PARIS, France -- Treating high-risk COVID-19 patients with the antidepressant fluvoxamine may reduce the risk of prolonged hospitalisation by up to a third, a large-scale study showed Thursday.

Authors said the research could help boost low-cost protection against severe sickness or death in countries that have yet to receive adequate vaccine doses during a grossly uneven rollout.

Fluvoxamine is traditionally used to treat mental health conditions such as depression and obsessive-compulsive disorders and was selected for trial due to its anti-inflammatory properties.

Many problems stemming from COVID are caused by swelling as the immune system over-reacts to the infection.

Writing in the journal The Lancet Public Health, researchers from North and South America, described results in nearly 1,500 COVID-19 outpatients in Brazil.

Of the 741 people that received fluvoxamine, 79 -- just over 10 percent -- had an extended stay in hospital.

Of the 756 who received a placebo, 119 (15.7 percent) were hospitalised.

Authors said that administering fluvoxamine resulted in a relative reduction in hospitalisations of 32 percent.

"COVID-19 still poses a risk to individuals in countries with low resources and limited access to vaccinations," said Edward Mills of McMaster University, co-principal investigator on the trial.

"Identifying inexpensive, widely available, and effective therapies against COVID-19 is therefore of great importance, and repurposing existing medications that are widely available and have well-understood safety profiles is of particular interest."

Although reducing deaths was not an intended area of focus, the study also found that 12 patients in the placebo group ended up dying, while just one from the fluvoxamine group succumbed to the virus.

The authors stressed that further evaluation is needed as fluvoxamine isn't on the World Health Organization's list of essential medicines and can be addictive.

"Given fluvoxamine's safety, tolerability, ease of use, low cost, and widespread availability, these findings may have an important influence on national and international guidelines on clinical management of COVID-19," said Gilmar Reis, study co-lead, based in Belo Horizonte, Brazil.