Mild Covid-19 cases more likely to lose their sense of smell, study finds

A patient in Mexico City, Mexico, underwent a nose-swab test in December 2020.

A patient in Mexico City, Mexico, underwent a nose-swab test in December 2020.
Photo: Rebecca Blackwell (AP)

For people with mild covid-19, the nose may know better than any other part of the body. New research released on Wednesday indicates that nearly 90 percent of people with mild illness experience that they lose their sense of smell, a higher percentage than those with more serious diseases. Although it is usually temporary, this odor loss can last six months and even longer.

The research, published in the Journal of Internal Medicine, looked at the medical records of more than 2,500 covid-19 patients who sought care in one of the 18 hospitals in Europe. Most were outpatients diagnosed with mild to moderate symptoms, while others were hospitalized with severe or critical covid-19.

A total of about three-quarters of the patients themselves reported an odor loss, medically known as anosmia. However, there was a wide gap in the presentation of this symptom at different levels of severity. Among patients with mild diseases, 86% odor loss reported, compared with only 4.5% of moderate patients and 6.9% of severe patients. If we only look at patients who confirmed their odor loss through objective tests, this gap has narrowed, but it remains: 54.7% of these milder cases confirmed odor loss, compared to 36.6% of the moderate to critical cases.

“Olfactory dysfunction is a common disease in covid-19 patients with a higher incidence in patients with mild forms of the disease,” the authors wrote.

The findings are in line with previous studies showing that anosmia is a very common symptom of covid-19 – although it was one underreport in the earliest days of the pandemic. Other research has suggested that milder cases are prone to anosmia more frequently. But the new study is one of the largest of its kind examining its prevalence and to have data from patients who had their anosmia confirmed with tests.

Although this study can not tell us why milder cases are more tend to lose their sense of smell, the authors do have theories. The main explanation is that anosmia is ironically a sign that the body is doing a decent job of repelling the coronavirus. In these patients, the infection never spreads far beyond the upper airways, thanks to a robust, localized immune response. But this reaction can lead to inflammation that damages or interferes with the nearby cells that help us process odor, at least for a short time.

Wrong reminders can also occur for more serious cases. Patients in the hospital often have very different symptoms, while the most critical patients require intensive interventions such as a feeding tube or ventilation. Ppatients with more severe covid-19 just may not notice it their anosmia as it happens. This may explain the higher anosmia rate of the study below moderateon-serious cases actually investigated for it.

Happy for most people who do experience kovid-related anosmia, it appears to be self-limiting. The average time it took in patients was about three weeks, and about one-third reported that it lasted only two weeks. But 15% of the patients examined objectively lost their sense of smell for at least two months, while the sixth still had just under 5%.month point. Those with worse anosmia at first it was likely that it would continue months later.

The authors theorize that cases of shorter duration of anosmia usually represents nothing more than the nasal congestion and swelling that is common with everyday colds and that can momentarily block our ability to sniff. But those with persistent anosmia probably have a more complicated problem on their hands, which means great damage to the olfactory cells, due to infection or inflammation. While most people who contract covid-19 will be able to smell to their heart’s content afterwards, the big pandemic – with 86 million confirmed cases worldwide as of early January – means that many people may permanently lose their sense of smell.

The authors hope that their research will lead to future studies that will monitor these long-term cases to better understand the phenomenon, as well as to find out how to to improve a person’s chance of recovery.

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