Covid-19 and Loss of Smell and Taste

Anosmia (complete olfactory loss) can be experienced after viral infections of the upper respiratory tract such as the common cold, flu and sinus infections, or after traumatic brain injury. Recovery from olfactory loss can be slow and typically starts with the onset of parosmia (olfactory distortion) a few months after the initial smell loss. Parosmia is usually a temporary phase that can happen during the recovery period as the olfactory nerves regenerate. The smell of common food items, body care products, household cleaners and things in the environment become distorted and invoke a strong sense of disgust. Eating can become a challenge, and anxiety, depression and loss of quality of life is experienced by many. However, the onset of parosmia is usually a good sign, as it indicates recovery of the olfactory neurons and in many cases is the start of recovery to a ‘new normal’.

We estimate that about 80-90% of those with post-Covid anosmia will regain their sense of smell within a few weeks⁵. However, an unlucky few will find that recovery of their sense of smell takes much longer. As those with post-Covid-19 anosmia start to recover, we are observing unusual fluctuations in symptoms, sometimes a temporary return to (near) normal, with the onset of parosmia in many cases, a few months down the line.

There is some controversy over the efficacy of smell testing in the general population as poor olfactory function is not an indication of a live virus, and can also be related to chronic rhinosinusitis, neurodegenerative aging, Alzheimer’s disease, Parkinson’s disease and head injury. However, a sensory panel is one group for which regular smell testing could be recommended. In general, loss of the ability to smell is not insignificant, and hard to miss. Most people with smell loss will discover they cannot smell their soap or shower gel, cannot taste minty toothpaste, and do not smell their coffee or toast. Usually within 20 minutes, you are very aware of the problem, there is no need for further testing, and most people will notify their employers. However, for a sensory panellist, where their job relies on their olfactory acuity, there might be the temptation to under-report. For those with full anosmia, this will soon become apparent by checking panellist performance data, but some may return with a weaker, or fluctuating olfactory function. With this in mind we should consider regular checks of panellist olfactory acuity using standardised methods.

Post-covid, a significant increase in olfactory dysfunction in the general population could be a problem. A recent study has shown that people tend to overestimate the recovery of their olfactory function. Objective tests showed that 55% of those reporting a return to a normal sense of smell still had a low olfactory function, and 5% were still functionally anosmic⁷ and this presents a potential problem for consumer trials. A typical consumer test using self-reported screening questions such as health information and familiarity with products, rather than sensory acuity, would not pick up those with low olfactory function, whether they were aware of it or not. If your consumer test focuses on product flavour, then it may be worthwhile to consider adding a smell test into your procedure. This could be as simple as identifying three familiar smells from a bottle, smelling strip or sniffing stick, or more complex sensory tests as required.