Los Angeles: About 23 per cent of people infected with the SARS-CoV-2 virus may develop long COVID with symptoms lasting for more than 12 weeks, according to a study. The research, published recently in the journal Scientific Reports, also identified predictors of who is likely to develop the sometimes-debilitating symptoms that can last for months.


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The study is unique because it accounts for preexisting symptoms such as fatigue and sneezing that are common to other conditions and may be mistaken for COVID symptoms, the researchers said.


"Long COVID is a major public health concern. Twenty-three per cent is a very high prevalence, and it may translate to millions of people," said study first author Qiao Wu, a doctoral candidate at the University of Southern California (USC) in the US.


"More knowledge on its prevalence, persistent symptoms and risk factors may help health care professionals allocate resources and services to help long haulers get back to normal lives," Wu said.


The study shows that obesity and hair loss at the time of infection are predictors of long COVID, but that other underlying conditions -- such as diabetes or smoking status -- have no discernable link to long-lasting symptoms.


While SARS-CoV-2 is typically an acute illness lasting about three weeks, some people with COVID-19 have symptoms that last months or longer.


The World Health Organization defines long COVID as symptoms that last 12 weeks or longer, a definition that the study's authors also used.


Estimates of the prevalence of long COVID range from 10 per cent to 90 per cent due to evolving diagnostic criteria and differences in study design, the researchers said.


For example, some studies have focused on hospitalised patients, which provided a limited perspective on long COVID in the broader population, they said.


The researchers used an internet-based national survey conducted by the Center for Economic and Social Research (CESR) at the USC with an estimated 8,000 respondents from across the US. From March 2020 to March 2021, researchers invited participants to answer biweekly questions about COVID.


Their final sample included 308 infected, non-hospitalised individuals who were interviewed one month before, around the time of infection and 12 weeks later.


After accounting for preexisting symptoms, about 23 per cent of the participants reported that they had experienced new-onset symptoms during infection which lasted for more than 12 weeks, meeting the study's definition of long COVID.


The chances of long COVID among people who experienced chest congestion were lower.


There was a lack of evidence relating the risk of long COVID to preexisting health conditions such as diabetes or asthma, or age, gender, race/ethnicity, education or current smoking status.


"The significant association between long COVID and obesity is consistent with previous studies," said Eileen Crimmins, a professor at USC.


"We differ from some existing studies in that we did not find a link between long COVID and any sociodemographic factors," Crimmins added.