Half of hospitalized COVID-19 patients developed long COVID

By Dr. Sanchari Sinha Dutta, Ph.D.Reviewed by Lauren HardakerDec 15 2025

A two-year study from Thailand reveals how often long COVID follows hospitalization, how long symptoms can persist, and why vaccination and disease severity make a critical difference to patients’ long-term quality of life.

Study: Prevalence and impact of long COVID on health-related quality of life in previously hospitalized COVID-19 patients: a 2-year follow-up study. Image credit: MW Shutter/Shutterstock.com

A new study published in Scientific Reports finds a high prevalence of long-COVID among patients in Thailand who had been hospitalized due to symptomatic coronavirus disease 2019 (COVID-19) in 2021.

Long-term COVID symptoms persist years after infection

The COVID-19 pandemic, caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has placed a significant burden on the global healthcare system since its emergence in 2019. The threat is continuing even in 2025, with a significant proportion of previously infected individuals still experiencing persisting symptoms of COVID-19, a condition medically termed as long-COVID.

The World Health Organization (WHO) defines long-COVID as the continuation or development of new symptoms three months after the initial SARS-CoV-2 infection, with these symptoms lasting for at least two months with no other explanation.        

The global prevalence of long-COVID has been estimated to be approximately 36 % to 43 %, with higher prevalence observed among hospitalized COVID-19 patients. Studies investigating long-COVID risk factors have identified female gender and acute COVID-19 severity as major predictors and COVID-19 vaccination as a major protector.

Studies analyzing the prevalence, predictors, and outcomes of long COVID have mostly been conducted in high-income countries. In low- and middle-income countries, especially in Southeast Asia, evidence is limited. The reported prevalence of long-COVID in Southeast Asia ranges from 3.4 % to 47 %. However, most of the studies lack long-term follow-up.

Given this gap in the literature, researchers at Mahidol University conducted a study in Thailand to determine the prevalence and predictors of long COVID-19 among hospitalized patients.

Tracking long COVID symptoms after hospital discharge

The study population included a total of 295 adult patients who were admitted to medical wards and intensive care units for symptomatic COVID-19 between August and November 2021.

The patients were followed up through telephonic interviews at three months, one year, and two years post-infection. During the interviews, patients were asked about long-COVID symptoms and whether these symptoms affected their daily living.

Patients’ demographic characteristics, such as gender, age at infection, body mass index, comorbidities, and vaccination status, as well as clinical information during hospitalization, including disease severity, were analyzed to identify predictors of long COVID.

Symptoms persisted up to two years post-infection

The study's estimations revealed that the prevalence of long-COVID among hospitalized patients was 49.8 % three months after acute SARS-CoV-2 infection. The most common symptoms were breathlessness, hair loss, fatigue, muscle pain, and chest pain.

The symptoms of long COVID persisted for one year among 64 % of patients who reported long COVID at three months, and for two years among 22 % of patients who remained symptomatic at one year, highlighting the conditional nature of long-term persistence. Notably, the study found an increasing prevalence of neurocognitive symptoms at one year post-infection. The five most reported persistent symptoms were breathlessness, memory disturbance, fatigue, cough, and peripheral numbness.  

The analysis of predictors and protective factors revealed that patients with severe to critical acute COVID-19 have a significantly higher risk of developing long-COVID. However, patients who have received a complete two-dose vaccination before the onset of COVID-19 have a significantly lower risk of developing long-COVID.

Severe illness raises risk while vaccination protects

The study reports a high prevalence of long-COVID among previously hospitalized patients in Thailand, with symptoms persisting in a substantial proportion of affected individuals over a period of two years. These findings are concerning as persisting symptoms of long-COVID have been found to affect the daily living of affected individuals.

Among reported symptoms, joint pain has shown a significant association with perceived impact on daily living. Other reported symptoms that frequently impact participants’ daily living include fatigue, sleep disturbances, decreased appetite, and muscle pain.  

According to the findings, having a severe to critical case of acute COVID-19 is a major risk factor for long-COVID, while complete vaccination protects against the condition. These two factors are also significant predictors of the perceived impact of long-COVID on daily living.

By analyzing demographic characteristics of participants who reported impaired daily living, the study finds that men are less likely to develop long-COVID with an impact on daily living than women. The differences in inflammatory profiles and airway morphology between genders may contribute to the higher risk of long-COVID among females.

Overall, the study's findings provide an overview of the long-term effects of COVID-19 among hospitalized Thai individuals, bridging a gap in the literature for low- and middle-income countries.

The study had a small sample size and a high loss-to-follow-up rate, which may limit the accuracy of prevalence estimates. The exclusive inclusion of hospitalized Thai patients may limit the generalizability of the findings to non-hospitalized patients and other nationals.

The study was conducted during the Delta variant outbreak, and the prevalence and impact of long COVID may differ from other SARS-CoV-2 variants. Moreover, the protective effect of vaccination against long-COVID symptoms may be underestimated in the study due to the lack of data on the timing between vaccination and the onset of infection.

The study lacks data on repeated COVID-19 infections and, therefore, could not reveal the impact of reinfection on long-COVID epidemiology. The absence of a comparator group of people without COVID-19 limits the affirmation that the reported symptoms are solely the sequelae of COVID-19.

Larger-scale, multicenter studies with appropriate comparator groups are needed to more accurately estimate the prevalence and more conclusively understand the health outcomes associated with long-COVID.  

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