New York, May 9 (IANS): Covid-19 deaths among elderly may be due to predetermined genetic limits on cell division, finds a new study.

Human immune system's ability to combat Covid, like any infection, largely depends on its ability to replicate the immune cells effective at destroying the SARS-CoV-2 virus that causes the disease.

These cloned immune cells cannot be infinitely created. Researchers at the University of Washington hypothesised that the body's ability to create these cloned cells falls off significantly in old age.

This genetically predetermined limit on your immune system may be the key to why Covid-19 has such a devastating effect on the elderly, said research professor James Anderson, in the paper published in The Lancet eBioMedicine detailing a model linking ageing, Covid and mortality.

"When DNA splits in cell division, the end cap, called a telomere, gets a little shorter with each division," explained Anderson.

"After a series of replications of a cell, it gets too short and stops further division. Not all cells or all animals have this limit, but immune cells in humans have this cell life," he added.

The average person's immune system coasts along pretty well despite this limit until about 50 years old. That's when enough core immune cells, called T cells, have shortened telomeres and cannot quickly clone themselves through cellular division in big enough numbers to attack and clear the Covid virus, which has the trait of sharply reducing immune cell numbers, Anderson said.

Importantly, he added, telomere lengths are inherited from parents. Consequently, there are some differences in these lengths between people at every age as well as how old a person becomes before these lengths are mostly used up.

"Depending on your parents and very little on how you live, your longevity or, as our paper claims, your response to Covid-19 is a function of who you were when you were born, which is kind of a big deal."

To build this model the researchers used publicly available data on Covid mortality from the Center for Disease Control and US Census Bureau and studies on telomeres, many of which were published by the co-authors over the past two decades.

Assembling telomere length information about a person or specific demographic, he said, could help doctors know who was less susceptible. And then they could allocate resources, such as booster shots, according to which populations and individuals may be more susceptible to Covid.