Researchers caution e-cigarette users as a recent study suggests that vaping may increase susceptibility to infection by SARS-CoV-2, the virus that spreads COVID-19.

The study conducted by the University of California, Riverside, found that typical ingredients of the vaping liquid - propylene glycol/vegetable glycerin - alone or with the use of nicotine enhanced susceptibility to COVID-19 infection through different mechanisms. The results were published in the journal of the American Journal of Physiology.

However, the addition of benzoic acid to e-liquids prevents the increased susceptibility associated with propylene glycol, vegetable glycerin, and nicotine, the researchers noted.

"Users who vape aerosols produced from propylene glycol/vegetable glycerin alone or e-liquids with a neutral to basic pH are more likely to be infected by the virus, while users who vape aerosols made from e-liquids with benzoic acid—an acidic pH—will have the same viral susceptibility as individuals who do not vape," said Rattapol Phandthong, first author of the study in a news release.

To understand the impact of vaping on susceptibility to COVID-19 infection, the team used airway stem cells from donors and created a 3D model of human bronchial epithelium. They then exposed these cells to JUUL and BLU electronic cigarette aerosols and observed an increase in the amount of ACE2, a host cell receptor for the SARS-CoV-2 virus. When the cells were further exposed to aerosols with nicotine, there was an increased activity of TMPRSS2, an enzyme essential for the virus to infect cells.

Based on their findings, researchers warn users to be cautious of the products used in vaping and encourage them to quit the habit.

"It would probably be best for vapers to quit vaping for the protection of their health and to stop nicotine dependency. If they cannot stop vaping, it is better to vape aerosols produced from an e-liquid with acidic pH or with benzoic acid to prevent the enhanced SARS-CoV-2 infection caused by nicotine, propylene glycol, and vegetable glycerin. However, inhalation of benzoic acid has its own risk, and data is still limited on this topic," said Prue Talbot, a study author.

Meanwhile, researchers acknowledge that the relationship between e-cigarettes and SARS-CoV-2 susceptibility is complex due to the availability of a wide range of e-liquids and different models of e-cigarettes.

"Our study only used Classic Tobacco Flavor JUUL e-cigarette and BLU Classic Tobacco e-cigarette. Even with just these two e-cigarettes, we found the aerosols and individual ingredients produced different effects on SARS-CoV-2 infection," Phandthong said.

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