TORONTO -- A new study has found that individuals with substance use disorder are more likely than others to have contracted a breakthrough COVID-19 infection.

Researchers based in Ohio published their findings in the journal World Psychiatry on Tuesday.

They looked at the anonymized health records of 579,372 individuals in the U.S. who were fully vaccinated between December 2020 and August 2021. Of those individuals, 30,183 had a diagnosis of substance use disorder.

A breakthrough infection of COVID-19 happens when a fully vaccinated person becomes infected with the SARS-CoV-2 virus.

These infections are rare. A study last month that looked at 1.2 million fully vaccinated people in the United Kingdom found that fewer than 0.2 per cent experienced a breakthrough infection.

The breakthrough infection rate in the Ohio study for individuals without substance use disorder was 3.6 per cent. That's much higher than the numbers from the U.K. study, but the Ohio study only looked at people who have had a recent encounter with the U.S. health-care system.

For individuals with cannabis use disorder, the breakthrough infection rate was 7.8 per cent.

Similarly, individuals with cocaine use disorder had a breakthrough infection rate of 7.7 per cent. For cocaine use disorder, the breakthrough infection rate was 7.7 per cent. For alcohol, the rate was 7.2 per cent. For opioids, it was 7.1 per cent. The breakthrough infection rate for people with tobacco use disorder was 6.8 per cent.

After controlling for comorbidities and socioeconomic conditions, the risk of breakthrough infection no longer differed significantly, with the exception of individuals with cannabis use disorder. These individuals were still 55 per cent more likely to experience a breakthrough infection.

Researchers say it's still unclear why people with cannabis addiction had a higher risk, even after adjusting for these confounding variables.

"This may indicate that additional variables, such as behavioral factors or adverse effects of cannabis on pulmonary and immune function, could contribute to the higher risk for breakthrough infection in this group," the authors wrote.

Paul Armentano, who is the deputy director for the U.S.-based National Organization for the Reform of Marijuana Laws, notes that the data only looked at people diagnosed with cannabis use disorder, who represent only a "small and distinct subset" of cannabis consumers.

"(The study) raises more questions than it answers," he said in a news release, stating that it's still unclear what the COVID-19 risks are for cannabis consumers who don't have substance use disorder.

"We have no idea from these findings what elevated risks, if any, these non-classified consumers – who comprise the overwhelming majority of marijuana users – possess," he added.

In addition, the researchers found that people with substance use disorder who received the Pfizer vaccine were slightly more likely to get a breakthrough infection compared to those who received Moderna, after adjusting for demographics, socioeconomic status and comorbidities. Breakthrough infection rates for Moderna recipients ranged from 4.9 to 7.3 per cent. For Pfizer recipients, it ranged between 7.3 to 9.7 per cent.

The study isn't without its limitations. Because this was an observational study, it's impossible to know whether substance use disorder may have been over- or underdiagnosed in the data. The data also only includes individuals who have had an encounter with the U.S. health care system and may not include people from rural areas, health populations or undocumented immigrants. In addition, there was no data on how severe the COVID-19 breakthrough cases were.​