The U.S. National Institutes of Health's $1 billion RECOVER Initiative has picked Pfizer Inc's antiviral drug Paxlovid as the first treatment it will study in patients with long COVID, organizers of the study said on Thursday.

The complex medical condition involves more than 200 symptoms ranging from exhaustion and cognitive impairment to pain, fever and heart palpitations that can last for months and even years following a COVID-19 infection.

According to details of the study, posted on Clinicaltrials.gov, the randomized, placebo-controlled trial will test Pfizer's treatment or a placebo in 1,700 volunteers aged 18 and older.

The Duke Clinical Research Institute is supervising the study, which is scheduled to start on Jan. 1.

The trial will investigate a leading theory of the cause of long COVID-19, which holds that fragments of the virus persist in the tissues of some individuals, causing prolonged symptoms.

Patients in several case studies have reported improvements in their symptoms after taking Pfizer's antiviral treatment, and several physicians have called for the drug to be studied in a large, scientifically rigorous study in patients with long COVID-19.

Paxlovid, which combines a new Pfizer pill with the old antiviral ritonavir, is currently authorized for use in the first days of a COVID-19 infection to prevent severe disease in high-risk patients.

Estimates of long COVID-19 prevalence range from 5 to 50% of people who have had a COVID-19 infection. It affects people who have had both mild and severe COVID-19, including children, and can be severe enough to keep people out of work.

Pfizer did not immediately respond to a request for comment.