Association of smoking status with hospitalisation for COVID-19 compared with other respiratory viruses a year previous: a case-control study at a single UK National Health Service trust

<ns3:p><ns3:bold>Background:</ns3:bold> It is unclear whether smoking increases the risk of COVID-19 hospitalisation. We first examined the association of smoking status with hospitalisation for COVID-19 compared with hospitalisation for other respiratory viral infections a year previous. Second, we examined the concordance between smoking status recorded on the electronic health record (EHR) and the contemporaneous medical notes.</ns3:p><ns3:p> <ns3:bold>Methods:</ns3:bold> This case-control study enrolled adult patients (446 cases and 211 controls) at a single National Health Service trust in London, UK. The outcome variable was type of hospitalisation (COVID-19 vs. another respiratory virus a year previous). The exposure variable was smoking status (never/former/current smoker). Logistic regression analyses adjusted for age, sex, socioeconomic position and comorbidities were performed. The study protocol and analyses were pre-registered in April 2020 on the <ns3:ext-link xmlns:ns4="http://www.w3.org/1999/xlink" ext-link-type="uri" ns4:href="https://doi.org/10.17605/OSF.IO/URFHN">Open Science Framework</ns3:ext-link>.</ns3:p><ns3:p> <ns3:bold>Results:</ns3:bold> Current smokers had lower odds of being hospitalised with COVID-19 compared with other respiratory viruses a year previous (OR<ns3:sub>adj</ns3:sub>=0.55, 95% CI=0.31-0.96, <ns3:italic>p</ns3:italic>=.04). There was no significant association among former smokers (OR<ns3:sub>adj</ns3:sub>=1.08, 95% CI=0.72-1.65, <ns3:italic>p</ns3:italic>=.70). Smoking status recorded on the EHR (compared with the contemporaneous medical notes) was incorrectly recorded for 168 (79.6%) controls (χ<ns3:sup>2</ns3:sup>(3)=256.5, <ns3:italic>p</ns3:italic>=&lt;0.001) and 60 cases (13.5%) (χ<ns3:sup>2</ns3:sup>(3)=34.2, <ns3:italic>p</ns3:italic>=&lt;0.001).</ns3:p><ns3:p> <ns3:bold>Conclusions:</ns3:bold> In a single UK hospital trust, current smokers had reduced odds of being hospitalised with COVID-19 compared with other respiratory viruses a year previous, although it is unclear whether this association is causal. Targeted post-discharge recording of smoking status may account for the greater EHR-medical notes concordance observed in cases compared with controls.</ns3:p>