Illustration explaining the concept ot viral disease airborne transmission. Credit: nobeastsofierce Science / Alamy Stock Photo

A mathematical model demonstrates how airborne, virus-loaded droplets move from the nose and throat to the deep lung and cause infection1.

When a person inhales the droplets, the virus is deposited in the mucous membrane that line the respiratory tract. It replicates in the throat epithelial cells, forms mucous droplets and is transported to the lungs via the lower respiratory tract.

Researchers at the Indian Institute of Technology Madras in Chennai used the model to propose a clinically plausible infection mechanism. Their model indicated that fluid dynamics play an important role in transporting the droplets through the respiratory tract.

The team, which included researchers at the Jadavpur University in Kolkata and Northwestern University in the United States, found that the lungs continuously wash out the virus. But, after a certain time, virus replication overtakes virus clearance rate, increasing its concentration in the mucous membrane.

In the deep lungs, the droplets deposit and release the virus, causing infection. The progress of this infection is independent of the deposited viral load. However, severe infection does depend on the viral load – a smaller viral load causes a milder infection.

The model predicted that severe infection can develop in the deep lungs between 2.5 and 7 days of initial symptom onset.

The researchers observed that stronger immune responses – from prior infection or induced by vaccination – significantly lower the risk of severe infection. This reinforces the need for vaccination to prevent severe infections from SARS-CoV-2.