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COVID-19 mostly kills through an overreaction of the immune system, whose function is precisely to fight infections, say scientists who have decoded the mechanisms, symptoms, and diagnosis of the disease caused by the SARS-Cov-2 coronavirus.

In a study published in the journal Frontiers in Public Health, the researchers explained step-by-step how the virus infects the airways, multiplies inside cells, and in severe cases causes the immune defences to overshoot with a "cytokine storm".

This storm is an over-activation of white blood cells, which release too-great amounts of cytokines -- inflammation-stimulating molecules --into the blood, they said.

"Similar to what happens after infection with SARS and MERS, data show that patients with severe COVID-19 may have a cytokine storm syndrome," said study author Daishun Liu, Professor at Zunyi Medical University in China.

"The rapidly increased cytokines attract an excess of immune cells such as lymphocytes and neutrophils, resulting in an infiltration of these cells into lung tissue and thus cause lung injury," Liu said.

The researchers explained that the cytokine storm ultimately causes high fever, excessive leakiness of blood vessels, and blood clotting inside the body.

It also causes extremely low blood pressure, lack of oxygen and excess acidity of the blood, and build-up of fluids in the lungs, they said.

The researchers noted that white blood cells are misdirected to attack and inflame even healthy tissue, leading to failure of the lungs, heart, liver, intestines, kidneys, and genitals.

This multiple organ dysfunction syndrome (MODS) may worsen and shutdown the lungs, a condition called acute respiratory distress syndrome, (ARDS), they said.

This, the researchers explained, happens due to the formation of a so-called hyaline membrane -- composed of debris of proteins and dead cells -- lining the lungs, which makes absorption of oxygen difficult.

Most deaths due to COVID-19 are therefore due to respiratory failure, they said.

The researchers explained that in the absence of a specific antiviral cure for COVID-19, the goal of treatment must be to the fight the symptoms, and lowering the mortality rate through intensive maintenance of organ function.

For example, an artificial liver blood purification system or renal replacement therapy can be used to filter the blood through mechanical means, they said.

The team noted that especially important are methods to supplement or replace lung function, for example with non-invasive mechanical ventilation through a mask, ventilation through a tube into the windpipe, the administration of heated and humidified oxygen via a tube in the nose, or a heart-lung bypass.

The researchers stressed the importance of preventing secondary infections.

They noted that SARS-Cov-2 also invades the intestines, where it causes inflammation and leakiness of the gut lining, allowing the opportunistic entry of other disease-causing microorganisms.

The researchers advocate that this should be prevented with nutritional support, for example with probiotics -- beneficial bacteria that protect against the establishment of harmful ones -- and nutrients and amino acids to improve the immune defences and function of the intestine.

"Because treatment for now relies on aggressive treatment of symptoms, preventative protection against secondary infections, such as bacteria and fungi, is particularly important to support organ function, especially in the heart, kidneys, and liver, to try and avoid further deterioration of their condition," Liu added.

New Delhi, Jul 21: The Centre has written to all states and union territories warning against the use of N-95 masks with valved respirators by people, saying these do not prevent the virus from spreading out and are "detrimental" to the measures adopted for its containment.

The Director General of Health Services (DGHS) in the Ministry of Health, in a letter to the Principal Secretaries of health and medical education of states, said it has been observed that there is "inappropriate use" of N-95 masks, particularly those with valved respirators, by the public other than designated health workers.

The DGHS referred to the advisory on the use of homemade protective cover for face and mouth available on the website of the Ministry of Health.

"It is to bring to your knowledge that the use of valved respirator N-95 masks is detrimental to the measures adopted for preventing the spread of coronavirus as it does not prevent the virus from escaping out of the mask. In view of the above, I request you to instruct all concerned to follow the use of face/mouth cover and prevent inappropriate use of N-95 masks," DGHS Rajiv Garg said in the letter.

The government had in April issued an advisory on the use of homemade protective cover for face and mouth, asking people to wear it, particularly when they step out of their residences.

The advisory stressed such face covers must be washed and cleaned each day, as instructed, and stated that any used cotton cloth can be used to make this face cover.

The colour of the fabric does not matter but one must ensure that the fabric is washed well in boiling water for five minutes and dried well before making the face cover. Adding salt to this water is recommended, it said.

It also listed the procedures of making such homemade masks, asking to ensure it fits the face well and there are no gaps on the sides.

It urges people to wash hands thoroughly before wearing the face cover, switching to another fresh one as the face cover becomes damp or humid, and never reusing it after single use without cleaning it.

"Never share the face cover with anyone. Every member in a family should have separate face cover," the advisory stated.

India's COVID-19 case tally crossed the 11-lakh mark on Monday, while the total number of recovered patients increased to over seven lakh, according to Union health ministry data.

The death toll due to the disease rose to 27,497 with 681 fatalities reported in one day.

The ministry data updated at 8 am on Monday showed that a record single-day jump of 40,425 COVID-19 cases had taken the total number of cases to 11,18,043.