Study identifies olfactory dysfunction as an early symptom of COVID-19

By Susha Cheriyedath, M.Sc.Reviewed by Aimee MolineuxMar 1 2022

In a study posted to the medRxiv* pre-print server, scientists from a Swedish University assessed olfactory loss and dysfunction among coronavirus disease 2019 (COVID-19) patients to objectively characterize it as a symptom of COVID-19. The study ascertains olfactory dysfunction as a predictive symptom of COVID-19.

This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources

To curb the spread of any infection, detecting early symptoms of a disease is essential. It is an important tool that helps reduce infectious diseases. In COVID-19 patients, cold and cough were outlined as early symptoms for monitoring non-hospitalized patients. However, with more than half the COVID-19-positive population reporting a loss of smell or reduced sense of smell, olfactory dysfunction soon surfaced as a symptom of interest.

According to a study by Hannum et.al, 2021 and Gerkin et.al, 2021,

nearly 50% of the individuals with confirmed COVID-19 infection report complete loss of olfaction function and an additional 10-20% report reduced olfactory sensitivity”.

With a high number of individuals with COVID-19 reporting reduced or loss of smell, olfactory dysfunction is a high probability symptom of COVID-19 compared to other early symptoms. This hypothesis has been so far only anecdotal, and studies lacked objective measures to support this.

About the study

In the present study, scientists from Sweden examined longitudinal data from a web-based interface that allows individuals to test their sense of smell. The assessment involved individuals logging into the interface and rating the intensity of the smell of select household odors, along with the reporting of other symptoms and results from potential COVID-19 tests.

At the beginning of the pandemic, about 5,000 individuals living in Sweden registered for this online assessment. These individuals were provided an online tool, a web-based data registry platform called smelltracker.org. The data for the study was obtained in a 10-month timeframe starting from April 2020 to February 2021. The participants were filtered to have 205 COVID-19-positive individuals and 156 pseudo-randomly matched control individuals without a positive COVID-19 test.

For the olfactory assessment, five items were chosen from a list of 71 everyday household items. During each assessment, the odors were rated for their intensity and pleasantness. Also, patients were asked to record any other symptoms that they were experiencing at the time, including cough, fever, shortness of breath, and other COVID-19-related symptoms. The data were collected both prior to and after taking the COVID-19 test, which helped hypothesize whether olfactory dysfunction occurs even before other COVID-19-related symptoms appeared.

Results

The study aimed to find out if odor perception was reduced before testing positive for COVID-19. The results showed that on average the group that tested COVID-19-positive showed a decline in the intensity of the odor six days prior to the test result date. This confirmed that a decline in odor perception is an early indicator of COVID-19.

The study also looked at the correlation between olfactory dysfunction and the onset of other early COVID-19 symptoms. It was observed that the impairment of olfactory function aligned with the progression of other symptoms of COVID-19.

Further, the researchers tested to see if the onset of odor intensity decline happened before the occurrence of non-odor-related symptoms. The findings showed that while odor intensity declined six days before the day of the test result, other initial symptoms like runny nose, aches, and sore throat also appeared around the same time. Fever was also one of the early symptoms but started only three days before the test result date. The study found that the highest balanced accuracy was 70% for olfactory dysfunction, followed by 69% for a runny nose.

Conclusion

The findings illustrate that olfactory dysfunction is an early sign of COVID-19. However, the symptom does not appear earlier than other symptoms during disease progression. This study objectively establishes that olfactory dysfunction demonstrates the highest COVID-19 predictability. This also corroborates with other studies where odor decline was measured subjectively.

Since the decline of odor intensity was also reported before the COVID-19 test results of the individuals, this helped to eliminate any bias in reporting based on the test results. This has clearly shown that olfactory dysfunction is an early sign as well as a common COVID-19 symptom. However, the study doesn’t factor in the delays from the date of infection to the date of testing.

The results also show that olfactory dysfunction seems to be a reliable indicator of COVID-19 only for the initial strains of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).

Since the testing time of the study participants spanned over a year, it wasn’t possible to determine what proportions of SARS-CoV-2 variants being monitored (VBMs) dominated the tested samples. It is still undetermined whether olfactory dysfunction is also an early indicator of COVID-19 for the Delta and Omicron VBMs.

This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources