About one in 20 people who contract the virus may suffer long-term damage to their senses, according to international estimates from a review of long COVID research, conducted by a team of experts at the National University of Singapore and published in the peer-reviewed medical journal, The BMJ, on Wednesday.
While the researchers noted that the studies relied on participants assessing their own ability to taste and smell and do not factor in different variants, they determined that loss of smell may persist in 5.6 percent of COVID-19 patients, while 4.4 percent may suffer long-term loss of taste.
Loss of taste or smell is acknowledged as one of the more common symptoms of COVID-19, alongside a cough, fever, sore throat, headache and fatigue. With more than 550 million confirmed infections worldwide to date, the research indicates at least 15 million and 12 million adults may experience-long term smell and taste deficiencies respectively.
Given the significant impact that loss of smell and taste can have on quality of life and general health and well-being, this could contribute to the rising burden of long COVID, warned the researchers.
Changes in smell and taste are common in people with COVID-19, with 40 to 50 percent of patients on average reporting these symptoms globally. However, little is known about the clinical course of these symptoms or how many patients develop persistent problems.
To address this knowledge gap, the international research team trawled databases for studies of adults with COVID-19-related changes to smell or taste and studies that described factors associated with these changes and time to recovery.
In all, 18 observational studies involving 3699 patients met their criteria. Four of the studies were conducted in the community setting and 14 studies in the hospital setting.
The researchers then used a mathematical technique known as cure modelling to estimate self-reported rates of smell and taste recovery and identify key factors associated with the duration and likelihood of recovery. The research was used to approximate how many people did not recover their sense of smell or taste after losing it during their infection.
The team found that 30 days after initial infection, only 74 percent of patients reported recovering their sense of smell, while 79 percent of patients reported recovering their ability to taste.
Recovery rates increased with each passing month, the researchers noted, reaching a peak of 96 percent for smell and 98 percent for taste after six months.
Women were less likely to recover their sense of smell and taste than men, while patients with nasal congestion and a more severe loss of smell were less likely to recover the sense. One patient the researchers spoke to said she has yet to recover her sense of smell, despite it being more than 27 months since her initial infection.
The researchers have acknowledged several limitations in their analysis. For instance, the included studies varied in quality and relied on self-reporting, which they said "may overestimate recovery, suggesting that the true burden of olfactory dysfunction is even greater".
However, this was a well-designed study with rigorous search methods, the authors noted, and the findings were unaltered after further analysis that excluded high-risk studies - suggesting that they are robust.
As such, the researchers said that while most patients are expected to recover their sense of smell or taste within the first three months, "a major group of patients might develop long lasting dysfunction that requires timely identification, personalised treatment, and long-term follow-up".
"Our findings are likely to be of substantial relevance to general doctors and otolaryngologists in the counselling of patients with smell and taste disorders post-COVID-19," they concluded.
In a linked editorial, experts cautioned that health systems are unprepared for the scale of the challenge the findings may present.
They warned that health leaders, policy makers and research funders "should realise the extraordinary importance of good chemosensory function for the wellbeing of humans, allocate adequate resources to support chemosensory research, and sustain medical specialists faced with an exceptional number of patients with smell and taste dysfunction".