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As many as 1.6 million people in the U.S. may have chronic olfactory dysfunction because of SARS-CoV-2, a new analysis suggested.

Based on new daily cases of COVID-19, acute incidence of COVID olfactory problems, and rates of recovery, more than 700,000 people -- and possibly up to 1.6 million -- may have chronic olfactory dysfunction lasting 6 months or longer, reported Jay Piccirillo, MD, of Washington University School of Medicine in St. Louis, and co-authors.

"The addition of 0.7 to 1.6 million new cases of chronic olfactory dysfunction represents a 5.3% to 12% relative increase," the researchers wrote in .

"COVID-19 affects a younger demographic group than other causes of olfactory dysfunction," Piccirillo and co-authors pointed out. "Thus, the lifelong burden of olfactory dysfunction will be much greater for the COVID-19 cohort than for patients in the older age groups."

Acute olfactory dysfunction is more prevalent in mild forms of COVID-19 than moderate-to-critical disease. While 95% of patients recover their sense of smell at 6 months post-infection, the 5% who don't make up a large number of people, Piccirillo and colleagues noted.

"To our knowledge, no study on long-term COVID-19 olfactory dysfunction has measured long-term recovery beyond 6 months," they observed. "Therefore, the rate and trajectory of recovery for COVID-19 chronic olfactory dysfunction is not known."

For their analysis, Piccirillo and co-authors estimated the number of daily new cases of COVID-19 based on national data from the from Jan. 13, 2020, to March 7, 2021. They incorporated two other factors: the COVID-19 olfactory dysfunction at 52.7% (95% CI 29.6-75.2) based on a recent meta-analysis, and the from olfactory dysfunction at 95.3% (95% CI 92.6-98.0), based on a prospective study.

The researchers created three estimates of the cumulative frequency of COVID-19 chronic olfactory dysfunction based on the confidence intervals. Intermediate estimates showed 712,268 people in the U.S. were expected to develop chronic olfactory dysfunction by August 2021. The lowest estimate put the number at 170,238; the highest estimate was 1,600,241.

"To put this number in context, before the COVID-19 pandemic, the National Institute on Deafness and Other Communication Disorders estimated that, among U.S. adults 40 years or older, measurable olfactory dysfunction was found in up to 13.3 million adults," Piccirillo and colleagues wrote.

Olfactory training with visual stimulation may help some people with persistent olfactory dysfunction, noted Jerome Lechien, MD, PhD, of Paris Saclay University in France, who wasn't involved with the study. "It's important to adhere to a good olfactory training program over the weeks following the persistence of loss of smell," he told MedPage Today.

"The patient has to smell odors found in daily life -- such as coffee, spices, or chocolate -- two times a day, with their eyes open," Lechien said. "The training is long and difficult, but improves recovery."

This study has several limitations, the researchers acknowledged. State-reported figures likely underestimate the true number of positive cases. Estimates of olfactory dysfunction are from relatively healthy, ambulatory patients, and olfactory dysfunction incidence may be higher among people hospitalized with SARS-CoV-2, Piccirillo and co-authors said.

"These data suggest an emerging public health concern of olfactory dysfunction and the urgent need for research that focuses on treating COVID-19 chronic olfactory dysfunction," they wrote.

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