Early last spring, [twin] sisters from Rochester, Mich., checked themselves into the hospital with fevers and shortness of breath. While Kelly was discharged after less than a week, her sister [Kimberly] ended up in intensive care, and spent almost a month in critical condition.
Nearly a year later, the sisters are bedeviled by the divergent paths their illnesses took.
“I want to know,” Kelly said, “why did she have Covid worse than me?”
Identical twins offer a ready-made experiment to untangle the contributions of nature and nurture in driving disease. With the help of twin registries in the United States, Australia, Europe and elsewhere, researchers are confirming that genetics can influence which symptoms Covid-19 patients experience.
These studies have also underscored the importance of the environment and pure chance: Even between identical twins, immune systems can look vastly different.
Last year, [epidemiologist Tim Spector] and his colleagues developed a symptom-tracking app. In a study that has not yet been published in a scientific journal, they reported that genetic factors might account for up to 50 percent of the differences between Covid-19 symptoms.
Still, Dr. Spector said, “It would be wrong to think we can answer this if we just crack the genes.”
