette Use Among Youth and Young Adults. A Report of the Surgeon General. 2016, Retrieved from Atlanta, GA: https://ecigarettes. surgeongeneral. gov ….
14. Proctor, R.N., The history of the discovery of the cigarette–lung cancer link: evidentiary traditions, corporate denial, global toll. Tobacco control, 2012. 21(2): p. 87-91.
15. Yassine, A., et al., Did JUUL alter the content of menthol pods in response to US FDA flavour enforcement policy? Tobacco control, 2022. 31(Suppl 3): p. s234-s237.
16. Hajek, P., et al., A Randomized Trial of E-Cigarettes versus Nicotine-Replacement Therapy. N Engl J Med, 2019. 380(7): p. 629-637.

NOT PEER REVIEWED
The study by Gallus et al. [1] sought to establish whether electronic cigarettes (ECs) and heated
tobacco products (HTPs) reduce or increase the probability of smoking in a cohort of Italian
participants and concluded that both EC and HTP use predict smoking initiation and relapse
among respondents. We would like to raise some concerns about the interpretation of the study
findings. The study suffers from a potentially crucial bias of the outcome being present at baseline, as
compared non-users with people who were already using products at baseline. Specifically,
smokers who were using ECs or HTPs at baseline may already represent failed attempts to quit at
baseline. Additionally, ex-smokers using these products may have already been in a trajectory to
relapse to smoking at, or even long before, baseline, and may in fact have initiated such product
use in order to avoid relapse. Still, this group may represent ex-smokers who were at higher risk
for relapsing at baseline compared to ex-smokers who did not use these products. Similarly,
never smokers who use novel nicotine products may represent individuals prone to the
engagement of an inhalational habit. Therefore, they would be more likely to initiate smoking.
The situation is very similar to assessing if people who drink beer at baseline are more likely to
drink whiskey at follow up compared to non-drinkers of beer. People who want to use alcohol
and are using alcohol at baseline, would be much more likely to use stronger liquor at follow up
compared to those who do not use alcohol. This represents a typical case of the common liability
model of addressing risk behaviors, which has been frequently misinterpreted as a “gateway”
theory. Moreover, the study design does not allow for any assessment of the motivations to use these
products. It is unclear if and how many participants were using nicotine products in an attempt to
quit (for smokers) or an attempt to prevent relapse (for ex-smokers) instead of simply
experimenting out of curiosity. Notably, while the study examined daily use through the
questionnaire, results were presented only for current users, a classification that includes both
occasional and daily users. Studies have shown that classifying daily and occasional users in the
same group can often be misleading (2-5), since frequency of use is an indirect indicator of
motivation for use (6), but also a determinant of their success as smoking substitutes (7). The study also suffers from the unpredictability of conducting surveys during COVID-19
lockdowns. Of note, there is convincing evidence that the coronavirus outbreak has had a
negative impact, inducing or exacerbating addictive behaviors as coping mechanisms. It is
possible that smokers preferred smoking – rather than alternatives – depending on their level of
distress (8,9).
Last but not least, the latest Cochrane Review finds high certainty evidence that nicotine e-cigarettes
are more effective than traditional nicotine-replacement therapy (NRT) in helping people quit
smoking (10). In conclusion, methodological issues of the study design makes it inappropriate to address the
research question.

