It's the first study to look at observational and genetic data on the relationship between smoking and coronavirus.
Smokers are 80% more likely to be admitted to hospital and significantly more likely to die from Covid-19 than non-smokers, new research led by Oxford University shows.
Until now, experts said, there has been conflicting data on the extent to which smoking affects the risk of hospital admission from Covid-19.
To address the issue, the team looked at primary care records, Covid test results, hospital admissions data and death certificates.
Overall, some 421,469 people were included in the research.
Among them, 1,649 tested positive for Covid, 968 were admitted to hospital related to the virus and 444 people died with Covid-19.
Most (59%) of the overall group had never smoked, over a third (37%) were former smokers and 4% were current smokers.
Among current smokers, most (71%) were light or moderate smokers (one to 19 cigarettes per day) and 29% were heavy smokers (20 or more per day).
The results showed that, compared with never-smokers, current smokers had an 80% higher risk of hospital admission with Covid-19.
They were twice as likely to die if they smoked one to nine cigarettes a day, while those smoking 10 to 19 cigarettes a day were almost six times more likely to die.
Among heavier smokers, people who smoked more than 20 a day were over six times more likely to die compared to people who had never smoked.
Genetic analysis showed that among those genetically more likely to smoke, there was a higher risk of Covid infection and hospital admissions.
The authors concluded that their “results from two analytical approaches support a causal effect of smoking on risk of severe Covid-19”.
Dr Ashley Clift, the lead researcher from Oxford, said: “Our results strongly suggest that smoking is related to your risk of getting severe Covid, and just as smoking affects your 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.
“So now might be as good a time as any to quit cigarettes and quit smoking.”
In an editorial accompanying the research, Dr Anthony Laverty and Dr Christopher Millett of Imperial College London said: “A respiratory pandemic should be the ideal moment to focus collective minds on tobacco control.”
The study was published in the journal Thorax.