Almost 90% of those who lost their taste and smell after falling ill with COVID-19 fully regained them or improved within a month. But about 10% of such cases never regained the two senses during the study’s observation period, highlighting long-term problems that the virus could bring.

An international team of researchers studied a group of 202 Italians that were infected but weren’t ill enough to be hospitalized. They were asked to rate their sense of smell or taste after being diagnosed and then again a month later through phone interviews. A six-point scale was used, scoring 0 for no problem and 5 for complete loss of the sense.

The loss of the sense of smell or taste was recognized as one of the core symptoms of people infected with the coronavirus. It was first observed in hospitalized patients but then also in those with mild disease. While understanding is limited, initial studies suggested that this could be caused by the disruption of cells that support olfactory neurons.

The study found that 113 of the patients – about two-thirds – reported loss of taste and/or smell up to two weeks before they tested positive. Of that total, 55 said they had recovered fully, 46 reported improvements in their symptoms and 12 said their symptoms were unchanged or worse.

“Although altered sense of smell or taste showed an improvement in most cases during the course of the disease, these symptoms were still the most frequently reported by patients with COVID-19 4 weeks after testing,” the researchers wrote. “However, the persistence of altered sense of smell or taste was not associated with the persistence of the SARS-CoV-2 infection at control swab”

The median length of time the patients went without being able to smell or taste at all was a little more than 11 days, according to the findings. There was no correlation between those patients who again tested positive for COVID-19 four weeks after their initial diagnosis and those who were still having trouble smelling or tasting at this point.

The researchers believe this means an active loss of one or both of those senses cannot be considered proof of an active COVID-19 infection, theorizing that it takes the body some time to “repair and regenerate” the senses regardless of whether the virus has left the body.

Age and sex were not found to be significant factors affecting the rate of recovery from anosmia or dysgeusia. However, those patients who reported the most significant losses of smell and/or taste also tended to report the worst recoveries after four weeks – again suggesting that it takes the body time to reverse the damage done to the senses by COVID-19.

“Even with a high rate of resolution, the staggering number affected by this evolving pandemic suggests an almost certain deluge of patients likely to present for the treatment of unresolved symptoms,” Dr. Joshua Levy of the Emory University School of Medicine in Atlanta and co-author. said in a statement

Levy suggested that in long-term cases, people could consider therapy such as smell-training to help restore the senses. UK Professor Claire Hopkins, one of the researchers behind the study, said the team is now conducting more research on people with long-lasting symptoms.

“For people who recover more quickly it is likely the virus has only affected the cells lining their nose,” she told the BBC. “For people who recover more slowly it may be that the virus has affected the nerves involved in smell, too. It can take longer for these nerve cells to repair and regenerate.”

The study was published in the journal JAMA.