Smell and Taste Changes: The Best Way to Guide Lockdown Policy?

by Tristan Wild, posted in Demystified

For the original article, please follow this link.

This article was reviewed by Dr. Emily Bowman, one of our subject matter experts.

Brief Summary: 

This study by Denis Pierron et al. investigated the reliability of self-reported smell and taste change numbers across France as an indicator to predict surges of COVID-19 hospital admissions and a basis for lockdown-related political decision-making. They found that smell and taste change report numbers were an earlier and more reliable indicator of surges in hospital admissions than currently used government indicators.

Research Re-hashed

Scientists around the world, and the governments that employ them, have been desperately trying to avoid severe strain on their healthcare systems due to surges of COVID-19 cases. Since the beginning of the pandemic, modelling disease spread using certain indicators has been essential for the planning process and, in some cases, has allowed for timely implementation of lockdowns and other measures. In this study, a group of French researchers evaluate the effectiveness of smell and taste change report numbers as an indicator to predict surges of COVID-19 hospital admissions and as a basis for lockdown-related political decision-making. 

During the pandemic, the French government classified areas as green (reducing strict lockdown policy) or red (keeping strict lockdown policy) based on three indicators: 

  1. Levels of SARS-CoV-2 active circulation, using the ratio of suspected COVID-19 consultations to general ER consultations to evaluate this.  
  2. Hospital strain  (CCRU – Critical Care Resuscitation Unit occupancy) 
  3. Testing capacity, based on the diagnostic tests availability
Areas Classified Green (reducing strict lockdown policy) or Red (keeping strict lockdown policy) across France (May 11th, 2020). Image obtained from: Pierron, D., Pereda-Loth, V., Mantel, M., et al. Nat Commun 11, 5152 (2020). https://doi.org/10.1038/s41467-020-18963-y (Fig. 1) 

These indicators have proven useful, but they work in hindsight as opposed to being predictive. As the researchers of this study and countless others have pointed out, smell and taste changes (i.e. the loss of smell and taste) are common early-onset symptoms that are more specific to COVID-19 than other symptoms like dry cough and fever.  Intuitively, these researchers decided to look at past data to determine whether self-reported smell and taste change numbers could be a reliable early indicator of COVID-19 hospital admission spikes and drops.

Data in each of the green and red areas were generated during March 1st to May 11th, 2020. In the graph below, the percentage of inhabitants (3832 subjects total) reporting smell or taste changes is on the x-axis (horizontal), and the percentage of inhabitants admitted to the CCRU is on the y-axis. The two variables are strongly related, meaning that areas with high numbers of smell/taste change reports also had a high percentages of CCRU admissions and vice versa. 

Image obtained from: Pierron, D., Pereda-Loth, V., Mantel, M., et al. Nat Commun 11, 5152 (2020). https://doi.org/10.1038/s41467-020-18963-y (Fig. 1)

The degree to which self-reported smell and taste change numbers are related to CCRU admissions (red bars) is actually stronger than for previous government indicators (blue bars) in both metropolitan regions (Reg) and Departments (Dep). Departments are the name for administrative regions in France, similar to provinces and territories in Canada or states in the United States. The correlation coefficient (y-axis) is a measure of the strength of the relationship (the higher the bar, the stronger the relationship). 

Image obtained from: Pierron, D., Pereda-Loth, V., Mantel, M., et al. Smell and taste changes are early indicators of the COVID-19 pandemic and political decision effectiveness. Nat Commun 11, 5152 (2020). https://doi.org/10.1038/s41467-020-18963-y (Fig. 1)

Not only were self-reported smell and taste change numbers a stronger indicator than the established government indicators, they also can provide warning of COVID-19 admissions surges a week in advance. Take the graph shown below. The peak of smell and taste reports (shown by the blue line) occurred 4 days after the lockdown, which was a whole week earlier than the peak of the ratio of suspected COVID-19 consultations to general ER consultations (a government indicator, shown by the dashed orange line). The lockdown, peak in smell/taste change reporting, and peak in the government ration indicator occurred March 17th, March 21st, and March 28th respectively. 

Not only were self-reported smell and taste change numbers a stronger indicator than the established government indicators, they also can provide warning of COVID-19 admissions surges a week in advance. Take the graph shown below. The peak of smell and taste reports (shown by the blue line) occurred 4 days after the lockdown, which was a whole week earlier than the peak of the ratio of suspected COVID-19 consultations to general ER consultations (a government indicator, shown by the dashed orange line). The lockdown, peak in smell/taste change reporting, and peak in the government ration indicator occurred March 17th, March 21st, and March 28th respectively. 

