There were loads of theories initially, some suggesting more likely to catch and it be very ill and others saying less likely to catch it, but higher chance of being very ill if you do.
Some said it was down to the lungs, some said it was because smokers touch their face more often and there was a theory that nicotine prevents it spreading. Clinical trials on nicotine are ongoing and results due later this year, but on the basis we haven’t heard anything about it, I’m assuming it’s a non-starter.