Symptoms of long Covid experienced by children and young people may change over time, according to the largest study of its kind published in The Lancet Regional Health - Europe.

Researchers asked children aged 11 to 17 about their health six months and 12 months after taking a PCR test between September 2020 and March 2021. They also asked them to recall their symptoms at the time of taking the test.

None of the children initially invited to participate in the study had been hospitalised.

The researchers examined data from 5,086 children, 2,909 of whom tested positive for Covid-19, and 2,177 of whom tested negative.

They asked the children and young people what they had experienced from a list of 21 symptoms, including shortness of breath and tiredness, as well as using validated scales to assess quality of life, mental health, wellbeing and fatigue.

The team found that, at time of testing, health issues were more common in children and young people who had tested positive for the virus compared to those who tested negative, as well as six months and 12 months post PCR test.

For example, among the test-positives, 10.9 per cent reported fatigue at all three time points while among test-negatives only 1.2 per cent reported fatigue at all three time points.

They also noted that the symptoms experienced changed over the course of a year. And while some of the children's original symptoms declined, new symptoms were reported.

This was also the case when researchers looked at scales measuring poor quality of life, experiencing emotional and behavioural difficulties, poor wellbeing and fatigue.

The researchers found that the symptoms of long Covid experienced by children and young people change over time and that clinicians should be aware of this. "Our research goes one step further than existing studies and indicates that researchers need to track individual trajectories using repeated measurement on the same children and young people over time," said study corresponding author Snehal Pinto Pereira from University College London Hospitals (UCLH), UK.

“Simply reporting repeated cross-sectional prevalences -- or snapshots -- of symptoms over time may obscure important information about long Covid in young people that has clinical relevance," Pereira said.

The study will continue to analyse survey results from participants for up to two years after the initial PCR test.

The authors acknowledged some limitations to the study.

The symptoms reported by participants at time of testing are subject to recall bias as they were reported at the time of first contact with the study -- at either three months or six months post-test.

However, six-month and 12-month symptoms were reported at the time they were being experienced, the researchers added.