As many as one in seven children who get coronavirus could have symptoms almost four months later, according to the world's largest study on long Covid in children.

People who tested positive were twice as likely to report three or more symptoms 15 weeks later than those who tested negative, research led by University College London and Public Health England found.

Lead author Professor Terence Stephenson said he feels "reassured" by the data, which he said shows it is "nowhere near what people thought in the worst-case scenario".

The researchers said their findings will be presented to the Joint Committee on Vaccination and Immunisation (JCVI) - which has yet to give a decision on extending the Covid-19 vaccine rollout to all 12 to 15-year-olds in England and Wales.

The research looked at almost 7,000 children aged 11 to 17, made up of those who had a positive PCR test result between January and March and a group who tested negative in the same period.

When surveyed around 15 weeks after their test, 14% more young people in the positive group had three or more symptoms, including unusual tiredness and headaches, than those who tested negative.

One in 14, or 7%, more in the positive group had five or more symptoms, the study showed.

Researchers said their data suggests that between September and March at least 4,000 - and possibly as many as 32,000 - teenagers of the total population of this age group who tested positive in England might have had three or more symptoms linked to the infection some 15 weeks later.

While there was little difference in the mental health and wellbeing scores between children who tested positive compared to those who tested negative, researchers said a high proportion in both groups reported being a bit or very worried, sad or unhappy.

This accounted for 41% of those who tested positive and 39% of those who tested negative.

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Prof Stephenson said: "There is consistent evidence that some teenagers will have persisting symptoms after testing positive for SARS-CoV-2.

"Our study supports this evidence, with headaches and unusual tiredness the most common complaints.

"The difference between the positive and negative groups is greater if we look at multiple symptoms, with those who had a positive test twice as likely to report three or more symptoms 15 weeks later."

The Children and young people with Long Covid (CLoCk) research will carry on, with analysis of results at six months, a year and two years from the time of the person's PCR test.

Prof Stephenson said while he is reassured by these early findings, he remains "very concerned" that there could be young people who are "severely affected".

He added: "That's something that we'll return to when we study young people at six months.

"But there will be some young people who are completely bedridden or remain very short of breath or have daily headaches, and I wouldn't want to diminish that, but we're reporting kind of aggregate numbers.

"I think overall it's better than people would have guessed back in December."

The study also involves researchers at the universities of Edinburgh, Bristol, Oxford, Cambridge, Liverpool, Leicester, Manchester as well as King's College London, Imperial College London, Public Health England, Great Ormond Street Hospital and University College London Hospitals.

Dr Liz Whittaker, senior clinical lecturer in paediatric infectious diseases and immunology at Imperial College London, said the JCVI's decision on extending the vaccine rollout is likely to be based on the risk of severe disease from the virus compared with risks of the jabs, rather than the data in this study which relates to long Covid.