Opinion | The mystery of long covid needs to be unraveled. We’re getting closer.

A strength of the Recover study is that it follows individuals from the time of first infection, an “acute cohort” that might be more accurate than studies that enroll people several months after infection. Also, it includes a group of uninfected individuals for comparison, which is key because many of the symptoms of long covid, such as fatigue, are felt by people with and without a past covid infection. The goal is to identify, as closely as possible, the symptoms that distinguish long covid.

An analysis of data from 9,764 participants identified 37 symptoms more often present six months or more after infection in those who caught covid, compared with those experienced by uninfected participants. A dozen such symptoms stood out: post-exercise malaise, fatigue, brain fog, dizziness, gastrointestinal symptoms, palpitations, changes in sexual desire or capacity, loss of or change in smell or taste, thirst, chronic cough, chest pain and abnormal movements. Other symptoms commonly reported included dry mouth, weakness, headaches, tremor, muscle and abdominal pain, fever, sweats, chills and sleep disturbances.

The researchers found that people who suffered infections in the early phase of the pandemic — before omicron — were more likely to have long covid. Among those who got the omicron variant, long covid was more prevalent among those who had been through multiple infections, compared with those who experienced just one. The researchers also found a modest reduction in long covid among those who were fully vaccinated.

Another question: How widespread is long covid? Different studies have provided varying results. A Dutch analysis of data collected before the delta and omicron waves, based on electronic health records, found the signs of long covid in about 1 in 8 people. Back then, few were vaccinated.

The Centers for Disease Control and Prevention found in a large study covering March 2020 to November 2021 that 1 in 5 adults from 18 to 64 years old who had covid, and 1 in 4 aged 65 years and over, experienced at least one persisting health condition related to their covid infection. The Recover report was based on self-reported symptoms and found 1 in 10 of the “acute cohort” first infected after Dec. 1, 2021 — during omicron — met the threshold for long covid. In this group, 92 percent were vaccinated.

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What all these studies suggest is that the long-covid problem could be quite sizable. Ten percent of the 662 million recovered cases worldwide would mean 66 million long-covid cases in the future. That suggests enormous economic costs are looming. Long covid might lead to changes in workplaces, economies and health care, and trigger cascading disability claims from workers who find they no longer have the stamina or good health they previously enjoyed. Those suffering from long covid might face not only unemployment but also lost health insurance to support their treatment.

A number of biomedical reports have suggested in recent months that the virus can spread throughout the body, including the brain, although it seems to reserve most of its damage for the respiratory system. The virus might cause long-term damage to the endothelial cells that line blood vessels, leading to persistent symptoms.

A committee of the National Academies is examining what should constitute a working definition of long covid. But that is just the start. A great deal of effort lies ahead to discover, diagnose and treat symptoms that persist in the wake of the covid catastrophe.