Twins with COVID help scientists untangle the disease’s genetic roots
After 35 years of sharing everything from a love for jazz music to tubes of lip gloss, twins Kimberly and Kelly Standard assumed that when they became sick with COVID-19 their experiences would be as identical as their DNA.
The virus had different plans.
Early last spring, the 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 ended up in intensive care.
Kimberly spent almost a month in critical condition, breathing through tubes and dipping in and out of shock. Weeks after Kelly had returned to their shared home, Kimberly was still relearning how to speak, walk and chew and swallow solid food she could barely taste.
Nearly a year later, the sisters are bedeviled by the bizarrely divergent paths their illnesses took.
“I want to know,” Kelly said, “why did she have COVID worse than me?”
Identical twins offer researchers 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 affect 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 — and continue to grow apart over the course of a lifetime.
Dr. Mishita Goel, one of the doctors who treated the Standard twins last spring, said she was surprised to see the virus chart such different medical trajectories in each sister.
Both sisters were carrying excess weight and had a history of metabolic conditions. If anything, Kelly, who has asthma and a more severe case of diabetes, might have had a slightly higher risk of entering intensive care, Goel said. But it was Kimberly who fared worse.
At least some of the factors that influence the severity of a COVID case are written into the genome. Recent studies suggest that people with type O blood, for example, might be at a slightly lower risk of becoming seriously sick with the coronavirus (though experts have cautioned against overinterpreting these types of findings). Other papers have homed in on genes that affect how cells sound the alarm about viruses, or raise their defenses to ward off invaders.
Still other genetic tweaks may make it easier for the coronavirus to break into cells. Some might even trigger an overzealous immune response to infection that damages healthy tissues alongside the sick ones — a common feature among the worst COVID-19 cases.
There even seems to be a measurable genetic influence on whether people experience symptoms such as fever, fatigue and delirium when stricken with the coronavirus, said Tim Spector, a public health researcher and the director of the Twins-UK registry based at St. Thomas’ Hospital in London. Last year, he and his colleagues developed a symptom tracking app to record how people dealt with the disease. 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.
Many of the conditions that can raise a person’s risk for severe COVID — excess weight, heart disease, diabetes, smoking — are highly influenced by environment and behavior, not just genetics. A person’s history of fighting off other coronaviruses, like those that lead to common colds, might also affect their likelihood of developing a serious case of COVID.
Some researchers have also floated the idea that the amount of coronavirus a person takes in may have an impact on the severity of disease, a trend that has been documented with other infections.