A new study by researchers at the UC Davis Comprehensive Cancer Center shows that current smokers have a 12% increased risk of laboratory-confirmed viral infection and a 48% increased risk of being diagnosed with respiratory illnesses. These results do not vary by type of virus, including a coronavirus.
In combination with past findings, current findings published today in the Nicotine and Tobacco Research review support urgent recommendations to increase tobacco control efforts to address COVID-19.
“Previous research has shown that smoking increases the risk of COVID-19 disease severity, but the risk of infection was less clear,” said Melanie Dove, a UC Davis tobacco researcher and lead author of the study. “The results of our study show that smokers have an increased risk of viral infection, including coronavirus and respiratory disease.”
Study results
The researchers reanalyzed data from the British Cold Study (BCS), a challenge study from 1986 to 1989 that exposed 399 healthy adults to 1 of 5 “cold” viruses. This included a common type of coronavirus (coronavirus 229E) that existed before the novel coronavirus (SARS-CoV-2 virus), which causes the disease COVID-19. Data from the British Cold Study can be found on Carnegie Mellon University’s The Common Cold Project website
The UC Davis researchers calculated the unadjusted and adjusted overall and coronavirus-specific relative risks for current smokers and each outcome (infection and disease), testing whether each association was modified by respiratory virus type.
The data showed that current smokers had an increased risk of respiratory viral infection and disease, with no significant difference between virus types. The increased associations for the 229E coronavirus alone did not reach statistical significance. This was likely due to the small sample size with only 55 participants, 20 of whom were smokers.
These findings are consistent with known harms caused by smoking on immune and respiratory defenses and some observational evidence of increased COVID-19 infection and disease progression in current smokers.
“As well as looking at associations by type of virus, one of the main reasons we re-analyzed the original UK cold study is to report a risk ratio instead of an odds ratio,” said explained Dove. “Odds ratios can overestimate the strength of an association if an event is not rare (>10%), so our results are somewhat lower (1.48 versus 2.1 in the BCS). The relative risks from this study can provide an estimate of the strength of the associations that can be used to guide tobacco control decisions.”
Compared to other study designs, the BCS is considered a high-quality study due to its randomized trial design, few missing data, clear definitions of smoking status, and laboratory-confirmed data. Observational studies have limitations. These include current smokers being more likely to get tested due to increased symptoms and the fact that smoking status is underreported in electronic health records. In addition, infected people who quit smoking immediately before testing or hospitalization are often registered as non-smokers or former smokers.
One of the main limitations of this study is that the benign common coronavirus 229E may have different biological and health effects than other coronaviruses, including SARS-CoV-2. In other words, the results may not be generalizable to other coronaviruses.
“These findings may have implications for population-level tobacco control as a strategy for preventing COVID-19 infection,” said Elisa Tong, lead author and professor in the Department of Internal Medicine at the UC Davis. “A quarter of the US population currently smokes or has high levels of cotinine, a metabolite of nicotine, and there is no safe level of smoke exposure for non-smokers. Global tobacco control is also of urgent importance, as many countries have even higher tobacco prevalence rates. “
Other UC Davis researchers who participated in the study include Bruce Leistikow and Nossin Khan from the Department of Public Health Sciences.
Dove was supported by the National Center for Advancing Translational Sciences, National Institutes of Health, through grant number UL1 TR001860 and related award KL2 TR001859.
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Material provided by University of California – Davis Health. Original written by Stephanie Winn. Note: Content may be edited for style and length.