A common argument against electronic cigarettes is the lack of long-term studies done on them - especially compared to federally approved products such as nicotine patches. After much time, a long term study on vaping has been published.

A five year study, focusing on the severity of COPD symptoms of smokers who transitioned to electronic cigarettes was released. Overall, they found improvements in severe COPD episodes and lung function in patients that were willing to stick with vaping, and that the improvements did not decline over time.

There was also a lot of other information included in the study that was worth noting.

Dual users were dropped from respiratory data. This was not always done in older studies, perhaps due to misunderstanding of vapes, but newer studies group them differently as they tend to have different behaviors and risks. It is noteworthy if a study that started five years ago grouped them differently.
Originally, there were 20 users in the electronic cigarette test group. By the end of the five year period, four users were dropped from the data - they either relapsed into using cigarettes or they quit vaping. At first glance, this seems like an amazing success rate - at the minimum, 80% of their subjects quit cigarettes for five years. However, while the number is high enough to warrant further investigation, it is not a reliable statistic as to whether or not vaping helps users quit cigarettes.

In a study where you want to study the effects of long-term vaping, it would be sensible to choose subjects that will continue vaping throughout your test period. Choosing a subject that is likely to quit vaping in two years is not helpful when you want to see how vaping for five years will affect them. So as a general measurement of how much electronic cigarettes contributed to quitting cigarettes, this is not a reliable number at all.

Lastly, they noted that the devices and nicotine changes over time. The study took place from September 2013–October 2019. Juul and the emergence of nicotine salts began in 2015. It’s a long term study, yes, but the vaping industry moves so fast it no longer even addresses the main concerns of anti-vapers anymore.