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Introduction The relationship between cigarette smoking status and SARS-CoV-2 infection and coronavirus disease 2019 (COVID-19) severity is highly debated. We conducted a retrospective cohort study of >2.4 million adults in a large healthcare system to evaluate whether smoking is associated with SARS-CoV-2 infection and disease severity. Methods This retrospective cohort study of 2,427,293 adults in KPNC from 3/5/2020 (baseline) to 12/31/2020 (pre-vaccine) included smoking status (current, former, never), socio-demographics, and comorbidities from the electronic health record. SARS-CoV-2 infection (identified by a positive PCR test) and COVID-19 severity (hospitalization, ICU admission or death ≤30 days of COVID-19 diagnosis) were estimated in time-to-event analyses using Cox proportional hazard regression models adjusting for covariates. Secondary analyses examined COVID-19 severity among patients with COVID-19 using logistic regression. Results During the study, 44,270 patients had SARS-CoV-2 infection. Current smoking was associated with lower adjusted rates of SARS-CoV-2 infection (aHR=0.64 95%CI:0.61-0.67), COVID-19-related hospitalization (aHR=0.48 95%CI:0.40-0.58), ICU admission (aHR=0.62 95%CI:0.42-0.87), and death (aHR=0.52 95%CI:0.27-0.89) than never-smoking. Former smoking was associated with a lower adjusted rate of SARS-CoV-2 infection (aHR=0.96 95%CI:0.94-0.99) and higher rates of hospitalization (aHR=1.10 95%CI:1.03-1.08) and death (aHR=1.32 95%CI:1.11-1.56) than never-smoking. Logistic regression analyses among patients with COVID-19 found lower odds of hospitalization for current versus never-smoking and higher odds of hospitalization and death for former versus never-smoking. Conclusions In the largest US study to date on smoking and COVID-19, current and former smoking showed lower risk of SARS-CoV-2 infection than never-smoking, while a history of smoking was associated with higher risk of severe COVID-19. IMPLICATIONS In this cohort study of 2.4 million adults, adjusting for socio-demographics and medical comorbidities, current cigarette smoking was associated with a lower risk of both COVID-19 infection and severe COVID-19 illness compared to never-smoking. A history of smoking was associated with a slightly lower risk of COVID-19 infection and a modestly higher risk of severe COVID-19 illness compared to never-smoking. The lower observed COVID-19 risk for current versus never-smoking deserves further investigation. Results support prioritizing individuals with smoking-related comorbidities for vaccine outreach and treatments as they become available.

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Background: Exposure to endocrine-disrupting chemicals (EDCs) during sensitive developmental windows, such as in utero, may influence disease later in life but direct measurement of fetal hormones is not feasible. The ratio of the length of the second finger digit to the fourth digit (2D:4D), a sexually dimorphic trait, is a biomarker of androgen levels and the androgen/estrogen balance in utero. However, it is unclear whether in utero EDC exposure might alter 2D:4D ratio. Aims: We examined 2D:4D ratio in Seveso children in relation to in utero exposure to a potent EDC, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) using linear regression. Study design: The Seveso Women's Health Study (SWHS) is a historical cohort study, following the health of women exposed to TCDD during a 1976 explosion in Seveso, Italy. Individual-level TCDD was measured for SWHS in serum collected soon after the accident. In 2014, the SWHS children born after the explosion were enrolled in the Seveso Second Generation Study. Subjects: 594 SWHS children born post-explosion to 397 mothers. Outcome Measures: Right hand 2D:4D ratio. Results: On average, 2D:4D ratio for males was significantly lower than for females (p<0.05). Overall, in utero TCDD exposure, either as maternal initial serum TCDD concentration or as TCDD extrapolated to pregnancy was not significantly associated with 2D:4D ratio in Seveso children. Results from all adjusted sensitivity analyses remained non-significant.