More Evidence Backs Loss of Smell as an Early Symptom of COVID-19

April 20, 2020

Study authors conducted a meta-analysis to determine how prevalent loss of smell was as a predictor for COVID-19.

For some, loss of smell is an increasingly recognized early symptom of coronavirus disease 2019 (COVID-19), according to a paper published in Laryngoscope.

Investigators from the University of Cincinnati conducted a review of literature regarding SARS-CoV-2 published in English or Chinese through the end of March to synthesize existing evidence about how loss of smell is related to COVID-19. The team found 19 studies that were suitable to their analysis, which included 14 published studies, 3 preprints, and 2 communicated studies.

“COVID-19 is not associated with the symptoms that are typically associated with a viral cold such as nasal blockage or mucus production,” Ahmad Sedaghat, MD, PhD, an associate professor in the University of Cincinnati College of Medicine's Department of Otolaryngology-Head and Neck Surgery and an UC Health physician specializing in diseases of the nose and sinuses, said in a press release.

“This distinction is also why it is fairly easy to distinguish COVID-19 from seasonal allergies.”

The investigators wrote that most of the virus’s transmission is through the upper respiratory tract (as was the case during the SARS-CoV-1 outbreak) but that nasal shedding of the live virus can occur early during the course of COVID-19. Shedding may even continue after the virus is no longer detected in the lower respiratory tract, they continued. One of the papers they analyzed showed higher viral load in nasal swabs when compared to throat swabs in a study of 17 patients.

Additionally, another team of investigators demonstrated the presence of risk for health care-associated infection posed to health care workers via nasal transmission during rhinology procedures.

During a single endoscopic pituitary case procedure in Wuhan, China—the epicenter of the coronavirus outbreak —up to 14 health care-associated infections were reported.

“At present, rhinology and otolaryngology societies from around the world are recommending that endonasal surgeries be approached as high‐risk procedures,” the study authors wrote.

The study authors also discovered a paper which detailed olfactory dysfunction as a symptom of COVID-19 out of Paris, where 55 patients presented with anosmia without nasal obstruction within 7 days of the occurrence of this symptom. Almost all of the patients were found to be COVID-19-positive via nasopharyngeal swabs and PCR testing. Those investigators found that the sense of smell begins to return between 5-10 days, but anosmia may persist longer in others.

The authors conducting the meta-analysis wrote that COVID-19-affected patients may have missed their anosmia, though, and it may be a uniquely described viral phenomenon in COVID-19. Still, it may be a predictive marker for COVID-19, especially in mild cases or patients who are otherwise asymptomatic and may not otherwise think they are infected, they added.

“The occurrence of sudden onset anosmia without nasal obstruction is highly predictive of COVID-19 and should trigger the individual to immediately self-quarantine with presumptive COVID-19,” Sedaghat said.

And as at least 90% of inhaled air — which may carry infected virus particles – enters the body through the nose, Sedaghat added, the nasal cavity is likely the major site of infection.

In their conclusion, the study authors noted that decreased sense of smell “may be an underappreciated symptom of COVID-19.”