Since the pandemic started to hit the US in full force in March, speculation about the link between vaping and Covid-19 has flourished. The Food and Drug Administration and the National Institute on Drug Abuse both issued warnings. Anecdotal reports of young vapers coming down with severe coronavirus infections began to crop up. But there was very little research to support a connection.

Now, a study published today in the Journal of Adolescent Health finally offers data that shows a relationship between e-cigarette use and Covid-19 risk. Researchers from Stanford University show that teenagers and young adults ages 13 to 24 who use e-cigarettes are five times more likely to be diagnosed with Covid-19 than their non-vaping peers. Those who are dual-users—people who smoke both traditional and electronic cigarettes—are seven times more likely to test positive for the virus, the researchers found.

“I knew there would be a relationship,” says co-author Bonnie Halpern-Felscher, a professor of pediatrics at Stanford University who studies youth tobacco use. “I did not expect it to be this strong of a relationship.”

Studies have already linked smoking with higher susceptibility to severe Covid-19 infections, but previously no population-based studies had examined the link between e-cigarette use and Covid-19 in teenagers and young adults. The question researchers wanted to answer was two-fold: Were e-cigarette smokers more likely to get tested for SARS-CoV-2? And were they more likely to test positive? “The answer is soundly yes” to both parts of the question, says Halpern-Felsher.

The researchers gathered their data through an online survey posted on spaces like social media and gaming sites. Over 4,000 teens and young adults from all 50 states responded, completing the roughly 15-minute survey. Researchers then weighted the samples to reflect the racial and ethnic, gender, LGBTQ status, and age makeup of the United States population.

The survey, which was sent out in early May, asked respondents whether they had ever used regular or electronic cigarettes; whether they had used them in the last 30 days; whether they had been tested for Covid-19; and whether their test results came back positive. The researchers also controlled for other Covid-19 risk factors like whether the respondents lived near a coronavirus hotspot; whether they were under- or overweight, which can affect lung function; and for their socio-economic status, which can affect how well people can socially distance. Ultimately, the researchers determined that dual-users who had smoked in the last 30 days were not only more likely to test positive, but they were also nine times more likely to get tested in the first place.

The survey did not explore why users decided to get tested. It’s possible that users confused the effects of vaping—extra phlegm, coughing, or shortness of breath—with Covid-19 symptoms. But the high rate of positive test results may indicate that vapers are more vulnerable to the virus itself.

That said, this study simply illustrates a correlation between e-cigarette and cigarette use and positive Covid-19 diagnoses. As the authors explain in the paper, their findings “show that e-cigarette use and dual use of e-cigarettes and cigarettes are significant underlying risk factors for COVID-19 that has previously not been shown.” But on its own, this paper can’t prove whether they may be more biologically susceptible to infection in the first place, or if they are more likely to have severe infections.

Still, Halpern-Felsher has a few theories for why this overlap might exist. Smokers may have more lung damage, making them more susceptible to the virus. Or they might be touching their hand to their mouth more often than other people, or sharing vapes, increasing their likelihood of being exposed in the first place. Or it could be that the virus is being spread through the aerosols vapers exhale. “Those are all hypotheses,” she says. “Someone needs to follow it up.”