A study of Californians who contracted the coronavirus found that 35 percent of them reported at least one symptom two months after they tested positive—a phenomenon known as “long COVID.” The most common lingering problems in the study—conducted before the vaccine were available and published this week by the U.S. Centers for Disease Control and Prevention—were fatigue and trouble breathing. The researchers found that women, people over 40, Black people, and those with pre-existing conditions were more likely to suffer from long COVID. They also noted the small size of the study sample—366 people—and said more research is needed.