As vaccination drives continue around the world scientists are trying to learn more about breakthrough COVID-19 cases, and it seems that vaccinated people with substance abuse are at a higher risk of catching the coronavirus, a new study shows.

Research conducted earlier in the pandemic showed that people with addictions were already more likely to contract, and experience serious complications from, COVID-19. Now, with the vaccines widespread, that same population is still at risk, according to the study, conducted by researchers at Case Western University and the National Institute on Drug Abuse.

The last year and a half has been particularly difficult for people in addiction, between the heightened risk for COVID-19 and a rapidly escalating overdose crisis – exacerbated by the pandemic – that killed more than 93,000 people in 2020 alone. Overdoses killed 1,214 Philadelphians in 2020, the second-worst year on record.

Though the vaccine is still highly effective and the risk of a breakthrough infection relatively low, researchers believe that the high prevalence of other co-occurring health issues among people with addiction may be behind the increased risk for a breakthrough infection.

When the study authors controlled for adverse socioeconomic health determinants – like issues with employment and housing – and comorbidities, the risk of breakthrough infections for people with and without a substance use disorder was the same.

The only exception was in patients diagnosed with a cannabis use disorder – a generally younger group of patients who were still found to be more likely to develop a breakthrough infection, even after researchers controlled for other factors.

"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 study authors wrote.

The study looked at more than 579,000 people fully vaccinated against COVID-19 between December 2020 and August 2021. Of that group, 30,183 patients had been diagnosed with a substance use disorder (SUD).

The study found that 7% of those with a SUD had a breakthrough infection at least 14 days after their second shot, compared to 3.6% of people without a substance use disorder.

People with a SUD were also more likely to experience a serious breakthrough infection, researchers found.

The study authors acknowledged some limitations, including that COVID-19 vaccines taken outside of health care organizations – like a mass vaccine clinic or a pharmacy – might not show up in the electronic health records used to conduct the study. And researchers couldn’t tell from the data whether a given breakthrough COVID-19 case was asymptomatic, symptomatic, or severe, or whether the cases they looked at were caused by the highly contagious delta variant.

Still, said Rong Xu, a professor of biomedical informatics at Case Western and one of the study’s authors, the findings show that an already vulnerable group is still at risk from the virus.

"The population with substance use disorder – they do have a lot of comorbidities, which makes them vulnerable to other diseases,” Xu said. It’s important, she said, to treat substance use from a physical-health perspective, as well as a behavioral one.

"We don’t focus as much on the long-term health consequences of substance use on much of our body systems, including the immune system,” she said.

In a press release last week, NIDA director Nora Volkow, another of the study’s authors, stressed that people with addiction should still get vaccinated and that the overall risk of COVID-19 among vaccinated members of the group is still very low.

"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,” she said.