Smokers are 60%-80% more likely to be admitted to hospital with Covid-19 and also more likely to die from the disease, data suggests.

study, which pooled observational and genetic data on smoking and Covid-19 to strengthen the evidence base, contradicts research published at the start of the pandemic suggesting that smoking might help to protect against the virus. This was later retracted after it was discovered that some of the paper’s authors had financial links to the tobacco industry.

Other studies on whether smoking is associated with a greater likelihood of more severe Covid-19 infection have produced inconsistent results.

One problem is that most of these studies have been observational, making it difficult to establish whether smoking is the cause of any increased risk, or whether something else is to blame, such as smokers being more likely to come from a lower socioeconomic background.

Dr Ashley Clift at the University of Oxford and colleagues drew on GP health records, Covid-19 test results, hospital admissions data and death certificates to identify associations between smoking and Covid-19 severity from January to August 2020 in 421,469 participants of the UK Biobank study – all of whom had also previously had their genetic makeup analysed.

Compared with those who had never smoked, current smokers were 80% more likely to be admitted to hospital and significantly more likely to die from Covid-19 if they became infected.

To investigate, Clift and his team used a technique called Mendelian randomisation, which uses genetic variants as proxies for a particular risk factor – in this case genetic variants that contribute to whether someone is more likely to smoke or to smoke heavily – to obtain further evidence for a causal relationship.

Even though the contribution of each of these variants is small and it is not necessarily understood why they increase a person’s chances of being a smoker, they avoid many of the limitations of observational studies and thereby help to give a clearer picture of whether there is a biological link between smoking and Covid-19.

“The study adds to our confidence that tobacco smoking does not protect against Covid-19, as their Mendelian randomisation analyses are less susceptible to confounding than previous observational studies,” wrote Dr Anthony Laverty and Prof Christopher Millett of Imperial College London in a linked editorial published in the journal Thorax.

The Mendelian randomisation analysis also supported the link between smoking and worse Covid-19 outcomes, finding that a genetic predisposition to smoking was associated with a 45% higher risk of infection and a 60% higher risk of hospital admission for Covid-19.

A genetic predisposition to heavy smoking was associated with a more than doubling in the risk of infection; a fivefold increase in the risk of hospital admission; and a tenfold increase in the risk of death from the virus, the team found.

Clift 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.”

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