Updated 16 December 2022 after a Facebook 'fact check'.
Studies which find smokers to be significantly less likely to get COVID-19 continue to be published, even if they are ignored by the media. There are so many of them that it's difficult to keep up.
The indefatigable Phil has found over 4,000 studies in which smoking status is recorded (see this heroic thread), although the vast majority do not look at smoking specifically as a risk factor, so we can only compare smoking rates among Covid patients/cases with the smoking rate of the general population. Most of them seem to show smokers significantly under-represented among Covid patients/cases, but it is a crude method and is not considered conclusive (interestingly, however, the over-representation of fat people in Covid wards is considered sufficient evidence to mark obesity as a risk factor).
It is better to focus on studies which use epidemiological methods and adjust for other factors. There are quite a few of them now, including a growing number looking at antibodies in unvaccinated populations to see who has had Covid. These studies help address sampling bias by testing whole populations; they do not rely on people's willingness to come forward or on their ability to identify their symptoms.
There is an ongoing meta-analysis, but it hasn't been updated since March (UPDATE: the seventh and final version has been published in Addiction). The main conclusion is that smokers are less likely to be infected and ex-smokers are more likely to be hospitalised if they catch the virus. The early finding that smokers are less likely to be hospitalised with Covid seems to be explained by them being less likely to be infected in the first place.
What follows is not a comprehensive list. These are just the studies that have crossed my radar. Please let me know about other evidence in the comments, including any studies that contradict the general conclusion that smokers are less likely to be infected with the virus. All of these studies have been published and peer-reviewed.
1. Lusignan et al. This British study published in the Lancet found that "active smoking was linked with decreased odds of a positive test result" with an odds ratio of 0.49 (0.34–0.71).
2. Hopkinson et al. This British study found that smokers were 27% less likely to test positive for COVID-19 although you have to look carefully to find the evidence because the lead author is the chairman on Action on Smoking and Health and he buries it as much as he can. The odds ratio is 0.73 (0.65-0.81).
3. Chen et al. This UK study didn't find an association between smoking or vaping and Covid infection but did find an association between vaping and smoking (i.e. dual use) and Covid infection (1.10 (0.89-1.36)).
4. Williamson et al. Meanwhile this British study published in Nature looked at the likelihood of dying from COVID-19 and found that smokers were slightly more at risk, or slightly less at risk, or neither, depending on how the figures were adjusted.
5. Rentsch et al. This study from the USA looked at 3,789 US military veterans aged between 54 and 75 who were tested for COVID-19, of whom 585 tested positive. Smokers were 55% less likely to test positive, with an odds ratio of 0.45 (0.35-0.57).
6. Ghinai et al. Studyy of homeless people in Chicago which found that smokers were less likely to be infected (0.71; 95% CI, 0.60-0.85).
7. Lan et al. Study from the USA finds smokers 90% less likely to test positive. Odds ratio: 0.1 (0.01-0.8).
8. Gu et al. American study found: 'Being a current smoker (self-reported in the latest EHR encounter) was
associated with a reduced chance of having positive test results (OR,
0.31 [95% CI, 0.20-0.48]; P < .001).'
9. Vila-Córcoles et al. Study from Spain finds smokers 57% less likely to be infected (0.43 (0.25-0.74)).
10. Miyara et al. French study finds that daily smokers are 76% less likely to be infected with COVID-19 (after adjusting for age and sex). Odds ratio for inpatients: 0.24 (0.14-0.40). For outpatients: 0.24 (0.12-0.48).
11. Kharroubi et al. Study of workers in a public health laboratory in Tunisia found that "tobacco smokers had a lower risk of infection (AOR = 0.54 [0.29–0.97])".
12. Paleiron et al. Interesting study of a Covid outbreak on a French Navy aircraft carrier. 76% of the crew members got the disease but smokers were 36% less likely to get it (0.36; 95% CI, 0.49-0.81). There was also a "trend towards a lower risk among e-cigarettes users". Risk of infection was even lower for heavy smokers than for those who smoked less than 10 cigarettes a day.
