Thursday, 4 March 2021

Smoking and COVID-19: new update

As North Carolina announces that smokers and ex-smokers can jump the queue for the SARS-CoV-2 vaccine, it's time to take another look at the evidence on smoking and COVID-19.

Regular readers will recall the evidence showing smokers heavily under-represented in Covid wards around the world. It made a few headlines last spring, but it has since faded away as a news story. Public Health England's false claim about smokers being 14 times more likely to develop severe COVID-19 remains online.

That doesn't mean the studies have dried up. The ongoing meta-analysis by the UCL academics is now in its eleventh edition and the song remains the same. With 121 studies under review, smokers are still less likely to get COVID-19. 
Current compared with never smokers were at reduced risk of SARS-CoV-2 infection (RR = 0.71, 95% Credible Interval (CrI) = 0.61-0.82, τ = 0.34). Data for former smokers were inconclusive (RR = 1.03, 95% CrI = 0.95-1.11, τ = 0.17) but favoured there being no important association (4% probability of RR ≥1.1).
My understanding of the evidence last year was that smokers were less likely to be hospitalised with Covid but more likely to suffer severely when they did. That wouldn't be surprising given the impact of underlying health conditions in worsening outcomes. But even that now looks doubtful. The outlook for ex-smokers is worse than for never-smokers, but it's far from clear that current smokers do any worse.
Former compared with never smokers were at increased risk of hospitalisation (RR = 1.19, CrI = 1.1-1.29, τ = 0.13), greater disease severity (RR = 1.8, CrI = 1.27-2.55, τ = 0.46) and mortality (RR = 1.56, CrI = 1.23-2, τ = 0.43). Data for current smokers on hospitalisation, disease severity and mortality were inconclusive (RR = 1.1, 95% CrI = 0.99-1.21, τ = 0.15; RR 1.26, 95% CrI = 0.92-1.73, τ = 0.32; RR = 1.12, 95% CrI = 0.84-1.47, τ = 0.42, respectively) but favoured there being no important associations with hospitalisation and mortality (49% and 56% probability of RR ≥1.1, respectively) and a small but important association with disease severity (83% probability of RR ≥1.1).

I'm not sure how 'important' that last finding is given that the result is not statistically significant. Since current smokers don't have a higher rate of Covid mortality, perhaps not much.

Fair play to the UCL researchers for sticking with this when a lot of people in 'public health' would have run a mile. Not that many people are paying attention to these inconvenient facts. 

There is a surprisingly large amount of evidence on this stuff once you start looking. This study from New York didn't find smoking to be a factor in any Covid outcomes. This study from Turkey found that there were fewer smokers with COVID-19 than would be expected from the general smoking rate and that smokers did no worse than nonsmokers if they contracted it. This meta-analysis found smokers to be 82 per cent less likely to be hospitalised with COVID-19! 
This study of grocery workers in the USA found 21 symptomatic cases, only one of whom was a smoker. The authors say: 

Our finding of fewer current smokers with a positive SARS-CoV-2 assay result, while in agreement with recent epidemiological studies, contradicts common perception and clinical recommendation on risks and effects of cigarette smoking on lung health warranting further research investigations.

It sure does. All sorts of possible explanations have been advanced for this.
On the other hand, this study from Brazil found smokers to be three times as likely to be hospitalised with Covid. And yet it found them no more likely to die from Covid and - most unusually - nor did it find obese people being more likely to die from it.

See this post of mine from last year for more studies or, if you want a deep dive, go through the studies listed in the living meta-analysis.

'What about vaping?' you may ask. Most vapers are former smokers so there is an obvious confounding factor waiting to trip up the sloppy epidemiologist. There hasn't been much research in this area, but a study published last month gives vaping the all clear. 

There were no differences in diagnosed/suspected Covid-19 between never, current and ex-vapers... Among UK adults, self-reported diagnosed/suspected Covid-19 was not associated with vaping status.
Hurrah! And...

Bayes factors indicated there was sufficient evidence to rule out small negative (protective) associations between vaping status and diagnosed/suspected Covid-19.


In conclusion, there isn't any obvious justification for making smokers a priority group for the vaccine, although there is a stronger case for ex-smokers. The system in North Carolina says that anyone who has smoked more than 100 cigarettes in their lives can jump the queue. It is impossible for authorities to verify this, so I guess they're relying on North Carolinians being very honest.

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