1. Gallus S, Stival C, McKee M, Carreras G, Gorini G, Odone A, van den Brandt PA,
Pacifici R, Lugo A. Impact of electronic cigarette and heated tobacco product on
conventional smoking: an Italian prospective cohort study conducted during the COVID19 pandemic. Tob Control. 2022 Oct 7:tobaccocontrol-2022-057368. doi: 10.1136/tc2022-057368.
2. Hitchman SC, Brose LS, Brown J, Robson D, McNeill A. Associations Between ECigarette Type, Frequency of Use, and Quitting Smoking: Findings From a Longitudinal
Online Panel Survey in Great Britain. Nicotine Tob Res 2015;17:1187-94.
3. Farsalinos, K. E.; Poulas, K.; Voudris, V.; & Le Houezec, J. Prevalence and correlates of
current daily use of electronic cigarettes in the European Union: analysis of the 2014
Eurobarometer survey. 2017, Internal and emergency medicine, 12(6), 757–763.
https://doi.org/10.1007/s11739-017-1643-7
4. Farsalinos, K. E.; & Barbouni, A. Association between electronic cigarette use and
smoking cessation in the European Union in 2017: analysis of a representative sample of
13 057 Europeans from 28 countries. 2021, Tobacco control, 30(1), 71–76.
https://doi.org/10.1136/tobaccocontrol-2019-055190
5. Farsalinos, K. E.; Polosa, R.; Cibella, F.; & Niaura, R. Is e-cigarette use associated with
coronary heart disease and myocardial infarction? Insights from the 2016 and 2017
National Health Interview Surveys. 2019, Therapeutic advances in chronic disease, 10,
2040622319877741. https://doi.org/10.1177/2040622319877741.
6. Amato MS, Boyle RG, Levy D. How to define e-cigarette prevalence? Finding clues in
the use frequency distribution. Tob Control. 2016 Apr;25(e1):e24-9. doi:
10.1136/tobaccocontrol-2015-052236
7. Harlow AF, Stokes AC, Brooks DR, Benjamin EJ, Leventhal AM, McConnell RS,
Barrington-Trimis JL, Ross CS. Prospective association between e-cigarette use
frequency patterns and cigarette smoking abstinence among adult cigarette smokers in the
United States. Addiction. 2022 Dec;117(12):3129-3139. doi: 10.1111/add.16009.
8. Avena NM; Simkus J; Lewandowski A; Gold MS; Potenza MN. Substance Use
Disorders and Behavioral Addictions During the COVID-19 Pandemic and COVID-19-
Related Restrictions. Front Psychiatry. 2021;12:653674.
HTTPS://doi.org/10.3389/fpsyt.2021.653674
9. Caponnetto, P.; Inguscio, L.; Saitta, C.; Maglia, M.; Benfatto, F.; & Polosa, R. Smoking
behavior and psychological dynamics during COVID-19 social distancing and stay-athome policies: A survey. 2020, Health Psychology Research, 8(1).
https://doi.org/10.4081/hpr.2020.9124
10. Hartmann-Boyce J; Lindson N; Butler AR; McRobbie H; Bullen C; Begh R; Theodoulou
A; Notley C; Rigotti NA; Turner T; Fanshawe TR; Hajek P. Electronic cigarettes for
smoking cessation. 2022, Cochrane Database of Systematic Reviews 2022, Issue 11. Art.
No.: CD010216. DOI: 10.1002/14651858.CD010216.pub7.
https://www.cochranelibrary.com/cdsr/doi/10.1002/14651858.CD010216.pub7/...
s

NOT PEER REVIEWED

In his rapid response to our paper, Dr Pesko expresses concern with the paper’s conclusions. Our study, conducted at the US population level, demonstrated that the prolonged decline in cigarette smoking among young adults was largely unaffected by the introduction of e-cigarettes. Dr Pesko reminds us that this conclusion is contrary to findings from several quasi-experimental designs, which he feels should have primacy over our population-level analysis of time trends. We acknowledge that there are potential threats to the validity and generalizability of the findings of our study, and indeed of every study. Hence, a conclusion of causality requires that an association be robust and replicable across settings and methods. Given the significant public policy implications of any finding that e-cigarette vaping has a role in reducing cigarette smoking prevalence, we feel that presentation of transparent and easily understood population trends have an important role to play in policy discussions. In particular, to support the argument that high e-cigarette taxes cause increased smoking among youth, it would be important to have an exemplar of one or more jurisdictions that imposed a high e-cigarette tax and subsequently experienced an increase in cigarette smoking.

A substantial portion of Dr Pesko’s critique of our paper is directed towards our commentary on a paper that he co-authored, Abouk et al. (1). This commentary was incorporated at the request of a reviewer. Abouk et al. studied surveillance data for US adolescents and concluded that “there were sizable positive cigarette cross-tax effects, suggesting economic substitution between cigarettes and ENDS for youth,” or in other words, that raising taxes on e-cigarettes would drive youth tobacco consumption away from e-cigarettes and towards traditional cigarettes. Our study considered the same jurisdictions used in Abouk et al., and looked for potential exemplars with substantial populations and with high e-cigarette taxes. We then reported cigarette and e-cigarette prevalence data for these exemplars for the 18-24-year-old population (i.e. for young adults still within the age window for smoking initiation).

