As the true number of COVID-19 cases and deaths is believed to be higher than reported, a new study suggests that the actual toll of long COVID may also double than the current estimates.The research, led by Mass General Brigham, found that many long COVID cases remain hidden because current surveillance systems rely heavily on diagnostic codes that fail to capture a large number of patients.Using a novel AI algorithm, researchers analyzed medical records of nearly 460,000 COVID-19 patients across 58 hospitals in the United States. The findings showed that approximately one in six people — around 16 per cent — developed long COVID, translating to more than 18 million Americans.The figures are nearly double current estimates and highlight the growing burden of chronic health conditions following COVID-19 infection. The study was published in JAMA Network Open.“Over 10 million people with long COVID would go entirely undetected by the diagnostic code that health systems and policymakers rely on to track the disease burden,” said corresponding author Hossein Estiri, a faculty member in the Mass General Brigham Department of Medicine.“The figures we uncovered are almost certainly an undercount,” he added.Researchers noted that current diagnostic coding systems, including the ICD code U09.9 for post-COVID conditions, identify fewer than 7 per cent of long COVID patients.What Did The Study Find?Also read: 16 Million Indians Die Due To Hypertension Every Year: AIIMS DocThe study analyzed electronic health records from 457,950 patients who had previously tested positive for COVID-19 across four US regions — New England, Southeast Texas, Southern California and Western Pennsylvania.Overall, 16.3 per cent of patients were identified with long COVID, with regional rates ranging from 13.6 per cent to 22.7 per cent.The researchers also found significant regional differences in long COVID symptoms and related conditions, including varying rates of prediabetes, which is emerging as a possible long-term effect of COVID-19.The study authors noted that undocumented infections — which became more common after widespread testing declined — were not included in the analysis. Patients without long-term medical records were also excluded, suggesting the actual burden of long COVID could be even higher.“These patients are not absent from clinical care; they are absent from the diagnostic code that would identify them as long COVID patients,” said lead author Jiazi Tian, a data scientist in the Clinical Augmented Intelligence Group at Mass General Brigham.“The cardiologist seeing new dysautonomia, the endocrinologist seeing new metabolic disease, the neurologist seeing unexplained cognitive complaints — some of these presentations are long COVID arriving without the label that would connect them to a COVID-19 infection,” Tian added.What Is Long COVID?Read More: Can Extreme Heat Trigger Heart Palpitations? Expert Explains RisksLong COVID refers to symptoms that continue for three months or longer after the initial COVID-19 infection.Common symptoms include:FatigueBrain fogHeadachesBreathlessnessAltered sense of smell or tasteResearchers say many long COVID conditions are still being studied, and some people may experience multiple symptoms at the same time.