Increases in smoking recorded in this study appear to conflict with official Canadian data
Table 2 shows marked increases in smoking in Canada between 2017 and 2018 among 16-19 year-olds:
Smoked in past 30-days: a 41% increase from 10.7% to 15.1%;
Smoked in the past week: a 57% increase from 7.6% to 11.9%;
Smoked ≥15 days in past 30 days: a 54% increase from 4.8% to 7.4%.
These (improbable) increases conflict with official data from Statistics Canada that was released shortly after the publication of the paper  drawn from the Canadian Community Health Survey, with a representative sample of 65,000. The age groups reported by Statistics Canada do not exactly match that reported in Table 2. However, the official data do suggest caution is essential in interpreting or acting on the findings in this survey:
Statistics Canada: Age group 12-17 years:
Current smoking declined by 9% from 3.5% to 3.2%;
Daily smoking declined by 18% from 1.1% to 0.9% (*estimated).
Statistics Canada: Age group 18-34 years:
Current smoking - unchanged at 19.2%;
Daily smoking - declined by 9% from 11.3 to 10.7%.
It is theoretically possible that the specific age group 16-19 used in the Hammond et al paper showed a sharp rise between 2017 and 2018 while there were compensating declines in the rest of 12-17 and 18-34 age groups and therefore the findings can be reconciled. But this seems very unlikely and would need further validation. More likely, the conflict arises from changes in the sample between 2017 and 2018 or other artefacts of the method.
The study uses a non-probabilistic sample. "Respondents were recruited from the Nielsen Consumer Insights Global Panel, which maintains panels in Canada, England, and the US, as well as their partners’ panels. The Nielsen panel is recruited using both probability and non-probability sampling methods in each country. Nielsen selected random samples from the online panels in each country."
Brown et al also register concerns about the English data in a response to this paper  adding to the concern about the reliability of some of the findings in this study. That would also have potential consequences for the measured increase in vaping.
 Statistics Canada. Table 13-10-0096-10 Smokers, by age group, 25 June 2019 https://www150.statcan.gc.ca/t1/tbl1/en/tv.action?pid=1310009610
 Statistics Canada. Table 13-10-0096-23 Current smoker, daily, by age group, 25 June 2019 https://www150.statcan.gc.ca/t1/tbl1/en/tv.action?pid=1310009623
 Brown J. et al. Smoking prevalence in England, 2017-2018. BMJ response, 3 July 2019 https://www.bmj.com/content/365/bmj.l2219/rr-1
Competing interests: No competing interests