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Long COVID symptoms may emerge months after SARS-CoV-2 infection, data from the prospective multicenter study suggested.

Symptom prevalence decreased over 1 year among long COVID patients, but persisted or emerged at different time points in some cases, reported Sharon Saydah, PhD, of the CDC's National Center for Immunization and Respiratory Diseases, and co-authors in the .

For about 16% of study participants, symptoms lasted 12 months after their initial SARS-CoV-2 test. At 3, 6, 9, and 12 months after testing, some people had ongoing symptoms, while others had emerging symptoms not reported previously.

"It was common for symptoms to resolve then re-emerge months later," noted co-author Juan Carlos Montoy, MD, PhD, of the University of California San Francisco.

"A lot of prior research has focused on symptoms at one or two points in time, but we were able to describe symptom trajectory with greater clarity and nuance," Montoy said in a statement. "It suggests that measurements at a single point in time could underestimate or mischaracterizes the true burden of disease."

INSPIRE was designed to assess long-term symptoms and outcomes among people with COVID-like illness who had a positive or negative SARS-CoV-2 test result at study enrollment. Participants who completed baseline and 3-, 6-, 9-, and 12-month surveys were included to identify emerging and ongoing symptoms.

A total of 1,741 people completed all quarterly surveys through 12 months, including 1,288 COVID test-positive and 453 COVID test-negative participants. Most participants were female.

Outcomes included self-reported symptoms in eight categories: extreme fatigue; cognitive difficulties; cardiovascular; pulmonary; musculoskeletal; gastrointestinal; constitutional; or head, eyes, ears, nose, and throat.

The prevalence of any symptom decreased substantially from baseline to 3-month follow-up -- from 98.4% to 48.2% for COVID-positive participants, and from 88.2% to 36.6% for COVID-negative participants.

Persistent symptoms decreased over the year. Emerging symptoms were reported for every symptom category at each follow-up period for both groups.

At 12 months, symptom prevalence was similar between groups, at 18.3% in the COVID-positive group and 16.1% in the COVID-negative group (P>0.05).

"We were surprised to see how similar the patterns were between the COVID-positive and COVID-negative groups," Montoy noted. "It shows that the burden after COVID may be high, but it might also be high for other non-COVID illnesses. We have a lot to learn about post-illness processes for COVID and other conditions."

In other research published in the , a national survey showed the prevalence of long COVID fell to 6.0%.

The survey also found that one in four people with long COVID (26.4%) had significant activity limitations, reported Nicole Ford, PhD, of the CDC's National Center for Immunization and Respiratory Diseases, and co-authors. The findings came from the Census Bureau's Household Pulse Survey from June 1-13, 2022 to June 7-19, 2023.

Among people who reported a history of previous SARS-CoV-2 infection, long COVID prevalence fell from 18.9% in 2022 to 11% in 2023. In the overall U.S. population -- irrespective of history of previous COVID-19 -- the prevalence of long COVID dropped from 7.5% to 6.0%.

Among both groups, prevalence declined from June 2022 through January 2023 before stabilizing.

The percentage of people with significant activity limitations didn't change over time, the researchers said. Only adults under age 60 experienced significant rates of decline (P<0.01).

"These findings highlight the importance of COVID prevention, including staying up to date with recommended COVID-19 vaccination, and could inform healthcare service needs planning, disability policy, and other support services for persons experiencing severe activity limitation from long COVID," Ford and colleagues wrote.

"Limited ability to carry out day-to-day activities because of long COVID symptoms can have a significant impact on quality of life, functional status, and ability to work or provide care to others," they added. "Long COVID in U.S. adults has also been associated with lower likelihood of working full time and higher likelihood of being unemployed."

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