13. Prinelli et al. Web-based study from Italy finds smokers were half as likely to get COVID-19 than nonsmokers. There was a dose-response relationship, with heavy smokers 62% less likely.
14. Lee et al. Study of 4,137 Covid patients in South Korea finds smokers 67% less likely to be infected (0.33, CI = 0.28–0.38).
15. Prats-Urbine et al. This study used UK Biobank data and found that current/former smokers are 45% more likely to test positive for SARS-CoV-2 (1.45 (95% CI 1.19 to 1.79)).
16. Fernandez-Fuertes et al. Study from Spain involving HIV patients. It found that 'active tobacco smoking was the only factor independently associated with lower risk of SARS-Cov-2 infection [Incidence rate ratio: 0.29 (95% CI 0.16–0.55)'.
17. Candel et al. Study of 10,614 nurses in Madrid finds smokers 77% less likely to have had COVID-19 (0.23 (0.20-0.27)). The finding is not mentioned in the abstract.
18. Cummins et al. UK study finds smokers are 27% less likely to be hospitalised with COVID-19 (0.63 (0.44-0.88)). The authors do not mention this finding in the text.
19. Wratil et al. Study of German health workers finds smokers are around half as likely to have antibodies for SARS-CoV-2.
20. Hull et al. Study of ethnic minorities in London finds that smokers are 40% less likely to be infected (0.60 (0.56 to 0.63)).
21. Barchuk et al. Study from Russia finds smokers are 54% less likely to have had Covid (as measured by antibody tests).
22. Jose et al. American study finds smokers are 57% less likely to be diagnosed with Covid. Vapers were no more and no less likely to get the virus than nonsmokers.
23. Holuka et al. Study from Luxembourg finds smokers are 50% less likely to have had Covid.
24. Iruretagoyena et al. Study of healthcare workers in Chile finds smokers are 62% less likely to have had Covid.
25. Li et al. Study in the Journal of the American Medical Association finds smokers to be 40% less likely to have had Covid.
26. Muro et al. Study from Spain finds smokers are 43% less likely to have had Covid.
27. Clift et al. Study from the UK finds no inverse relationship between having a genetic liability for smoking and SARS-CoV-2 infection. Naturally, this one got plenty of media attention. I wrote about it here. When using conventional epidemiology, the study found that "heavy smoking (≥20 cigarettes/day) was associated with a reduced risk of confirmed infection when adjusting for age and sex (OR 0.50, 95% CI 0.29 to 0.89)".
28. Maraqa et al. Study from Palestine finds that smokers are half as likely to have been infected with SARS-CoV-2 (0.47 (0.31-0.72)).
29. Duszynski et al. Study of 8,124 people in Indiana, USA, finds that smokers - but not vapers - are half as likely to get COVID-19 (0.49: 0.32-0.74).
30. Pagani et al. Italian study finds that smokers are 58% less likely to get SARS-CoV-2 (OR 0.42: 0.29–0.60). Unusually, it also finds that former smokers are at reduced risk (OR 0.49: 0.33–0.75).
31. Mutevedski et al. Study from South Africa finds that daily smokers are half as likely to be infected with SARS-CoV-2 as non-smokers (0.50: 0.38-0.67).
32. Rosoff et al. Mendelian Randomisation study finds that smoking is not associated with lower risk of infection. Although the study is titled 'Smoking is significantly associated with increased risk of COVID-19 and other respiratory infections', the text makes it clear that the association is only with people with a "genetic liability for smoking". Strangely, it is only MR studies that find such an association with 'smoking' (see 27 above).
33. Egede et al. Study of 31,549 people tested in Wisconsin, USA last year finds smokers 66% less likely to have COVID-19 (0.34; significant at p < 0.001). Former smokers were 21% less likely. Those who did were more likely to be hospitalised. Smokers with Covid were no more likely to die of it than nonsmokers, but former smokers were.
