Friday 24 April 2020

Two more studies show smokers at less risk from COVID-19

Some researchers in London have looked at the evidence on COVID-19 and smoking and created a rapid evidence review. They do a better job than Stanton Glantz did last week, but they make the same mistake of not splitting up the current smokers from the ex-smokers. There isn't really any evidence that ex-smokers are more resistant to the coronavirus and there is no reason to think they would be. The hypothesis is that nicotine confers a benefit, and ex-smokers aren't getting any nicotine.

If you bundle the ex-smokers together with the smokers, you muddy the picture. Sure enough, the UK review finds 'substantial uncertainty arising from the recording of smoking status on whether current and/or former smoking status is associated with SARS-CoV-2 infection, hospitalisation or mortality'.

Nevertheless, it alerted me to this study which I had not seen before. It looks at 3,789 US military veterans aged between 54 and 75 who have been tested for COVID-19, of whom 585 tested positive. Smokers were heavily under-represented with an odds ratio of 0.45 (95% CI 0.35-0.57), ie. they were 55 per cent less likely to test positive.

Further evidence was published yesterday by some French epidemiologists. Their study involved pupils, parents, siblings, teachers and non-teaching staff at a high school in an area of Northern France that has been badly hit by COVID-19. Of the 661 people tested for COVID-19 antibodies, 26 per cent of them tested positive, of whom 5.3 per cent were hospitalised and none died. These are important findings in themselves, but there are also figures on smoking prevalence which, once again, are rather striking.

Smoking was found to be associated with a lower risk of infection (7.2% versus 28.0% for smokers and non-smokers, respectively), and this association remained significant after adjustment for age (OR = 0.23; 95% CI = 0.09 –0.59) or occupation (OR = 0.27; 95% CI = 0.10 –0.71).

This suggests that smokers are 70-80 per cent less likely to test positive for COVID-19, in line with a previous study from France. The authors note that...

Earlier studies in China and the U.S. have documented a low proportion of smokers among COVID-19 patients (6% of 191 hospitalised patients in Wuhan, and 1% of 7162 patients in the U.S.). The protection associated with smoking in our study was very substantial (75% decrease in risk of infection), and deserves full attention. One possible explanation would be the downregulation of ACE2, the SARS-CoV-2 receptor, by nicotine.

Taken together with the rest of the evidence, the association between current smoking and lower COVID-19 risk is starting to look statistically and clinically meaningful. There are now two big questions.

Firstly, is it the nicotine or is it something else? Trials underway in France should help us answer that.

Secondly, are smokers/nicotine-users more likely to fight off the disease when they get it or are they less likely to get it in the first place? Data on hospital admissions can be used to support either hypothesis, but the two studies mentioned above suggest the latter.

There is more to come, I'm sure. Watch this space.

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