As many as 30 per cent people infected with COVID-19 developed Long COVID, a set of symptoms that persist for months beyond the initial phase of a SARS-CoV-2 infection, according to a study conducted in the US.

Researchers from the University of California, Los Angeles (UCLA) in the US found that people with a history of hospitalisation, diabetes, and higher body mass index were mostly likely to develop Post Acute Sequelae of COVID-19 (PASC), commonly known as Long COVID.

The study, published in the Journal of General Internal Medicine, found that ethnicity, older age, and socioeconomic status were not associated with the syndrome even though those characteristics have been linked with severe illness and greater risk of death from COVID-19.

Of the 309 people with long COVID studied, the most persistent symptoms were fatigue and shortness of breath (31 per cent and 15 per cent, respectively) in hospitalised persons, and loss of sense of smell (16 per cent) in outpatients.

"This study illustrates the need to follow diverse patient populations longitudinally to understand the Long COVID disease trajectory and evaluate how individual factors such as pre-existing co-morbidities, sociodemographic factors, vaccination status and virus variant type affect type and persistence of Long COVID symptoms," said Sun Yoo, health sciences assistant clinical professor at UCLA.

"Studying outcomes in a single health system can minimise variation in quality of medical care," Yoo said in a statement.

The researchers wanted to evaluate the association of Long COVID with demographics and clinical characteristics in order to devise the most effective treatments.

They studied 1,038 people who were enrolled in the UCLA COVID Ambulatory Program between April 2020 and February 2021. Of those, 309 developed Long COVID.

A person was determined to have the syndrome if they reported persistent symptoms on questionnaires 60 or 90 days after infection or hospitalisation.

Potential weaknesses in the study include the subjective nature of how patients rated their symptoms, the limited number of symptoms the researchers evaluated, and limited information about patients' pre-existing conditions.

"Because persistent symptoms can be subjective in nature, we need better tools to accurately diagnose Long COVID and to differentiate it from exacerbations of other emerging or chronic conditions," said Yoo.

"Finally, we need to ensure equitable access to outpatient Long COVID care," Yoo added.