Smoking is likely to increase the severity of COVID-19 and the risk of dying from the viral infection, according to a study published on 28 September.

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Studies carried out early on in the pandemic reported a lower prevalence of active smokers among people admitted to hospital with COVID-19 than in the general population. However, other population-based studies have suggested that smoking is a risk factor for infection. Most of the research to date, however, has been observational in nature and so unable to establish a causal effect.

The latest study, published in the journal Thorax, is the first of its kind to pool observational and genetic data on smoking and COVID-19 to strengthen the evidence. "Our results strongly suggest that smoking is related to your risk of getting severe COVID, and just as smoking affects the risk of heart disease, different cancers, and all those other conditions we know smoking is linked to, it appears that it's the same for COVID," said lead researcher Ashley Clift.

"So now might be as good a time as any to quit cigarettes and quit smoking," said Clift, from the University of Oxford in the UK. The team of researchers from Oxford, the University of Bristol, and the University of Nottingham drew on linked primary care records, COVID-19 test results, hospital admissions data and death certificates.

They looked for associations between smoking and COVID-19 infection severity from January to August 2020 in 421,469 participants of the UK Biobank, all of whom had had their genetic make-up analysed when they agreed to take part in 2006-10. The researchers found that compared with people who had never smoked, current smokers were 80 per cent more likely to be admitted to hospital and significantly more likely to die from COVID-19.

They used Mendelian randomisation to assess whether a genetic predisposition to smoking and heavy smoking might have a role in COVID-19 severity among 281,105 of the original participants living in England. Mendelian randomisation is a technique that uses genetic variants as proxies for a particular risk factor -- in this case genetic variants that make someone more likely to smoke or to smoke more heavily -- to obtain genetic evidence in support of a causal relationship.

The technique revealed that a genetic predisposition to smoking was associated with a 45 per cent higher risk of infection and a 60 per cent higher risk of hospital admission for COVID-19. Having these genetic factors to smoke more heavily was associated with over twice the risk of infection, a five-fold increase in the risk of hospital admission, and a 10-fold increase in the risk of death from the virus, the researchers found.

While the researchers acknowledge that they relied only on hospital COVID-19 test data rather than on more representative community data, they point to the similarity of the findings in both sets of analyses. Both the observational analyses indicating associations with recent smoking behaviours and Mendelian randomisation analyses indicating associations with a lifelong predisposition to smoking and smoking heaviness support a causal effect of smoking on COVID-19 severity, they said.