Marijuana use doubles people's risk of breakthrough COVID-19 cases, study shows

The three available COVID-19 vaccines in the United States have proven to be highly effective at preventing infections and severe health complications caused by the virus. But some fully-vaccinated people could be at a greater risk of experiencing breakthrough COVID-19 cases because they use marijuana, experts say.

While the overall risk of a vaccinated person testing positive for COVID-19 is 3.6%, researchers found that nearly 8% of vaccinated people considered to have marijuana substance abuse disorders had breakthrough infections of the coronavirus, according to a study published in the medical journal World Psychiatry earlier this month.

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Vaccinated people who are the heaviest marijuana users are more likely to have breakthrough infections than vaccinated people with other substance abuse disorders.

Though those with marijuana use disorders tend to be younger with fewer underlying medical conditions than those with other drug addictions, researchers said the adverse effects of marijuana on the lungs and the immune system could contribute to the increased risk of a breakthrough COVID-19 infection.

It's unclear how the study's findings pertain to recreational or medical marijuana users, as it focused on people with a drug addiction. A substance use disorder is a disease that affects a person's brain and behavior and causes one to be unable to control his or hers use of a legal or illegal drug.

Overall, 7% of fully-vaccinated people who abuse drugs got infected compared to those without substance use disorders. And people suffering from drug addiction also have an increased risk of severe illness, or worse, after breakthrough COVID-19 infections.

"We must continue to encourage and facilitate COVID-19 vaccination among people with substance use disorders, while also acknowledging that even after vaccination, this group is at an increased risk and should continue to take protective measures against COVID-19," the National Institute on Drug Abuse's director Nora D. Volkow said.

Researchers from the National Institute on Drug Abuse and Case Western Reserve University in Cleveland analyzed the health records of 580,000 Americans with and without substance use disorders, who were fully vaccinated against COVID-19 between December 2020 and August 2021. Each study participant had also not been infected with COVID-19 before vaccination.

Scientists attributed the increased likelihood of breakthrough COVID-19 cases to underlying health conditions and adverse socioeconomic factors. But when these factors were controlled for, people with most substance use disorders no longer faced an increased likelihood of experiencing a breakthrough COVID-19 infection — except for pot users.

This latest study piggybacks on research from earlier in the pandemic, which found that people with substance use disorders were more likely to contract COVID-19 and have severe complications once infected. Researchers found this to be especially true among Black people who suffered from drug addiction.

"From previous studies, we knew that people with substance use disorders may be particularly vulnerable to COVID-19 and severe related outcomes," Case Western Reserve University professor Rong Xu said. 

"These results emphasize that, while the vaccine is essential and effective, some of these same risk factors still apply to breakthrough infections. It is important to continuously evaluate the effectiveness of COVID-19 vaccines and the long-term effects of COVID-19, especially among people with substance use disorders."