Case studies can't spot harm faster than the harm occurs. The main harm from smoking is a significantly increased risk of lung cancer and other illnesses which can take decades to show up. That's why even with modern medicine advancing over the last century it took so long to nail down the health risks of smoking.

I used to smoke, and now I vape. My lungs function so much better now that it’s easily discernible to me that smoking irritates lungs far more than vaping.

Now, you could argue that there's little evidence of nicotine being harmful aside from being highly addictive (although even then there would be an economic argument to be made), but if there is any harm, the addictive nature will make sure that harm is long-lasting.

Modern laboratory science is no help in figuring out fast whether vaping is harmful, because we don't understand how the human body works well enough to do laboratory tests that will tell us. So we're reduced to basically what we used to show that smoking was harmful: long-term data collection and analysis.

> My lungs function so much better now that it’s easily discernible to me that smoking irritates lungs far more than vaping.

Saying that vaping is less harmful than smoking (which is probably true) is a much weaker statement than saying that there's zero indication that vaping is harmful, so there's no reason to worry about it, which is the claim I was responding to.