34. Salerno et al. Study of nearly half a million dialysis patients in the USA finds that tobacco use is associated with a 16% reduction in risk of COVID-19 diagnosis (OR 0.84: 0.81-0.87).
35. Vallarta-Robledo et al. Study from Switzerland finds smokers are 56% less likely to be infected with SARS-CoV-2 after adjusting for potentially important confounders (OR 0.44 (0.35-0.77).
36. Modenese et al. Study from Italy finds smokers 30% less likely to be diagnosed with SARS-CoV-2 (0.70 (0.54–0.91)).
37. Ozcifci et al. Study of Behçet's syndrome patients finds smokers are 34% less likely to be diagnosed with SARS-CoV-2 (0.66 (0.47-0.93)).
38. Gashi et al. Study of municipal workers in Kosovo finds smokers are half as likely to have had SARS-CoV-2 (0.52, 0.28-0.97).
39. Siller et al. Study from Austria finds that smokers are 61% less likely to have had SARS-CoV-2 than nonsmokers (0.39 (0.27-0.56)).
40. Madhi et al. Study in the New England Journal of Medicine looking at Gauteng, South Africa finds that daily smoking is associated with a 14% reduction in infection risk (0.86 (0.82-0.90)). Note that this is the first study of this kind involving Omicron.
41. Verburgh et al. Study from the Netherlands finds that smoking is associated with a 58% reduction in infection risk (0.42 (0.18-0.99).
42. Coppeta et al. Study of healthcare workers in Italy finds that smokers are more likely to get SARS-CoV-2 than nonsmokers (1.6 (1.1–2.4).
43. Zuñiga et al. Study from Chile find that tobacco consumption is associated with lower risk of SARS-CoV-2 infection, with effects ranging from 0.62 to 0.85.
44. Constantino et al. French study of patients with rheumatic diseases finds that smokers are much less likely to test positive for COVID-19 (0.18 (0.03–0.61)).
45. Coric et al. Study from Serbia finds that smokers are 71% less likely to have COVID-19 (0.29 (0.15–0.55)).
46. Morino et al. Seroprevalence study from Spain finds that "users who smoked had a lower prevalence of [SARS-CoV-2] antibodies than non-smoking users, with a PR = 0.60 (0.39, 0.92)".
47. Warszawski et al. Study from France finds a "strong inverse correlation between the presence of SARS-CoV-2 antibodies and smoking... as in other studies". Nonsmokers were about twice as likely to test positive.
48. Sandal et al. Study of Turkish healthcare workers finds smokers 62 per cent less likely to get COVID-19 (0.38 (0.23-0.63)).
49. Rojanaworarit et al. Study from New York finds that current smokers are 40 per cent less likely to get COVID-19 (0.60 (0.49-0.74)). The authors note that "consistently low prevalences of smoking have been observed among patients hospitalized with COVID-19 in several studies. In an outpatient setting, smokers have been shown to have lower odds of SARS-CoV-2 test positive results".
50. Fuente et al. Study from Spain concludes: "Smoking was the only factor associated with a decreased risk of SARS-CoV2 infection of any grade [odds ratio (OR) 0.491; 95% CI 0.275–0.878; p = 0.017]."
51. Halili et al. Study from Kosovo finds lower rate of infection among smokers (0.56 (0.31, 0.99)), but the association is no longer statistically significant after adjustments (0.55 (0.31-1.01).
52. Vial et al. Seroprevalence study from Chile finds that "Smoking showed a negative association with SARS-CoV-2 infection." The authors note that "the protective effect of tobacco use has also been evidenced in European countries, reporting a significant negative association between smoking prevalence and the prevalence of COVID-19 across the 38 European nations after controlling for confounding factors (p = 0.001). Some authors propose that nicotine prevents infection by competing with the virus with the ACE2 receptor. In this sense, epidemiological and experimental evidence has been found." Indeed.
