the loss of smell or taste is one of the first occurred COVID-19 symptoms

SARS-CoV-2 like SARS-CoV and MERS-CoV could infect the nervous system through neurons in the nasal passage targeting olfactory bulb

Experiment on transgenic mice expressing human ACE2 showed SARS-CoV enter CNS through neurons in the nose. 

SARS-CoV-2 could work its way up to the olfactory mucosa, which consists of epithelium cells, blood vessels, and the axons from olfactory neurons, which lead to the frontal lobe.

Loss of sense could be caused by inflammation as the body fights the virus

SARS-CoV-2 infects medulla oblongata— part of the brain stem that serves as the control center for the heart and the lungs—causing acute respiratory failure for patients e.g. loss of involuntary breathing

SARS-CoV-2 cross BBB by circulating through the blood and then attack ACE2 receptors in the endothelia that lines blood capillaries in the brain, causing swelling and compression of brain stem