Introduction:
Because of the alleged protective effect of nicotine on SARS-CoV-2 infection driven by the relatively low smoking prevalence among COVID-19 patients, some researchers, often linked with the tobacco industry, speculated that e-cigarettes (e-cigs) containing nicotine could lower the risk of SARS-CoV-2 infection.

Material and Methods:
We conducted a systematic literature review (updated to June 2022) on all the studies providing data on the link between e-cig and heated tobacco product (HTP) use, and COVID-19 incidence and progression, using the main scientific databases (i.e., PubMed/MEDLINE and Web of Science).

Results:
Starting from 422 non-duplicate publications, we found 14 original publications, including 10 on SARS-CoV-2 infection, 4 on hospitalization, 3 on severity and 1 on mortality. Ten studies provided data on the risk of SARS-CoV-2 infection for current/ever e-cig users vs. never/non-users: none of them found a significant decreased risk of SARS-CoV-2 infection among e-cig users, while four studies found a significant increased risk for (exclusive or dual) e-cig users. Three studies showed that e-cig users had more frequent and persistent COVID-19 symptoms. A recent large cohort study did not find any relationship between e-cig use and COVID-19 progression. A couple of cross-sectional studies investigated the association between HTP use and COVID-19 incidence, finding no significant association.

Conclusions:
The growing evidence on the role of e-cig on COVID-19 incidence, shows that, if any, e-cig use is associated with an increased risk of SARS-CoV-2 infection when compared to non-use. E-cig use might increase the risk of severe forms of COVID-19, but the studies on this topic are few and conflicting. No significant association was found in the few studies on the association between HTP use and COVID-19.