53. Richard et al. Study from Switzerland finds that smokers are 64% less likely to test positive for SARS-CoV-2 (0.36, 95% CI 0.23–0.55).
54. Study from the USA finds: "Smoking was associated with a lower risk of
SARS-CoV-2 infection with a RR of 0.25 (95% CI: 0.06, 0.99)."
55. Cherif et al. Seroprevalence study from Tunisia finds that "current tobacco smokers had lower SARS-CoV-2 seroprevalence than non-smokers (OR = 0.5 (0.4–0.6)".
56. Haller et al. Study from Switzerland looking primarily at face masks finds that the only factor that has a statistically significant negative association with COVID-19 is active smoking.
57. Kahlert et al. Another study from Switzerland finds smokers to be 60% less likely to get COVID-19 than nonsmokers (aOR 0.4, 95% CI 0.2–0.7).
58. Gornyk et al. Seroprevalence study from Germany finds smokers were 60% less likely to have had Covid (0.4 (0.3-0.7)).
59. Shafrir et al. Study from Israel finds smokers are 46% more likely to test positive (and are no more likely to suffer from severe disease if they do).
60. Tang et al. Study from Canada finds smokers are less likely to be infected but the difference is not statistically significant.
61. Villarroel et al. Study from Bolivia finds that "tobacco smoking was negatively associated with seropositivity (25.4%, OR: 0.420; 95% CI 0.24–0.74)".
62. Yeung et al. Another Mendelian Randomisation study (see 27 and 32 above) which found that people who have a genetic predisposition to smoke (but who do not necessarily smoke) are 19% more likely to get COVID-19 (1.19 (1.11-1.27)). My commentary on this method is here.
63. Gao et al. Massive study of nearly 8 million people in the UK finds smokers to be less likely to be hospitalised with COVID-19 and much less likely to enter ICU. Former smokers were more likely.
64. Basso et al. Study from Italy looking at breakthrough infections in vaccinated healthcare workers finds infections are 20% less common among smokers.
65. Badial et al. Study from Switzerland finds smokers 70% less likely to get COVID-19 (0.3 (0.2-0.4)).
66. Schonfield et al. Study from Argentina finds that "those with a history of smoking showed the lowest seroprevalence, a finding also reported in an Argentine case study and in other epidemiological studies."
67. Zhong et al. Study from China finds that "smokers were less likely to develop COVID-19 (OR = 0.224, 95% CI = 0.084–0.592 p = 0.003)." This is a risk reduction of 78%.
68. Erber et al. Study of hospital workers in Germany finds smokers are 48% less likely to develop COVID-19 (OR=0.52 (0.26–0.94)) (see table 2).
69. Shanaube et al. Study from Zambia finds that daily smokers are 53% less likely to have had COVID-19 (0.25-0.90).
70. West et al. Swiss study of health and social care workers finds that smokers were much less likely to have had COVID-19 (OR 0.26; 95% CI 0.097–0.696).
71. Nguyen et al. US study finds that "Smoking history, adjusting for medical conditions, appears to be protective for COVID-19 outcomes for <65 year olds". These outcomes include not only infection, but hospitalisation, ICU admission and (for those under 50) death. Above the age of 65, these outcomes worsened for smokers, although there was no increased risk of infection.
72. Djukic et al. Study from Serbia finds that "smoking was associated with a decreased risk of COVID-19 development (OR = 0.22, 95%CI: 0.14–0.35, p < 0.001)."
73. Young-Wolff et al. Massive US study involving 2,427,293 people in California finds that smoking was associated with lower risk of infection, hospitalisation, ICU admission and death from COVID-19. "Current smoking was associated with lower adjusted rates of SARS-CoV-2
infection (aHR=0.64 95%CI: 0.61-0.67), COVID-19-related hospitalization
(aHR=0.48 95%CI:0.40-0.58), ICU admission (aHR=0.62 95%CI:0.42-0.87),
and death (aHR=0.52 95%CI:0.27-0.89) than never-smoking." In other words, smokers were a third less likely to catch it and half as likely to die from it.
