These long-lasting side-effects are happening to young people as well as old, and can affect otherwise asymptomatic individuals. The explanation interests me and I've added a paragraph from that part of the article below...
`Parosmia: 'Since I had Covid, food makes me want to vomit''
LINK
...So what causes parosmia? The prevailing hypothesis is that it results from damage to nerve fibres that carry signals from receptors in the nose to terminals (known as glomeruli) of the olfactory bulb in the brain. When these regrow - whether the damage has been caused by a car accident or by a viral or bacterial infection - it's thought the fibres may reattach to the wrong terminal, Parker says. "They are in the wrong meeting room! This is referred to as cross-wiring and it means the brain doesn't recognise the smell, and is perhaps programmed to think of it as danger." The theory is that in most cases the brain will, over time, correct the problem, but Parker is reluctant to say how long it will take...
From my food science background I can add another factor that could help explain changed responses to odours. What we think of as the `smell' of a chemical in food, scent etc changes with the concentration of that chemical in the air reaching our nose. For example the odour of chocolate at a much higher concentration than usual can be detected by our brains as `faecal'. (That's a favourite example used by sensory analysis experts!) we all know the bad eggs smell of hydrogen sulphide but at a higher concentration of the gas we can't even detect it (which is why the gas is dangerous). This shows that our sense of smell is complicated and easily altered. Perhaps anosmia occurs when the virus blocks the receptors and then parosmia occurs afterwards when the receptors are free again but have become more sensitive?
The article doesn't mention the ACE-2 receptors in the nose that are the entry point for the covid-19 virus. These same receptors are part of the mechanism by which we detect odours and it's perhaps not surprising that the covid infection affects our sense of smell. This article was published last August...
`Odor-sensing cells in nose seen as key entry point for SARS-CoV-2: Compared to cells in other parts of the respiratory tract, cells associated with smell have up to 700 times more ACE2 receptors, which bind to the virus that causes COVID-19'
LINK
....Scientists at Johns Hopkins Medicine, experimenting with a small number of human cell samples, report that the "hook" of cells used by SARS-CoV-2 to latch onto and infect cells is up to 700 times more prevalent in the olfactory supporting cells lining the inside of the upper part of the nose than in the lining cells of the rest of the nose and windpipe that leads to the lungs. These supporting cells are necessary for the function/development of odor-sensing cells. The findings, from a preliminary study of cells lining both the nose and trachea, could advance the search for the best target for topical or local antiviral drugs to treat COVID-19, and offer further clues into why people with the virus sometimes lose their sense of smell....
I noticed too this sentence in the article...
Cells from children were not examined for this study, in part because they tend to have low ACE2 levels in the cells lining the nose, which may contribute to generally less severe illness among children infected with the SARS-CoV-2 virus.