Loss of taste and smell may be most reliable COVID-19 symptoms for digital surveillance

Caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), as the coronavirus disease 2019 (COVID19) pandemic has evolved, diagnostic testing capacity expanded and guidelines also changed from testing for just fever and respiratory symptoms to testing for a wider range of symptoms. Several large scale, participatory digital surveillance platforms were also developed worldwide to complement the information obtained from laboratories and some clinical studies. These platforms offered a real-time understanding of community-wide epidemiology of COVID-19. In both clinical and syndromic surveillance analyses, loss of taste or ageusia and loss of smell or anosmia were identified as symptoms that can strongly predict COVID-19 infection. These symptoms have been used to tweak testing policy changes.

Determining consistent associations between COVID-19 symptoms and SARS-CoV-2 test status

A team of researchers from various institutions in London, Israel, and the USA recently tested the existence of consistent links between COVID-19 symptoms and SARS-CoV-2 test status in 3 national surveillance platforms. They sought to identify symptoms consistently linked to a positive SARS-CoV-2 test and those that are not over time and across 3 surveillance platforms in the US, the UK, and Israel. They also wanted to determine if inconsistencies could lead to a better understanding and inform future studies as the pandemic progresses. Their study has been published on the preprint server, medRxiv*.

The cross-sectional and longitudinal platforms used in this study are web and smartphone-based. The duration of the study was 4 months, starting  April 1 2020 and ending July 31 2020. It covered fluctuating COVID-19 prevalence at the end of the first wave and, in some places, the start of the second wave, targeting communities in Israel, UK, and the US.

Moreover, the study period overlaps the time of expansion of test access and test seeking. These analyses tracked and highlighted the significance of each symptom in predicting SARS-CoV-2 test positivity under a wide range of conditions. Logistic regression of self-reported symptoms on SARS-CoV-2 test status was adjusted for sex and age in each study cohort. Odds ratios over time were compared to testing policy changes and fluctuations in the incidence of COVID-19.

Anosmia/ageusia was consistently the strongest predictor of COVID-19 infection across all platforms

Despite differences in surveillance methodology, disease prevalence, and access to testing, loss of sense of smell (anosmia) or loss of taste (ageusia) was consistently found to be the strongest predictor of COVID-19 infection across all platforms over time, based on 658,325 tests (5% positive) from more than 10 million respondents in 3 digital surveillance platforms.

Overall, anosmia/ageusia was an order of magnitude more common among those reporting positive (US 43%, UK 44%, Israel 13.9%) compared to negative (US 5%, UK 3%, Israel 0.17%).”

As access to testing improved, the relevance of COVID symptoms, and the consistency of their ability to predictive became clear.

“Anosmia/ageusia was, at all times, across all platforms, the symptom with single highest OR for self-reported SARS-CoV-2 test positivity.”

However, confidence intervals widened with a decrease in COVID-19 incidence. Hence, in order to achieve robust COVID-19 prediction based on symptoms, models must consider surveillance data during higher incidence periods with improved test access, and effect estimates that replicate across various epidemiologic conditions and platforms.

These findings show the power of citizen science in the COVID-19 response.”

Findings demonstrate the generalizability of anosmia and ageusia as predictive symptoms
of COVID-19

The findings of the study confirm the validity of anosmia and ageusia as a reliable symptom of COVID-19, regardless of the surveillance platform used or the testing policy. This analysis shows that precise effect estimates and an understanding of testing patterns to interpret differences are best done only during high-incidence periods. The study results strongly support the need to keep testing access open for both public health utility and real-time epidemiologic investigations.

Future directions that build on these findings include the incorporation of cross-platform, spatio-temporal and multivariate effect estimates in the development of global prediction public health tools.”

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
Susha Cheriyedath

Written by

Susha Cheriyedath

Susha has a Bachelor of Science (B.Sc.) degree in Chemistry and Master of Science (M.Sc) degree in Biochemistry from the University of Calicut, India. She always had a keen interest in medical and health science. As part of her masters degree, she specialized in Biochemistry, with an emphasis on Microbiology, Physiology, Biotechnology, and Nutrition. In her spare time, she loves to cook up a storm in the kitchen with her super-messy baking experiments.

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