74. Kleynhans et al. Study from South Africa finds that nonsmokers are nearly four times more likely to get COVID-19 (OR=3.9).
75. Tomaselli et al. Seroprevalence study from Italy finds that smokers are 77% less likely to have had COVID-19 (0.23 (0.12-0.45)). Smoking status was verified by testing for cotinine.
76. Peremiquel-Trillas et al. Seroprevalence study from Spain finds smokers are 62% less likely to have had COVID-19 (0.38 (0.18-0.79)).
77. Hausfater et al. Study of healthcare workers in France finds that "current smoking was associated with reduced risk (0.36 [0.21; 0.63])".
78. Nernani et al. Study of long term patients in psychiatric hospitals in New York finds that smokers are 25% less likely to get COVID-19 (0.75 (0.60-0.90)). The authors do not mention this finding in the text.
79. Ramirez et al. Seroprevalence study from Switzerland finds that smokers are 19% less likely to have had COVID-19, but the difference is not statistically significant (0.81 (0.53-1.22)). This finding is tucked away in the supplementary material (p. 6).
80. Wessendorff et al. Study from Germany finds that smokers are 68% less likely to get COVID-19: "we observed a reduced risk of infection (OR 0.32, 95% CI 0.12 to 0.81) even after adjustment for ‘time spent outside’".
81. El-Ghitany et al. Seroprevalence study from Egypt finds that ("surprisingly") smokers were 81% less likely to have had COVID-19.
82. Zar et al. Study from South Africa finds smokers 57% less likely to have COVID-19 (0.43 (0.28-0.66)).
83. Huynen et al. Study from Belgium finds no statistically significant difference between seropositivity of smokers and nonsmokers (adjusted OR=0·43, 95% CI: 0·27; 0·69, p<0.001).
84. Wei et al. Study of essential workers in the USA finds that smokers were 73% less likely to be infected (0.27 (0.12-0.61)).
85. Abdi et al. Study of HIV patients in South Africa finds that smokers are 43% less likely to have had COVID-19 (0.57 (0.36, 0.90)).
86. Leclercq et al. Study from Belgium finds that "being a smoker (OR 0.36 (95% CI 0.18–0.72)) was negatively associated with having... antibodies".
87. Javed et al. Study from Pakistan finds that smokers were 45% less likely to have been infected with SARS-CoV-2 (0.550 (0.424–0.712)).
88. Weber et al. Study of healthcare workers in France finds that smokers are 40% less likely to have had COVID-19 (0.6 (0.4-0.9)).
89. Rahman et al. Study from Bangladesh finds that "smoking (OR 0.70; 95% CI 0.55 to 0.89) was associated with lower risk of seropositivity".
90. Laval et al. Study of an outbreak of COVID-19 on (another) French aircraft carrier finds that "smoking was associated with reduced infection (adjusted odds ratio (ORa): 0.57; 95% confidence interval (CI): 0.44–0.73)".
91. Colaneri et al. Study from Italy finds that "being a current smoker was negatively associated" with SARS-CoV-2 infection (0.43 (0.23–0.80)).
92. Dadgari et al. Study from Iran finds no association either way between smoking and COVID-19 (1.08 (0.84-1.35)).
93. Mostafa et al. Study from Egypt finds positive associations with both former and current smoking which disappear after adjustment for confounding factors. Current smokers: 0.65 (0.38-1.09).
94. Dörr et al. Study from Switzerland finds that smokers are 32% less likely to be infected, but neglects to mention it in the text (OR = 0.68 (0.52-88)).
