About 400 million people worldwide have been afflicted with long COVID-19, according to new research.

The estimated economic cost, stemming from factors like healthcare services and patients unable to return to work, is about $1 trillion globally each year, or about 1 percent of the global economy, according to the research published on Friday in the journal Nature Medicine.

The research aims to summarize the knowledge about and effects of long COVID-19 across the globe four years after it first emerged. It also seeks to "provide a roadmap for policy and research priorities," said one author, Ziyad Al-Aly, the chief of research and development at the V.A. St. Louis Health Care System and a clinical epidemiologist at Washington University in St. Louis.

Al-Aly wrote the paper with several other leading long COVID-19 researchers and three leaders of the Patient-Led Research Collaborative, an organization formed by long COVID-19 patients who are also professional researchers.

Other conclusions include that about 6 percent of adults globally have experienced long COVID-19; many people have not fully recovered; and treatment remains one of the biggest challenges.