New York, Aug 4 (IANS): One of the immune system's oldest branches, called complement, may be influencing the severity of Covid-19 disease, say researchers.

The study, published in the journal Nature Medicine, found that people with age-related macular degeneration -- a disorder caused by overactive complement -- are at greater risk of developing severe complications and dying from Covid-19. The connection with complement suggests that existing drugs that inhibit the complement system could help treat patients with severe disease.

The authors also found evidence that clotting activity is linked to Covid severity and that mutations in certain complement and coagulation genes are associated with hospitalisation of Covid patients.

"Results provide important insights into the pathophysiology of Covid-19 and paint a picture for the role of complement and coagulation pathways in determining clinical outcomes of patients infected with SARS-CoV-2," said study author Sagi Shapira from the Columbia University Vagelos in the US.

Coronaviruses are masters of mimicry, particularly with proteins involved in coagulation and proteins that make up a complement, one of the oldest branches of the human immune system, the researchers said. Complement proteins work a bit like antibodies and help eliminate pathogens by sticking to viruses and bacteria and marking them for destruction. Complement can also increase coagulation and inflammation in the body.

"Unchecked, these systems can also be quite detrimental. The new coronavirus -- by mimicking complement or coagulation proteins -- might drive both systems into a hyperactive state," Shapira said.

According to the study, if complement and coagulation influence severity of COVID, people with pre-existing hyperactive complement or coagulation disorders should be more susceptible to the virus. That led the research team to look at Covid-19 patients with macular degeneration, an eye disease caused by overactive complement, as well as common coagulation disorders like thrombosis and haemorrhage.

Among 11,000 coronavirus patients with suspected Covid-19, the researchers found that over 25 per cent of those with age-related macular degeneration died, compared to the average mortality rate of 8.5 per cent and roughly 20 per cent required intubation.

"Complement is also more active in obesity and diabetes, and may help explain, at least in part, why people with those conditions also have a greater mortality risk from Covid-19, the authors wrote.

People with a history of coagulation disorders also were at increased risk of dying from Covid infection, they said."I think our findings provide a stronger foundation for the idea that coagulation and complement play a role in Covid and will hopefully inspire others to evaluate this hypothesis and see if it's something that can be useful for fighting the ongoing pandemic," the team noted.