Kimberly and Kelly Standard, who are twins, assumed that when they became ill with Covid-19, their experiences would be as identical as their DNA.
The virus had different plans.
In early spring, the sisters from Rochester, Michigan, were admitted to the hospital with fever and shortness of breath. Although Kelly was discharged in less than a week, her sister ended up in the ICU and spent nearly a month in critical condition.
Almost a year later, the sisters are plagued by the divergent paths their illnesses have taken.
“I want to know,” said Kelly, “why did she have Covid worse than me?”
Identical twins offer an experiment ready to unravel the contributions of nature and cultivation in the management of diseases. With the help of twin records in the United States, Australia, Europe and elsewhere, researchers are confirming that genetics can influence what symptoms Covid-19 patients have.
These studies also underscored the importance of the environment and pure chance: even among identical twins, the immune system can be very different.
But at least some of the factors that influence the severity of a Covid-19 case are written in the genome. Recent studies suggest that people with type O blood, for example, may have a slightly lower risk of becoming seriously ill (although experts have warned against over-interpreting these types of findings). Other work has focused on genes that affect how cells alarm about viruses.
There even appears to be a measurable genetic influence on whether patients experience symptoms such as fever, fatigue and delirium, said Tim Spector, an epidemiologist and director of the TwinsUK registry based at St. Thomas’ Hospital in London.
Last year, he and his colleagues developed a symptom tracking application. In a study that has not yet been published in a scientific journal, they reported that genetic factors may be responsible for up to 50 percent of the differences between Covid-19 symptoms.
Still, Dr. Spector said, “It would be wrong to think that we can answer that if we just break the genes.”