Image obtained from: Pierron, D., Pereda-Loth, V., Mantel, M., et al. Smell and taste changes are early indicators of the COVID-19 pandemic and political decision effectiveness. Nat Commun 11, 5152 (2020). https://doi.org/10.1038/s41467-020-18963-y (Fig. 1)

Interestingly, trends in Google searches followed this trend as well. One day after lockdown (March 18th), there was a peak of Google searches related to early, non-specific COVID-19 symptoms (fever, dry cough, body aches). On March 21st, searches related to “smell loss” and “taste loss” peaked. One week after on March 28thcame the spike in searches related to “shortness of breath,” the symptom often proceeding CCRU admissions which peaked March 31st.

Then, the researchers looked at the relationship between strict lockdown procedures and confirmed COVID-19 cases (red dotted lines represent the daily proportion of confirmed COVID-19 cases) in France, Italy, and the United Kingdom. As shown below, a decrease in reports of smell and taste change corresponds to increasing strict government lockdown policy (measured by a stringency index, with green representing less strict to red representing more strict). The United Kingdom had the least strict lockdown policy which was also implemented the latest out of the three countries; corresponding to this, the decrease in smell and taste reporting (blue line) is less prominent in the UK (peaking at week 12 and actually re-peaking at week 15). 

Image obtained from: Pierron, D., Pereda-Loth, V., Mantel, M., et al. Smell and taste changes are early indicators of the COVID-19 pandemic and political decision effectiveness. Nat Commun 11, 5152 (2020). https://doi.org/10.1038/s41467-020-18963-y (Fig. 2)

Summary: Findings, Limitations, and Implications

Using data from various reliable sources, such as the European Centre for Disease Prevention and Control (ECDC), these researchers made several different conclusions that could have significant implications on the ways countries approach this pandemic. They were successful in demonstrating that: 

(1) Increases in smell and taste change self-reporting has a stronger relationship with the number of CCRU admissions than established government indicators. 

(2) Increases in the smell and taste change self-reporting precedes increases in government indicators (i.e. the ratio of suspected COVID-19 consultations to general ER consultations) by around a week. 

(3) The daily proportion of self-reported smell and taste change correlates to government policy strictness.

The increased timing and correlation of self-reported smell and taste change numbers show that it may be a superior method of predicting impending surges of COVID-19 hospital admissions. A week of advanced notice is a considerable amount of time for governments in positions where swift and challenging decisions need to be made to ensure its people’s safety.

The research was published on October 14th, coming at a time when many countries, like Canada, are experiencing subsequent waves of COVID-19 cases; findings like this will surely help countries make more proactive decisions during these instances of uncertainty. Many countries already have online systems to self-report symptoms, including smell and taste loss. Using this data is a practical and low-cost method to add to the arsenal of tools government health officials have.

Of course, this research was done in France, Italy, and the United Kingdom, all developed countries where its citizens have access to online websites, mobile applications and testing centres to report symptoms. These Westernized countries also have strongly-equipped scientific communities required to collect and interpret this data. Therefore, analyzing self-reported taste and smell loss numbers may not be the most reliable and practical strategy for low-income countries. Furthermore, many of us are experiencing “pandemic fatigue” and are currently less fearful of COVID-19 than the start of the infection — this has implications in fewer people reporting symptoms, which could weaken the relationships mentioned above between smell and taste change self-reporting and hospital admissions.

No study is without its limitations, and these findings are incredibly promising for countries that are closely tracking symptom reports. In conclusion, if you are experiencing symptoms, please report, monitor, and get tested. Not only will it help you protect your family and friends, but it will also help government health officials make more proactive and informed decisions regarding the safety of your larger community. 

References

  1. Pierron, D., Pereda-Loth, V., Mantel, M., et al. Smell and taste changes are early indicators of the COVID-19 pandemic and political decision effectiveness. Nat Commun 11, 5152 (2020). https://doi.org/10.1038/s41467-020-18963-y

Published by Tristan Wild

I am a fourth-year student studying Microbiology and Immunology at Western University. My educational background combined with the experience of interning at Sanofi Pasteur, the world's largest vaccine manufacturer, has garnered my interest in infectious disease and medical interventions. It is my hope that the articles my fellow peers and I write give you, the reader, some well-needed understanding about the science that is often misrepresented on news and social media.

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