Since the beginning of the pandemic, a sizable minority of coronavirus infections have resulted in long COVID, a perplexing collection of lingering and often disabling symptoms that persist weeks, months, or years after the initial infection.

The condition has been reported in both children and adults; in those with preexisting conditions and those in good health; in COVID-19 patients hospitalized; and in those who had only mild symptoms during their initial infection.

A new study from USC researchers sheds some light on the prevalence of long COVID and provides some early indicators of who is more likely to develop long-term symptoms. The study, which was published in Scientific Reports this month, also discovered that 23% of people who had coronavirus infections between March 2020 and March 2021 were still reporting symptoms up to 12 weeks later.

Approximately 8,000 people, some infected and some not, were recruited to answer biweekly questions about their overall health and COVID-19 status. They had a sample of 308 people who had gotten the disease at some point during the year by the end of the yearlong survey period.

After excluding respondents who experienced symptoms such as headache and fatigue prior to infection as a result of unrelated conditions such as seasonal allergies, the researchers discovered that nearly one in every four COVID-19 patients was still experiencing symptoms 12 weeks after becoming infected.

“These people are not necessarily able to do all of the activities they would like to do, not able to fully work and take care of their families,” said Eileen Crimmins, a demographer at USC’s Leonard Davis School of Gerontology and study co-author.

Demographers and healthcare providers have struggled to determine who is at higher risk for long-term COVID.

Several previous studies have found that women are more vulnerable. However, the USC study found no correlation between long COVID and age, gender, race, or preexisting health conditions such as cancer, diabetes, hypertension, or heart disease in its sample.

It did note an increased risk in patients who were obese prior to infection. It also discovered some links between certain symptoms people experienced during their initial infection and the likelihood of developing long COVID. Patients who reported sore throats, headaches, and, interestingly, hair loss after testing positive were more likely to have symptoms that lasted months later.

The study does not account for two major developments: vaccines and variants, because it only covers the first year of the pandemic. During the study period, none of the COVID-19 patients in the sample were eligible for vaccines, and all were infected before the Alpha variant from the United Kingdom arrived in the United States.

While the study’s 308 participants were representative of the population, no snapshot of a few hundred people can tell the entire story of the approximately 200 million people in the United States who have had the virus, according to CDC estimates.

Headache, nasal congestion, abdominal pain, fatigue, and diarrhea were the most commonly reported long COVID symptoms. However, the study did not address many of the symptoms that people with long COVID describe as the most debilitating, according to Hannah Davis, a co-founder of the Patient-Led Research Collaborative, a research group devoted to the condition.

Those attempting to track or treat long COVID face difficulties in defining it. COVID-19 is a chimerical beast; symptoms evolve over time and vary greatly between patients.

The fluidity of long COVID makes estimating its prevalence difficult. According to various studies, the percentage of people reporting persistent symptoms 12 weeks after their initial infection ranges from 3% to 50%.

The lack of strict diagnostic criteria is another major issue for patients seeking treatment. According to Melissa Pinto, an associate professor of nursing at UC Irvine who studies the condition, long COVID is currently considered a “exclusionary diagnosis,” meaning one that is given only after all other valid possibilities have been ruled out. In the United States, this can entail a lengthy and costly process of submitting to various tests and specialists.

For many long-term COVID patients, 12 weeks marks the start of a months- or years-long ordeal.