but quitting smoking is likely to lower the risk of developing more severe or fatal cases of the infection, according to research from several recent papers.
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Interest in how tobacco use affects COVID-19 infection rates stems from research showing that men at the epicenter of the outbreak in China having a higher early mortality rate. Early reports from China showed a case fatality rate of 4.7% for men, compared with 2.8% for women, according to the World Health Organization. The virus that causes COVID-19, severe acute respiratory syndrome coronavirus 2, is suspected to enter a cell using the ACE2 receptor. Since smoking up-regulates this receptor, one popular theory is that smoking can increase the risk of COVID-19 or exacerbate symptoms of an existing infection (Eur Respir J. 2020 Apr 8. doi: 10.1183/13993003.00688-2020). In China, about half of men are active smokers, compared with 2.7% of women (Transl Lung Cancer Res. 2019;8[Suppl 1]:S21-30), so this association would explain the severe cases and increased mortality in this group. In response to potential risk for public health, the World Health Organization, Centers for Disease Control and Prevention, the Attorney General of Massachusetts, and other organizations have warned that smoking may increase one’s risk of transmitting and developing COVID-19 or may worsen the infection.
“While it is easy to jump to the conclusion that more ACE2 means more susceptibility to severe infection, there is no evidence to support this,” Brandon Michael Henry, MD, of the cardiac intensive care unit and the Heart Institute at Cincinnati Children’s Hospital Medical Center, said in an interview. “Moreover, some would argue (including myself) that increased ACE2 may in fact be protective, as ACE2 decreases the levels of angiotensin-2 which likely plays a significant role in the pathophysiology of ARDS.”
Some researchers have examined the limited evidence of smoking on COVID-19 risk and come to preliminary conclusions. In a letter to the editor recently published in the European Journal of Internal Medicine, Dr. Henry and Giuseppe Lippi, MD, of the section of clinical biochemistry in the department of neuroscience, biomedicine, and movement at the University of Verona (Italy), performed a meta-analysis of papers examining smoking and COVID-19 up to March 9, 2020 and identified five articles with 1,399 COVID-19 cases (Eur J Intern Med. 2020 Mar 16. doi: 10.1016/j.ejim.2020.03.014).
“Given the fact that COVID-19 is a primarily respiratory illness, smoking was one of first risk factors we examined,” Dr. Henry said.
They noted that a study by Liu et al. in the Chinese Medical Journal was the only paper that showed a significant association between smoking status and COVID-19 case severity (Chin Med J [Engl]. 2020 Feb 28. doi: 10.1097/CM9.0000000000000775), while the four other studies showed no significant association. The pooled data of all five studies showed an association that was not statistically significant (odds ratio, 1.69; 95% confidence interval, 0.41-6.92; P = .254). When Dr. Lippi and Dr. Henry performed the analysis again after removing a paper by Guan et al. (N Engl J Med. 2020 Feb 28. doi: 10.1056/NEJMoa2002032) comprising 89.5% of patients in the pooled analysis, there was no significant association (OR, 4.35; 95% CI, 0.86-21.86; P = .129).
Constantine I. Vardavas, MD, FCCP, of the department of oral health policy and epidemiology at Harvard School of Dental Medicine, Boston, and Katerina Nikitara, of the University of Crete in Heraklion, Greece, also published a systematic review in Tobacco Induced Diseases of five studies evaluating smoking and COVID-19 (Tob Induc Dis. 2020. doi: 10.18332/tid/119324). Of the studies chosen for the review, four were shared with the paper by Dr. Lippi and Dr. Henry. They found “a higher percentage of smokers” made up severe COVID-19 cases, but acknowledged the majority of these were from the largest study by Guan et al. Overall, they calculated smokers carried a risk ratio of 1.4 (95% CI, 0.98-2.00) for developing severe COVID-19 symptoms, and were over twice as likely to be admitted to an ICU, require a mechanical ventilator, or die from COVID-19, compared with patients who did not smoke (RR, 2.4; 95% CI, 1.43-4.04).
“Although further research is warranted as the weight of the evidence increases, with the limited available data, and although the above results are unadjusted for other factors that may impact disease progression, smoking is most likely associated with the negative progression and adverse outcomes of COVID-19,” Dr. Vardavas and Ms. Nikitara concluded.
However, the association between smoking and severe disease was not significant, and it is not immediately clear how the analysis was performed based on the details in the editorial. “Both of our reports were limited by a lack of data adjusted for age, sex, and comorbidities which may influence any analysis on smoking,” Dr. Henry said.
Some researchers have proposed collecting information on smoking status and conducting further research on whether vaping devices like e-cigarettes also impact COVID-19 cases. An editorial by Samuel Brake and colleagues published in the Journal of Clinical Medicine proposed the ACE2-receptor binding site as an area of interest for COVID-19 and as a potential therapeutic target (J Clin Med. 2020 Mar 20. doi: 10.3390/jcm9030841).
Ultimately, whether smoking itself is associated with COVID-19 is still an open question. Nonetheless, encouraging patients to quit smoking should be a priority because long-term sequelae of smoking have been linked to worsened or fatal COVID-19 cases, said Dr. Henry.
“There is a lack of definitive data on smoking to date. Nonetheless, we do know that many illnesses associated with smoking, such as [chronic obstructive pulmonary disease, hypertension, and heart disease are all strong risk factors for severe and fatal COVID-19,” he said. “Thus, absolutely we should encourage the public to quit smoking, especially for older individuals and those with comorbidities.”
The papers by Lippi et al., Vardavas et al., and Brake et al. had no funding source, and the authors reported no relevant conflicts of interest.