Over 35 per cent of people had at least one long-COVID symptom diagnosed in the 3-6 month period after COVID-19 infection, finds a new study.

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According to researchers, including Paul Harrison from the University of Oxford, the most common symptoms were breathing problems, abdominal symptoms, fatigue, pain and anxiety/depression. "Research of different kinds is urgently needed to understand why not everyone recovers rapidly and fully from COVID-19. We need to identify the mechanisms underlying the diverse symptoms that can affect survivors," Harrison, who headed the study, said in a statement.

For the study, the team investigated long-COVID in over 2,70,000 people recovering from COVID-19 infection, using data from the US-based TriNetX electronic health record network. The study reported on how commonly nine core long-COVID symptoms were diagnosed and how this rate compared to people recovering from influenza.

The nine core long-COVID symptoms, occurring 90-180 days after COVID-19 was diagnosed, comprise -- abnormal breathing - 8 per cent, abdominal symptoms - 8 per cent, anxiety/depression - 15 per cent, chest/throat pain - 6 per cent, cognitive problems ('brain fog') - 4 per cent, fatigue - 6 per cent, headache - 5 per cent, myalgia (muscle pain) - 1.5 per cent, other pain - 7 per cent, any of the above features - 37 per cent.

Higher rates were seen if the whole 1-180 day period after COVID-19 infection was included. The severity of infection, age, and sex affected the likelihood of long-COVID symptoms: long-COVID symptoms were more frequent in those who had been hospitalised, and they were slightly more common in women.

These factors also influenced which of the symptoms people were most likely to experience. For example, older people and men had more breathing difficulties and cognitive problems, whereas young people and women had more headaches, abdominal symptoms and anxiety/depression.

Many patients had more than one long-COVID symptom, and symptoms tended to co-occur more as time progressed. The study also looked at the same symptoms in people recovering from influenza. Long-COVID symptoms did occur after influenza, but were 1.5 times more common after COVID-19.