Smokers at increased risk of severe COVID-19 according to a large population study

A new UK study available on the preprint server medRxiv* screened 2.4 million users of a symptom reporting application during the coronavirus disease (COVID-19) pandemic, revealing that smokers are at higher risk of increased symptom burden and more prone to hospitalization.

The COVID-19 pandemic, caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), remains a threat to the capacity of health systems worldwide, especially as communities are releasing social distancing measures introduced after the first wave of the pandemic.

A proportion of individuals with COVID-19 develop progressive lung involvement and respiratory failure, with potentially extensive systemic consequences and death. Groups at highest risk for complications are elderly and those with long-term medical conditions (including diabetes, cardiovascular diseases, and chronic lung conditions).

Tobacco smoking and COVID-19

It is already known that tobacco smoking represents a significant risk factor for both bacterial and viral infections of the respiratory tract. Smokers are five times as likely to get infected with the influenza virus, and twice as likely to develop pneumonia. Therefore, it may play a substantial role in complicating COVID-19 as well.

And indeed, there is ample evidence from case series that smoking is associated with more severe forms of the disease, a higher risk of ICU admission, as well as excess mortality in individuals with COVID-19 admitted to hospital; conversely, a potential protective effect of nicotine has been recently suggested by some studies.

In order to appraise the impact of current smoking more precisely, the researchers from Imperial College London, King's College London, and Zoe Global Ltd analyzed the data from a population COVID-19 symptom reporting app.

These scientists hypothesized that current smokers may be at increased risk of developing COVID-19 symptoms, and would also experience higher symptom burden. Smoking also causes vascular endothelial damage, which is a hallmark of severe SARS-CoV-2 infection.

Symptom study app yields a significant sample size

Adult individuals were invited to download the COVID Symptom Study App (advertised on various venues) for study purposes, resulting in a study population of UK residents who had provided baseline data (including anthropometrics, demographics, medical conditions, and smoking status) via the app between March 24 and April 23, 2020.

Study participants who reported feeling physically unwell were taken through a series of questions, including fourteen potential COVID-19 symptoms and any hospital attendance or admittance.

In order to explore differences in the COVID-19 course between smokers and non-smokers, the researchers used the number of reported COVID-19 symptoms as a proxy for disease severity, assuming that a higher symptom burden was suggestive of a more severe course of the disease.

Additionally, the association of subcutaneous adipose tissue expression of ACE2 (which is both the receptor for SARS-CoV-2 and a potential mediator of disease severity) with participants' smoking status was evaluated in a subset of 541 twins from the TwinsUK Registry, i.e., the largest cohort of community-dwelling adult twins in the UK.

Smoking as a significant risk factor

"The main finding of this study from a large prospective population cohort was that current smoking was associated with a substantially increased risk of developing symptoms suggestive of COVID-19", report study authors in their paper currently available on medRxiv preprint server.

Moreover, current smoking was linked to an increased symptom burden, suggesting an impact on disease severity, while smokers testing positive for SARS-CoV-2 were more likely to end up in hospitals.

Also, ACE2 expression in adipose tissue was found to be lower with further impact on disease severity mechanisms and increased risk of systemic disease in smokers with COVID-19.

And the final number of study participants was not negligible. More specifically, data were available on 2.4 million participants with a mean age of 43.6 years; 35% reported feeling unwell and entered at least one symptom in the app. There were a total of 63% female participants, while the overall smoking prevalence was 11%.

Implications for public health efforts

"Our results provide compelling evidence for an association between current smoking and individual risk from COVID-19, including symptom burden and risk of attending hospital", emphasize study authors.

These data suggest that smoking cessation should be taken into account as an indispensable element in tackling the COVID-19 pandemic. It seems that smoking increases both the likelihood of symptomatic disease ("classic" symptoms of cough, fever, and breathlessness) and the severity of disease (number of symptoms).

"Combined with this, a reduction in population smoking rates is likely to reduce the health system burden from other conditions that require hospitalization such as acute vascular events and exacerbations of lung disease, as well as improving resilience by reducing overall sickness absence among key-workers", say study authors.

In conclusion, smoking cessation efforts should be integrated into public health and health communication campaigns, but also within other efforts that are aiming to address critical issues around the ongoing COVID-19 pandemic.

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
Dr. Tomislav Meštrović

Written by

Dr. Tomislav Meštrović

Dr. Tomislav Meštrović is a medical doctor (MD) with a Ph.D. in biomedical and health sciences, specialist in the field of clinical microbiology, and an Assistant Professor at Croatia's youngest university - University North. In addition to his interest in clinical, research and lecturing activities, his immense passion for medical writing and scientific communication goes back to his student days. He enjoys contributing back to the community. In his spare time, Tomislav is a movie buff and an avid traveler.

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