THIS IS INTERESTING: Smokers seem less likely than non-smokers to fall ill with covid-19.

Smokers are almost certainly not protected from initial infection by SARS-CoV-2. In fact, because they first handle and then puff on cigarettes, they may be especially susceptible—for transmission often takes place through the mouth’s mucous membranes. What seems to be happening is that infected smokers are less likely to develop symptoms, or, if they do develop them, are more likely than non-smokers to have symptoms which are mild. That means they are more likely to stay home and not to show up in hospital statistics.

All this suggests that something in tobacco smoke is having a protective effect. The best guess is that the something in question is nicotine.

Note, however, that there are umpteen compounds in tobacco smoke, and nicotine is just the most obvious.