New insights into the paradox between smoking and the risk of SARS-CoV-2 infection (COVID-19): Insufficient evidence for a causal association.

AIMS: Previous studies have reported a 'smoker's paradox', where people who smoke appear to be protected against Severe Acute Respiratory Syndrome CoronaVirus-2 (SARS-CoV-2) infection (COVID-19). This conflicts with well-established evidence that people who smoke are generally more vulnerable to respiratory infections. In this study, we aimed to validate the association between smoking and SARS-CoV-2 infection in a general Dutch population, and to evaluate the evidence underlying the possible causal relationship between smoking and SARS-CoV-2 infection by applying a modern adaptation of the Bradford Hill criteria. METHODS: In total, 57,833 participants from the Lifelines Cohort Study were included in the analysis. Smoking status, including never smoker, current smoker, and former smoker, was derived from the Lifelines general assessment between 2014 and 2017, while SARS-CoV-2 infection status was derived from an additional COVID-19 questionnaire from 2021 to 2022. Logistic regressions were used for the association between smoking status and infection status. The adapted Bradford Hill's criteria, including the strength of association (including an analysis of plausible confounding), plausibility, temporality and study design suitability, were applied to evaluate the existing literature. RESULTS: We found, compared with never smokers, an increased risk of SARS-CoV-2 infection for former smokers (odds ratio (OR)=1.07, 95% confidence interval (CI)=1.01-1.13), but a reduced risk for current smokers (OR=0.85, 95% CI=0.79-0.92), after adjusting for several relevant covariates. However, we discerned a possible explanation of the smoker's paradox since we observed that current smokers were more likely to be non-responders to the COVID-19 questions and, more importantly, these non-responders were more likely to have other established risk factors for SARS-CoV-2 infection. CONCLUSIONS: There is insufficient evidence to suggest that smoking protects against SARS-CoV-2 infection. According to the adapted Bradford Hill's criteria, we observed a high inconsistency between study results, a high possibility for residual confounding and no clear evidence for biological plausibility. Future studies should include linkage with the confirmed testing results from national healthcare registries to mitigate avoidable bias.

Using data on biomarkers and siblings to study early-life economic determinants of type-2 diabetes.

We study the effect of economic conditions early in life on the occurrence of type-2 diabetes in adulthood using contextual economic indicators and within-sibling pair variation. We use data from Lifelines: a longitudinal cohort study and biobank including 51,270 siblings born in the Netherlands from 1950 onward. Sibling fixed-effects account for selective fertility. To identify type-2 diabetes we use biomarkers on the hemoglobin A1c concentration and fasting glucose in the blood. We find that adverse economic conditions around birth increase the probability of type-2 diabetes later in life both in males and in females. Inference based on self-reported diabetes leads to biased results, incorrectly suggesting the absence of an effect. The same applies to inference that does not account for selective fertility.

Sarcopenia augments the risk of excess weight on COVID-19 hospitalization: A prospective study using the Lifelines COVID-19 cohort.

OBJECTIVE: We investigated the associations of sarcopenia alone, overweight or obesity, and sarcopenic overweight or obesity with COVID-19 hospitalization. METHODS: Participants from the Lifelines COVID-19 cohort who were infected with COVID-19 were included in this study. Sarcopenia was defined as a relative deviation of muscle mass of ≤ -1.0 SD from the sex-specific mean 24-h urinary creatinine excretion. Overweight or obesity was defined as a body mass index ≥ 25 kg/m2. Sarcopenic overweight or obesity was defined as the presence of overweight or obesity and low muscle mass. COVID-19 hospitalization was self-reported. Logistic regression models were used to analyze the associations of sarcopenia alone, overweight or obesity, and sarcopenic overweight or obesity with COVID-19 hospitalization. RESULTS: Of the 3594 participants infected with COVID-19 and recruited in this study, 173 had been admitted to the hospital. Compared with the reference group, individuals with overweight or obesity and sarcopenic overweight or obesity were 1.78-times and 2.09-times more likely to have been hospitalized for COVID-19, respectively, whereas sarcopenia alone did not increase the risk of COVID-19 hospitalization. CONCLUSIONS: In this middle-aged population, sarcopenic overweight or obesity elevated the risk of hospitalization for COVID-19 in those infected with COVID-19 more than overweight or obesity alone. These data support the relevance of sarcopenic overweight or obesity as a risk factor beyond the geriatric setting and should be considered in risk stratification in future public health and vaccination campaigns.