Fig. 5.

The possible mechanism underlying the association of COVID-19 severity with anosmia. SARS-CoV-2 can invade the central nervous system (CNS) via the olfactory pathway (straight purple line leading up towards the brain) or spread to the lower respiratory tract via inhalation after entering the nasal cavity (straight purple line pointing downward). However, viral invasion activates the host immune system and recruits inflammatory mediators, which can cause damage to the olfactory epithelium (red spot), leading to anosmia. This process can prevent viral entry into the CNS by blocking its transmission to the olfactory bulb (green dotted line pointing towards the brain), thereby prevents the infection of respiratory centres in the brain. When this innate immune response is triggered, it can destroy the virus and limit viral propagation to the lower respiratory tract (green dotted line pointing towards the lower respiratory tract). Mucociliary clearance is another nasal defence mechanism that clears the particles that enter the lower respiratory tract by expelling them into the oropharynx from where they are either expectorated or swallowed (straight purple lines pointing toward the oral cavity and oesophagus). However, individuals with risk factors may have compromised nasal defences mechanism, allowing the virus to enter the lower respiratory tract through aspiration into the lungs, resulting in lower respiratory tract infection (dotted purple lines)