Table 1 of Abouk et al. outlines the 13 jurisdictions that taxed e-cigarettes between 2015 and 2019, providing average tax figures for the first two quarters of the years 2015, 2017 and 2019. Unfortunately, none of the high-quality US tobacco-use surveillance systems (including the two used by Abouk et al.) draws representative samples at the county or city levels. Thus, in three of their jurisdictions, it is not possible to present valid estimates of the change in tobacco use; simply raking the larger sample to the population distribution within the county or city does not make it representative of that jurisdiction. An additional five states did not meet the minimum population criterion of our paper (5 million), needed for reasonably tight confidence limits on the state-level population estimates. Therefore, only the following five states of the original 13 jurisdictions could be considered as possible exemplars in our paper: California, Minnesota, North Carolina, New Jersey, and Pennsylvania.

Dr Pesko’s suggestion in his critique that we should have used 2019 rather than 2017 e-cigarette tax data is problematic, because the most recent population survey with state-level estimates of tobacco use (TUS-CPS) collected two-thirds of its data in 2018. A fundamental principal of causal inference asserts that to test whether an exposure influenced an outcome, the exposure must have been in place before the outcome was assessed. (2) Accordingly, we used the 2017 e-cigarette tax data, so that the taxes were assessed prior to the prevalence estimates of 2018.

From Abouk et al. table 1, during the first two quarters of 2017, e-cigarettes taxes (per milliliter) for the five states under consideration were: Minnesota ($2.50), Pennsylvania ($1.05), California ($0.72), North Carolina ($0.05) New Jersey ($0). Pesko argues that we should have focused on the top three states as possible exemplars. However, California introduced a major increase in all tobacco taxes in the latter half of 2017 and, importantly, the tax rate on non-cigarette tobacco products was required to be equivalent to the tax imposed on cigarettes. (3) Thus, the effect of e-cigarette taxes on cigarette smoking in California is confounded by the concurrent increases in cigarette taxes. Accordingly, we concluded that there were only two possible exemplars from the Abouk et al. paper: Minnesota and Pennsylvania.

In the supplement to our paper (eTable 3) we presented data on exclusive e-cigarette vaping for each of the 24 most populous US states, for both 2014/15 and 2018/19. For Minnesota, exclusive e-cigarette vaping remained constant at 2.8% in both years, in contrast to a 2.7-fold increase in its comparable group of states. For Pennsylvania, exclusive vaping declined from 3.3% to 0.9%, in contrast to an over twofold increase in its comparable group of states. This aligns with economic theory, (4) suggesting that in each of these two states higher taxes are associated with lower demand for e-cigarettes. Thus, both of these states can be used as transparent examples of the Abouk et al. hypothesis. However, in our paper, we noted that smoking prevalence among 18-24-year-olds across all of the 24 most populous US states declined by 43% from 12.9% (95% CI: 12.2%,13.7%) in 2014/15 to 7.4% (95% CI: 6.7% to 8.1%) in 2018/19. Pesko rightly points out the wide confidence limits for individual state smoking prevalence estimates in each of these years. However, there was no discernible evidence, not even suggestive evidence, indicating that, in either of these two exemplar states, the rate of decline in cigarette smoking was slower compared to other US states.

In summary, it is important to have robust and replicable results before making conclusions that associations are causal. Before making an argument that states should refrain from increasing e-cigarette taxes because such taxes will slow down the decline in smoking initiation, it is particularly important to have transparent examples demonstrating such a cause and effect. None of the 13 jurisdictions used in the Abouk et al. study provides such a demonstration.

References
1. Abouk R, Courtemanche C, Dave D, Feng B, Friedman AS, Maclean JC, et al. Intended and unintended effects of e-cigarette taxes on youth tobacco use. J Health Econ. 2023 Jan 1;87:102720
2. Bradford Hill A. (1965). The Environment and Disease: Association or Causation?. Proceedings of the Royal Society of Medicine. 58 (5): 295–300. doi:10.1177/003591576505800503.
3. California State Board of Equalization New Tax Rate on Other Tobacco Products Effective July 1 2017 through June 30, 2018. Available at: https://www.cdtfa.ca.gov/formspubs/l502.pdf
4. International Agency for Research on Cancer (IARC). Handbooks of Cancer Prevention, Tobacco Control, Vol. 14. Effectiveness of Tax and Price Policies for Tobacco Control. Lyon, France., 2011