Delta variant

Is loss of sense of smell or taste much less common in Omicron-related Covid-19 compared to earlier strains of SARS-CoV-2?

FA Manian MD, MPH, , , , , , , , , , , , Leave a comment

Absolutely! Although loss of smell was a cardinal symptom of Covid-19 with earlier strains of SARS-CoV-2 (eg, Wuhan, alpha, delta), on average omicron causes olfactory dysfunction in only 13% of patients, 3-4 times lower than the earlier strains.1

But why is omicron less likely to causes loss of smell or taste? There may be at least 2 explanations. First explanation revolves around the solubility of omicron in the olfactory mucus. Recall that to access the olfactory epithelium, viruses and other pathogens have to first dissolve in and penetrate the mucus layer that not only allows odorants to reach the olfactory receptors but also protects the olfactory epithelium from toxins and pathogens. Hydrophilic and acid proteins can penetrate the mucus barrier more easily because they are more soluble in the mucus layer.1

What does this have to do with omicron? Well, it turns out that omicron with all its mutations in the spike protein is actually more alkaline than the Wuhan and delta strains. This means that omicron may have lower solubility in mucus and have a harder time reaching and infecting the olfactory epithelium. 1 Since the composition of olfactory mucous differs significantly from other mucus layers in the respiratory tract, omicron may still cause disease.2

Another potential mechanism may be related to the inefficiency of omicron in other steps necessary to infect nonneuronal cells of the olfactory epithelium within the nasal cavity, such as the endosomal route. 1 It turns out that cells of the olfactory epithelium express less of the endosomal membrane fusion proteases (cathepsins) which omicron prefers for cell entry! Fascinating! 

Bonus Pearl: Did you know that only 5-10% of functional olfactory neurons are required for a relatively normal sense of smell? This means that SARS-CoV-2 needs to eliminate at least 90% of all support cells of the olfactory neurons within a 3-4 day period (before their regeneration) for the host to notice anosmia?

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References

  1. Butowt R, Bilinska K, von Bartheld C. Why does the omicron variant largely spare olfactory function? Implications for the pathogenesis of anosmia in coronavirus disease 2019. J Infect Dis 2022;226:1304-1308. Why Does the Omicron Variant Largely Spare Olfactory Function? Implications for the Pathogenesis of Anosmia in Coronavirus Disease 2019 – PubMed (nih.gov)
  2. Yoshikawa K, Wang H, Jaen C, et al. The human olfactory cleft mucus proteome and its age-related changes. Sci Rep 2018;8:17170. The human olfactory cleft mucus proteome and its age-related changes – PMC (nih.gov)
  3. Harding JW, Getchell TV, Margolis FL. Degeneration of the primary olfactory pathway in mice. V. Long-term effect of intranasal ZNS04 irrigation on behavior, biochemistry and morphology. Brain Res 1978;140:271-85. Denervation of the primary olfactory pathway in mice. V. Long-term effect of intranasal ZnSO4 irrigation on behavior, biochemistry and morphology – PubMed (nih.gov)

Disclosures: The listed questions and answers are solely the responsibility of the author and do not necessarily represent the official views of Mercy Hospital-St. Louis, Massachusetts General Hospital, Harvard Catalyst, Harvard University, their affiliate academic healthcare centers, or its contributors. Although every effort has been made to provide accurate information, the author is far from being perfect. The reader is urged to verify the content of the material with other sources as deemed appropriate and exercise clinical judgment in the interpretation and application of the information provided herein. No responsibility for an adverse outcome or guarantees for a favorable clinical result is assumed by the author. Thank you!

Is loss of sense of smell or taste much less common in Omicron-related Covid-19 compared to earlier strains of SARS-CoV-2?

Why is the Delta variant of SARS-CoV-2 increasingly becoming a “variant of concern” in the current Covid-19 pandemic?

