Here are the seven reasons the coronavirus causing COVID-19 hits some people harder than others, why it kills some and spares others.
Though scientists at first thought age was the dominant factor, with young people avoiding the worst outcomes, new research has revealed a suite of features impacting disease severity.
These influences could explain why some perfectly healthy 20-year-old with the disease is in dire straits, while an older 70-year-old dodges the need for critical interventions.
Underlying health conditions are thought to be an important factor influencing disease severity.
Indeed, a study of more than 1.3 million COVID-19 cases in the United States, published June 15 in the journal Morbidity and Mortality Weekly Report, found that rates of hospitalisations were six times higher and rates of death were 12 times higher among COVID-19 patients with underlying conditions, compared with patients without underlying conditions.
The most commonly reported underlying conditions were heart disease, diabetes and chronic lung disease.
In general, risk factors for more severe COVID-19 outcomes include: Age, Diabetes (type 1 and type 2), Heart disease and hypertension and Smoking.
The other risk factors are Blood type, Obesity and Genetic factors.
About 8 out of 10 deaths associated with COVID-19 in the U.S. have occurred in adults ages 65 and older, according to the U.S. Centers for Disease Control and Prevention (CDC).
The risk of dying from the infection, and the likelihood of requiring hospitalization or intensive medical care, increases significantly with age. For instance, adults ages 65-84 make up an estimated 4-11% of COVID-19 deaths in the U.S, while adults ages 85 and above make up 10-27%.
The trend may be due, in part, to the fact that many elderly people have chronic medical conditions, such as heart disease and diabetes, that can exacerbate the symptoms of COVID-19, according to the CDC.
The ability of the immune system to fight off pathogens also declines with age, leaving elderly people vulnerable to severe viral infections, State News reported.