People with a genetic predisposition to heavy smoking are ten times more likely to die from COVID-19, according to research published yesterday in the journal Thorax. The study authors also found that smoking significantly increases an individual’s chances of becoming infected with the virus and being hospitalized.

The relationship between tobacco smoking and COVID-19 outcomes has proven difficult to establish, largely because most of the studies conducted to date have been observational. In addition to generating confusing and contradictory results, these studies have failed to identify any biological mechanism that might explain how the two are related.

To resolve this, the authors of this latest study combined observational analyses with genetic screening, thereby establishing a link between the genes that contribute to cigarette use and COVID-19. Scanning through data on more than 420,000 people in the UK, they discovered that current smokers were 80 percent more likely to end up in hospital than non-smokers after becoming infected.

Mortality rates were also closely tied to smoking habits, with individuals who smoked between one and nine cigarettes a day being 2.14 times more likely to die from the virus than non-smokers. People who smoked between 10 and 19 a day, meanwhile, were 5.91 times more likely to die, while those who smoked more than 20 cigarettes a day had a more than six-fold increase in their chances of succumbing to the illness.

To add some weight to these observational statistics, the researchers then harvested genetic data from the UK biobank study in order to determine how genes associated with smoking affect COVID-19 severity. After studying the genomes of over 280,000 people, they determined that those with a genetic predisposition to smoking were 45 percent more likely to catch the virus and 60 percent more likely to end up in hospital than those lacking this genetic susceptibility.

Furthermore, genes linked to heavy smoking were associated with a 2.5 fold increase in likelihood of infection. Individuals carrying these genetic variants were also five times more likely to be hospitalized with the illness and ten times more likely to die.

While it isn’t clear how these genes influence a person’s susceptibility to smoking, the study authors insist that their effect is robust, explaining that “[g]enetic variants predicting lifelong differences in smoking initiation ... and smoking heaviness ... were strongly associated with observed smoking behaviours.”

Summing up their findings, they write that “[t]ogether, the results from our two analytical approaches support a causal effect of smoking on the risk of severe COVID-19.”

Such a conclusion should go some way to clearing up the confusion regarding the impact of smoking on virus symptomology. This uncertainty has not been helped by the publication of two studies early in the pandemic which claimed that cigarettes protect against COVID-19, but which were later retracted after it emerged that the authors had connections to the tobacco industry.

Highlighting the importance of this new research, study author Dr Ashley Clift explained that “[o]ur results strongly suggest that smoking is related to your risk of getting severe COVID,” adding that “now might be as good a time as any to quit cigarettes and quit smoking.”