Cigarettes seem like an unlikely ally against a respiratory virus. Tobacco smoke damages the tiny air sacs in the lungs where oxygen enters the blood, and slows down the hairs lining the body’s air passages that gently waft along mucus. Smokers are affected more severely by colds and flu, and years of smoking can lead to a type of lung failure known as emphysema, which is a form of chronic obstructive pulmonary disease.
But data emerging from the countries first hit by coronavirus gave doctors pause: the proportion of smokers among those being hospitalised for covid-19 was lower than in the general population. In China, for example, about 8 per cent of people in hospital with covid-19 were smokers, while 26 per cent of the general population smoke. The equivalent figures for Italy are 8 and 19 per cent respectively.
“The data seem to be repeated in different countries,” says Alberto Nájera at the University of Castilla-La Mancha in Spain, whose team analysed figures from 18 of the first such reports (Preprints, doi.org/dv8f). He says nicotine may reduce the immune system’s tendency to overreact to the virus and cause a cytokine storm, an inflammatory response that can be deadly.
A different idea, put forward by Jean-Pierre Changeux at the Pasteur Institute in Paris, is that nicotine lowers the amount of a molecule on lung cells called ACE2, which the coronavirus uses to gain entry into those cells (Qeios, doi.org/dv8h).
But many of the reports that find lower rates of smoking among covid-19 patients also suggest that smoking is more common among people who get sickest and die (Qeios, doi.org/dv8j). This is hard to explain if nicotine really protects against the coronavirus.
Questions have also been raised about the accuracy of the initial reports. Some people recorded as non-smokers may have recently stopped due to the pandemic, says Nick Hopkinson at Imperial College London. There has been speculation that in places like Italy where there weren’t always enough intensive care beds to go round, some may have lied about smoking, he says.
Perhaps the biggest concern is that it isn’t valid to compare smoking rates among coronavirus patients with the general population, says Eleanor Murray at Boston University. Most people who have severe enough covid-19 infections to end up in hospital are older, and older people have lower smoking rates.
Rather than looking at smoking rates in hospital patients, Hopkinson is using an app downloaded by 1.5 million people in the UK to track their cases. His findings aren’t yet published, but suggest that smokers are about 25 per cent more likely to develop covid-19 symptoms – although this is judged by users’ self-reports, not medical tests.
A recent study looked at the health records of 17 million people in England to establish risk factors linked with dying from covid-19 among (medRxiv, doi.org/dt9z).
The results for smoking change depending on which other risk factors are included. But the most logical analysis according to Hopkinson, adjusting for age and sex only, finds smokers at a somewhat higher risk of dying from covid-19. “The claim that smoking is protective is interesting and perplexing, but it doesn’t stand up to scrutiny,” he says.
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