INTRODUCTION: ENDS can benefit those who use combustible tobacco if they transition completely to ENDS. ENDS can also result in nicotine addiction among nicotine-naïve people. METHODS: ENDS-related tobacco-use transitions were assessed among U.S. youth and adults using weighted Population Assessment of Tobacco and Health Study Waves 4 (2016-2017) and 5 (2018-2019) adult and youth data. A beneficial transition was defined as those who used combustible tobacco and transitioned exclusively to ENDS use or quit with the help of ENDS. A harmful transition was defined as (1) nonusers of any tobacco product who initiated ENDS (with or without combustible tobacco co-use) or (2) those who exclusively used ENDS and then added or transitioned to combustible tobacco use. Sensitivity analyses were conducted to examine modified definitions of beneficial and harmful transitions based on different assumptions. The analyses were conducted between August 2024 and November 2024. RESULTS: Total sample size (N=31,733) represented ∼256 million people. For those using a combustible tobacco product in Wave 4, 2.1 million (∼4.6%) transitioned to exclusive ENDS use or to ENDS-assisted cessation of a combustible tobacco product (benefit). In addition, 4.6 million (∼%2.2) transitioned from nonuse to ENDS or, among people who use ENDS exclusively in Wave 4, added combustible or transitioned to combustible tobacco use in Wave 5 (harm). CONCLUSIONS: For every 1 beneficial transition, ENDS use was associated with 2.15 harmful transitions
this ratio ranged from 0.75 to 2.77 in sensitivity analyses. With effective restrictions on ENDS access and marketing for tobacco-naïve people, the population benefits of ENDS could outweigh population harms.