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FORECAST: loss of a sense of smell reliable indicator of Covid-19

Tue, 10/06/2020 - 12:07
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Researchers at the University College London and University College London Hospitals NHS Foundation Trust), London, UK, have found that the vast majority of participants with new onset loss of smell were seropositive for SARS-CoV-2 antibodies, and this acute loss of sense of smell needs to be considered globally as a criterion for self-isolation, testing and contact tracing in order to contain the spread of COVID-19. Indeed, the authors noted that the loss of a sense of smell may be a more reliable indicator of Covid-19 than cough or fever. The study published in the paper, ‘Seroprevalence of SARS-CoV-2 antibodies in people with an acute loss in their sense of smell and/or taste in a community-based population in London, UK: An observational cohort study’, in PLOS Medicine, is the first to accurately establish the proportion of those who had experienced loss of smell and or taste as having COVID-19.

Professor Rachel Batterham
Professor Rachel Batterham

“As we approach a second wave of infections, early recognition of Covid-19 symptoms by the public together with rapid self-isolation and testing will be of vital importance to limit the disease’s spread,” said lead author, Professor Rachel Batterham. “While people in the UK who experience sudden onset loss of smell or taste are advised to self-isolate and seek a test, at a global level few countries recognise this symptom as a Covid-19 indicator: most focus on fever and respiratory symptoms.”

The Finding Out if COVID-19 Infection Can be pREdicted by ChAnges in Smell and/or Taste (FORECAST) study, led by Professor Rachel Batterham, wanted to determine the seroprevalence of SARS-CoV-2 antibodies in a community-based population with acute loss of smell and/or taste and compare the frequency of COVID-19 associated symptoms in participants with and without SARS-CoV-2 antibodies. It also evaluated whether smell or taste loss are indicative of COVID-19 infection.

The observational cohort study looked at two cohorts of patients presenting with COVID-19: 1) a general public cohort, that investigated if new loss or reduced sense of smell and/or taste are early signs of COVID-19 and, 2) a hospital cohort, that investigated if taste/smell changes can predict the clinical course of a COVID-19 infection.

Using the unique Covid Symptom Study app, 33,650 text messages were sent by four London-based primary care centres inviting adults with loss of smell and/or taste in the preceding month to participate in the study from 23 April 2020 to 14 May 2020. A total of 590 participants enrolled via a web-based platform (developed and hosted by Dendrite Clinical Systems) and responded to questions capturing their sex, age, ethnicity, smoking status, previous COVID-19 testing, questions about their smell and taste symptoms, as well as other symptoms of COVID-19.

Participants were subsequently sent a point-of-care testing kit detecting the presence of immunoglobulin M (IgM) and immunoglobulin G (IgG) antibodies to SARS-CoV-2. Participants with a prior COVID-19 PCR test result were offered antibody testing irrespective of their result. Testing was carried out between 24 April 2020 and 22 May 2020.


Ninety percent (n 531) of participants reported loss of their sense of smell (complete smell loss by 69.9% (n=371) and as a partial smell loss by 30.1% (n=160)) and 89.8% (n=530) of participants reported loss of their sense of taste (complete taste loss by 47.4% (n=251) and partial by 52.6% (n=279)). Combined loss of smell and taste was reported by 80.0% (n=472).

Of 590 eligible participants, 567 participants (96.1%) underwent SARS-CoV-2 testing, with 77.4% (n=439) having a positive SARS-CoV-2 result (IgG (n=303), IgG and IgM (n=122) and IgM (n=14). One additional participant was included with a positive PCR result (77.6% (n = 440) positive). The researchers reported that participants with and without SARS-CoV-2 antibodies were comparable in terms of age, sex, ethnicity, smoking status, and frequency of other reported symptoms. Importantly, 52.1% (n=229) of the participants with SARS-CoV-2 antibodies had no history of cough, and 39.8% (n=175) had neither a fever nor a cough.

Among the participants who underwent SARS-CoV-2 antibody testing (n=567), 89.9% (n=510) reported loss of their sense of smell and 89.7% (n=509) taste loss. A total of 80.4% of participants with smell loss and 86.0% (n=307) of those with complete smell loss had a positive test result. A total of 77.8% of participants with taste loss and 86.0% of those with complete taste loss (n=209) had a positive test result.

Participants with loss of smell alone were nearly three times more likely than participants with isolated taste loss to have SARS-CoV-2 antibodies (p<0.001) and participants with combined loss of smell and taste were four times more likely to have SARS-CoV-2 antibodies (p<0.001). These findings remained unchanged after adjusting for sex, age, ethnicity and smoking status.

“Our research suggests a key public health message should be: people who notice a loss in their ability to smell everyday household odours such as garlic, onions, coffee, and perfumes should self-isolate and seek a coronavirus PCR swab test,” added Professor Batterham. “Our findings show that loss of smell and taste is a highly reliable indicator that someone is likely to have Covid-19 and if we are to reduce the spread of this pandemic, it should now be considered by governments globally as a criterion for self-isolation, testing, and contact tracing.”

The study was funded by the National Institute for Health Research Biomedical Research Centre at UCLH.

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