Study shows lower SARS-CoV-2 infection rates in smokers

Coronavirus disease 2019 (COVID-19) primarily infects the lungs, later spreading through the bloodstream. The worst prognoses are mostly seen in ‘at-risk’ groups, such as the immunocompromised, elderly, or those suffering from respiratory conditions.

At first glance, one might expect smokers to be more likely to contract the disease than non-smokers, as smokers have higher levels of respiratory disease, are more likely to be taking immune-suppressing drugs for cancer treatment, and have already damaged lungs.

Early data seemed to support this, showing smokers in China as overrepresented among severe cases of COVID-19. However, little data was gathered on mild and asymptomatic cases, and more recent studies have shown conflicting results, even suggesting that smoking can offer a certain level of protection against contracting the disease.

Researchers from Indiana University have been investigating transmission to those who consume nicotine in different manners as well as self-reported health status. Their work is published in the journal BMC Public Health.

The researchers pooled data from two state-wide testing initiatives that were part of seroprevalence studies conducted in April and June 2020. Participants were tested for active infection and antibodies against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). In each study, participants were randomly selected from tax records, and then a non-random group of racial and ethnic minorities were included to reduce under-representation in testing efforts.

All individuals were asked to report their demographic, health status, and primary nicotine and tobacco habits. Tobacco habits were split into multiple categories: cigarettes, chewing tobacco, and vaping, and individuals were asked to categorize their consumption as every day, some days, or not at all.

Testing for active SARS-CoV-2 presence was completed by taking nasopharyngeal swabs and using RT-PCR testing, while 2-3mL of blood was tested for antibody presence using a chemiluminescent microparticle immunoassay. Data gathered was analyzed using combined samples from both waves, with a bivariate logistic regression model controlling for age, sex, ethnicity, random/non-random status, and nicotine use.

A total sample size of 8,214 individuals was collected, with over 1,300 active tobacco users. 11% of these smoked cigarettes every day, ~3% smoked cigarettes on some days. Chewing tobacco was less common, showing at 1.4% and 0.9%, respectively. Vaping showed similar results at 1.3% daily and 1.5% some days. For self-reporting of health, 16% qualified themselves as ‘excellent’, 38% as ‘very good’ and 35% as ‘good’.

The researchers found that individuals who smoked cigarettes regularly were more likely to be protected from COVID-19, showing lower current infection as well as previous infection than non-smokers. Vaping/e-cigarettes showed no association, and chewing tobacco was positively associated with infection.

These results are backed up by previous studies showing decreased transmission rates to smokers. Those that reported their health as ‘excellent’ showed reduced positivity rates compared to ‘good’ and. ’fair’

The authors suggest that the reduced risk of transmission that smoking daily seems to provide is likely due to the tendency of nicotine to bind with the angiotensin-converting enzyme 2 (ACE2) receptor. This receptor is a key target for SARS-CoV-2; the S1 subunit of the spike protein contains a receptor-binding domain that binds to ACE2 in order to allow viral cell entry.

Thus, nicotine may bind to these, reducing the number of sites available to SARS-CoV-2 and preventing infection. However, smokers who have already contracted more severe respiratory diseases are at much greater risk if they do contract the disease, which is why the initial research from China showed inflated figures of smokers suffering from COVID-19 that required hospitalization.

The authors highlight that their study did not include institutionalized individuals, removing the bias in many other studies that showed smokers at increased risk. However, the authors note that the decreased transmission risk they identified in their research is only valid for asymptomatic and mildly symptomatic coronavirus infections.

In recent years, electronic/vaping equipment has dramatically increased in popularity in Indiana, whose smoke rates are higher than in the US as a whole. These results could be vital for shaping public health policy. In order to effectively target restrictions and prevent the spread of COVID-19 to those at greatest risk, lawmakers and public health officials must understand the factors that affect transmission, as well as the likelihood that COVID-19 will escalate to more severe prognoses that may require hospitalization.

With the rising threat of variants of concern such as the Delta strain, this information is more valuable than ever.

Journal reference:
Sam Hancock

Written by

Sam Hancock

Sam completed his MSci in Genetics at the University of Nottingham in 2019, fuelled initially by an interest in genetic ageing. As part of his degree, he also investigated the role of rnh genes in originless replication in archaea.

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