Heavy marijuana users who are also vaccinated may be more susceptible to breakthrough cases of COVID-19, a new study found. The study, published last Tuesday in World Psychology, found that those with a substance use disorder (SUD) — a dependence on marijuana, alcohol, cocaine, opioids and tobacco — were more likely to contract the coronavirus after receiving both of their vaccination shots. Those without a SUD saw a 3.6 percent rate of breakthrough infections, compared to a 7 percent rate in those with a SUD.