Does smoking improve the chance of COVID-19 hospitalization?

A study conducted by UK researchers found that patients hospitalized with Coronavirus Disease 2019 (COVID-19) are less likely to be currently smoking than patients who were infected with other types of viral infections in the past year (2019) Respiratory tract.

The researchers also found that the prevalence of smoking in patients hospitalized with COVID-19 was significantly lower than expected in the local general population.

Worryingly, the team reports that smoking status data was poorly recorded, with information in electronic health records (EHRs) differing significantly from information in medical notes.

Robin Bailey of the London School of Hygiene and Tropical Medicine and colleagues say this poor record is a problem, especially for studies that rely solely on EHRs as a source of information on smoking status.

A pre-print version of the paper is available on the medRxiv * server while the article is being peer-reviewed.

It is uncertain whether smoking increases the risk of COVID-19 hospitalization

COVID-19 is a respiratory disease caused by the highly infectious severe acute respiratory syndrome Coronavirus 2 (SARS-CoV-2).

To infect cells, SARS-CoV-2 uses a surface virus protein to bind to the angiotensin converting enzyme 2 (ACE2) of the human receptor and fuse with the host cell membrane.

Some studies have shown that ACE2 is expressed in higher concentrations in the respiratory tract of smokers, possibly increasing the chances of contracting the virus. However, other studies have shown that smoking actually decreases the expression of ACE2.

A review of 279 studies found that current smokers were at lower risk of testing positive for SARS-CoV-2 than never-smokers, while ex-smokers were at higher risk of serious illness and hospitalization than never-smokers.

Most of these studies, however, have been limited by the lack of appropriate control groups, poor tracking of smoking status, and insufficient adaptation to potential confounders, say Bailey and the team. Many studies also used routine EHRs to obtain data on an individual level on demographics, comorbidities, and smoking status.

"Previous research suggests that smoking status data obtained through EHRs is incomplete or completely inaccurate," the researchers write.

Because hospital populations differ from the general population in terms of age and gender, comparisons of current and past smoking prevalence in hospital and non-hospital populations are likely to be limited.

"There is therefore an urgent need for alternative study designs with relevant comparison groups (e.g. populations treated in hospital with a similar type of viral respiratory infection) and for adaptations for covariates to relate smoking status to the results of COVID -19 disease better to understand, "he tells The Team.

What did the researchers do?

In a retrospective case-control observational study conducted at an NHS hospital in London, the team examined the association between previous and current smoking and hospitalizations for COVID-19 compared to hospitalizations for other respiratory virus infections a year earlier.

Authorization flowchart for controls (left side) and cases (right side).

Authorization flowchart for controls (left side) and cases (right side).

The researchers also obtained data on smoking prevalence in London from the representative annual population survey.

The data were analyzed using logistic regression analysis to adjust for age, gender, socio-economic position and comorbidities.

The study included 446 hospitalized COVID-19 patients (cases) who had data collected between March 1, 2020 and August 26, 2020. These patients were compared to 211 patients enrolled with other respiratory viral infections (controls) for whom data were collected between January 1, 2019 and January 1, 2019 to December 31, 2019.

What did the study find out?

Hospitalized COVID-19 patients were 45% less likely to be smokers than those admitted with other respiratory viral infections the previous year.

No significant difference in the likelihood of being a former smoker was observed between the two groups.

The prevalence of current smoking in the cases was 9.4% compared to 12.9%, significantly lower than was to be expected based on the age- and gender-adjusted local smoking prevalence.

The low prevalence of current smoking in cases appeared to be due to the fact that current smokers were underrepresented among 45 to 59 year olds. In all other age groups (30-44 years, 60-74 years and 75 years plus) the prevalence was similar to the expected London prevalence.

"It should be noted that cases between the ages of 45 and 59 are more likely to occur in less-favored areas than in control areas," says Bailey and colleagues. "As smoking status is strongly linked to socio-economic position, this may partly explain the lower prevalence of smoking in this age group."

Smoking status was poorly recorded

Smoking status was poorly recorded, with the status recorded in the summary EHRs differing significantly from that recorded in medical notes.

This poor record was more pronounced in controls than in cases, which the researchers believe can probably be explained by observing discharged COVID-19 patients visiting a follow-up clinic where they were specifically asked about their smoking status.

"The observed discrepancy between smoking status recorded in summary EHRs and concurrent medical notes is a problem, particularly in studies that rely solely on EHRs as a source of information on smoking status," warn the researchers.

Representative longitudinal studies at the population level in multiple locations are required

"To the best of our knowledge, this is one of the few studies specifically designed to examine the relationship between smoking status and hospitalization with COVID-19," the team writes.

Bailey and colleagues say that a better understanding of the role of smoking as a risk factor for COVID-19 hospitalization requires longitudinal studies at a representative population level in multiple locations.

"Targeted tracking of exact smoking status is needed to characterize the role of smoking in COVID-19 disease outcomes," they add.

* Important NOTE

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

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