Impact of Smoking on Household SARS-CoV-2 Transmission

<b>Background/Objectives:</b> The role of smoking in household SARS-CoV-2 transmission is controversial. The objective of this study was to analyze the impact of smoking on SARS-CoV-2 index cases and transmission to household contacts. <b>Methods:</b> Prospective cohort study of 227 index cases and 332 household contacts in Catalonia and Navarre (May 2022-December 2024). The primary outcome measure was SARS-CoV-2 infection in contacts, confirmed by rapid antigen and polymerase chain reaction testing. Adjusted odds ratios (aOR) and 95% confidence intervals (CI) for the smoking-infection association were calculated using logistic regression, adjusting for age, vaccination, and previous infection. <b>Results</b>: The cumulative infection incidence in contacts was 38.6%, and was higher in people aged ≥65 years (60.6%) and in smokers (48.6%) vs. non-smokers (33.8%). In the multivariate analysis, index case vaccination (aOR = 0.27; 95%CI: 0.11-0.63) and previous contact infection (aOR = 0.49; 95%CI: 0.30-0.81) were associated with a lower probability of transmission, while smoking by household contacts (aOR = 2.09; 95%CI: 1.19-3.65) and age ≥ 65 years (aOR = 5.13; 95%CI: 2.18-12.09) were associated with an increased risk of infection. The index case smoking status was not statistically significant. <b>Conclusions:</b> Smoking by cohabitants and age ≥ 65 years increase the risk of SARS-CoV-2 infection. Index case vaccination and previous contact infection are associated with reduced intra-household transmission.