95. Alasmari et al. Seroprevalence study from Saudi Arabia finds that smokers are half as likely to have been infected (OR = 0.48 (0.29–0.78)).
95. Ferrara et al. Study from Italy finds no statistically significant reduction in infection risk among smokers (OR = 0.61 (0.35-1.06)).
96. Örtqvist et al. Study of pregnant women in Sweden and Norway finds that: "Smoking was associated with decreased odds of test-positivity in women under non-universal testing (aOR 0.46, 95% CI 0.31–0.70), although this association was not observed in analyses of women under universal testing (aOR 1.06, 95% CI 0.72–1.56)".
97. Dev et al. Study from India finds an association between smoking and infection but this disappears after adjustment.
98. Adorni et al. Italian web-based study finds that 'current smoking (aOR 0.66, 95% CI 0.50-0.87) was associated with decreased odds' of having COVID-19.
99. El Moussaoui et al. Study of healthcare workers in Belgium finds that fewer smokers tested positive for SARS-CoV-2 but the difference was not statistically significant (0.57 (0.29–1.1)).
100. Zar et al. Study of mothers in South Africa finds that "current smoking was associated with seronegativity (adjusted OR=0·43, 95% CI: 0·27; 0·69, p<0.001)", i.e. the smokers were 57% less likely to have had Covid.
101. Whittemore et al. Study from New York finds that smokers/vapers were 17% less likely to test positive for COVID-19 (0.83 (0.80-0.87)). The finding appears in Table 2 but is not mentioned in the text.
102. Green et al. Study from Israel involving patients with bronchial asthma concludes: 'A significantly higher proportion of smokers was observed in the COVID-19–negative group than in the COVID-19–positive group (4734 [13.45%] vs 103 [4.55%]; P < .001).'
103. Gu et al. Study from Michigan finds that smokers were much less likely to test positive for COVID-19 (0.31 (0.20-0.48)).
104. Lombardi et al. Seroprevalence survey of Italian healthcare workers finds smokers were 59% less likely to test positive (0.41 (0.27-0.61)).
105. Holuka et al. Study from Luxembourg finds that smokers are half as likely to test positive for COVID-19 (0.50 (0.30–0.83)).
There is also an ecological study that compared smoking rates and COVID-19 mortality rates in 38 European countries and found an inverse relationship.
There is also a large study from the USA which doesn't look at prevalence but, very interestingly, finds that smokers with COVID-19 who are given nicotine in hospital have a better survival rate and COVID-19 vaccines have more effect on smokers than on nonsmokers.
As mentioned above, there are numerous studies comparing the number of smokers in hospital with COVID-19 with the number of smokers in the general population. They mostly produce a similar finding to the majority of studies listed above. Some of these studies are well done - like this one from France - but I don't consider the methodology to be quite rigorous enough to justify inclusion in the full list. (Note that a simple comparison between the % of patients and the % of the general population was enough for people in public health to acknowledge that obese people were over-represented while the under-representation of smokers continues to be ignored.)
Finally, the UK's weekly ONS infection survey consistently shows that smokers about half as likely to catch the virus as nonsmokers. This is almost never remarked upon.
As for why this finding keeps emerging, opinions differ. One suggestion is that smokers spend more time outside, but this sounds rather like a cope given the size of the effect. Smokers do not spend all that much more time outside and it seems unlikely that this would give frontline healthcare workers, for example, significantly more protection.
It may be due to the nicotine, although it is notable that study 23 and study 29 did not find any benefits from vaping. This study discusses some possible biological mechanisms.
Various biological mechanisms have been proposed which are beyond my level of scientific understanding. In any case, the consistency of these findings is very interesting and the lack of interest shown towards them by the public health establishment is revealing.
Meanwhile, the legacy 'public health' research community is annoyed that people are freely sharing such information on Twitter.
PS. This is too complicated for me to understand but it offers a scientific explanation for why smokers are less likely to get SARS-CoV-2.
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