FA Manian MD, MPH, , , , , , , , , , , , , , , Leave a comment

The Delta variant (B.1.617.2, formerly India variant) has become an increasingly prevalent strain of SARS-Cov-2 causing Covid-19 in many countries outside of India, including the United States and United Kingdom, particularly affecting younger unvaccinated persons.  Several features of the Delta variant are of particular concern. 1-7

  1. Delta virus appears to be more transmissible when compared to previously emerged variant viruses. Data from new Public Health England (PHE) research suggests that the Delta variant is associated with a 64% increased risk of household transmission compared with the Alpha variant (B.,1.1.7, UK variant) and 40% more transmissibility in outdoors. 1,8  
  2. Delta virus is also associated with a higher rate of severe disease, doubling the risk of hospitalization based on preliminary data from Scotland. In vitro, it replicates more efficiently than the Alpha variant with higher respiratory viral loads.5
  3. Delta virus may also be associated with reduced vaccine effectiveness with increased vaccine breakthroughs. One study found that Delta variant is 6.8-fold more resistant to neutralization by sera from Covid-19 convalescent and mRNA vaccinated individuals.5 Fortunately, a pre-print study released by PHE in May 2021 found that 2 doses of the Pfizer vaccine were still 88% effective against symptomatic infection with Delta variant  (vs 93% for the Alpha variant) and 96% effective against hospitalization; 1 dose was only 33% effective against symptomatic disease (vs 50% for the Alpha variant). 6   Two doses of Astra Zeneca vaccine were 60% effective against symptomatic disease from the Delta variant.8 
  4. Aside from its somewhat unique epidemiologic features, Covid-19 caused by Delta variant seems to be behaving differently (starting out as a “bad cold” or “off feeling”), with top symptoms of headache, followed by runny nose and sore throat with less frequent fever and cough; loss of sense of smell was not common at all based on reported data to date.1

What the Delta variant reminds us is, again, the importance of vaccination, masks and social distancing. The pandemic is still with us!

Bonus Pearl: Did you know that, on average, a Delta variant-infected person may transmit it to 6 other contacts (Ro~6.0) compared to 3 others (Ro~3) for the original SARS-CoV-2 strains found during the early part of the pandemic?1

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References

  1. https://www.bbc.com/news/health-57467051
  2. Knodell R. Health Advisory: Emergence of Delta variant of coronavirus causing Covid-19 in USA. Missouri Department of Health & Senior Services. 23 June, 2021. https://health.mo.gov/emergencies/ert/alertsadvisories/pdf/update62321.pdf
  3. Kupferschmidt K, Wadman M. Delta variant triggers new phase in the pandemic. Science 25 June 2021; 372:1375-76. https://science.sciencemag.org/content/sci/372/6549/1375.full.pdf
  4. Sheikh A, McMenamin J, Taylor B, et al. SARS-CoV-2 Delta VOC in Scotland: demographics, risk of hospital admission, and vaccine effectiveness. Lancet 2021; 397:2461-2. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8201647/
  5. Mlcochova P, Kemp S, Dhar MS, et al. Sars-Cov-2 B.1.617.2 Delta variant emergence and vaccine breakthrough. In Review Nature portfolio, posted 22 June, 2021. https://www.researchsquare.com/article/rs-637724/v1
  6. Bernal JL, Andrews N, Gower C, et al. Effectiveness of Covid-19 vaccines against the B.1.617.2 variant. MedRxiv, posted May 24, 2021. https://www.medrxiv.org/content/10.1101/2021.05.22.21257658v1 vaccine efficacy
  7. Allen H, Vusirikala A, Flannagan J, et al. Increased household transmission of Covid-19 cases associatd with SARS-Cov-2 variant of concern B.1.617.2: a national case control study. Public Health England. 2021. https://khub.net/documents/135939561/405676950/Increased+Household+Transmission+of+COVID-19+Cases+-+national+case+study.pdf/7f7764fb-ecb0-da31-77b3-b1a8ef7be9aa  Accessed June 27, 2021.
  8. Callaway E. Delta coronavirus variant: scientists brace for impact. Nature. 22 June 2021. https://www.nature.com/articles/d41586-021-01696-3 

Disclosures: The listed questions and answers are solely the responsibility of the author and do not necessarily represent the official views of Mercy Hospital-St. Louis or its affiliate healthcare centers, Mass General Hospital, Harvard Medical School or its affiliated institutions. Although every effort has been made to provide accurate information, the author is far from being perfect. The reader is urged to verify the content of the material with other sources as deemed appropriate and exercise clinical judgment in the interpretation and application of the information provided herein. No responsibility for an adverse outcome or guarantees for a favorable clinical result is assumed by the author.

Why is the Delta variant of SARS-CoV-2 increasingly becoming a “variant of concern” in the current Covid-19 pandemic?