Although most children with Covid-19 recover within a week, a small percentage of more than 1,700 UK children will experience long-term symptoms, according to a new study of more than 1,700 UK children. The researchers found that 4.4 percent of children have symptoms that last four weeks or more, while 1.8 percent have symptoms that last eight weeks or more.

The results suggest that what has sometimes been referred to as “long covid” is less common in children than adults. In a previous study, some of the same researchers found that 13.3 percent of adults with Covid-19 had symptoms that lasted for at least four weeks and 4.5 percent had symptoms that lasted for at least eight weeks.

“It is comforting that the number of children with long-term symptoms of Covid-19 is low,” said Dr. Emma Duncan, King’s College London endocrinologist and lead author on the study, in a statement. “Even so, some children suffer from long-term illness with Covid-19, and our study confirms the experiences of these children and their families.”

The study, published Tuesday in The Lancet Child & Adolescent Health, is based on an analysis of data collected by the smartphone app Covid Symptom Study. The paper focuses on 1,734 children between the ages of 5 and 17 who tested positive for the virus and developed symptoms between September 1 and January 24. Parents or carers reported the children’s symptoms in the app.

In most cases the illness was mild and brief. The children were sick for an average of six days and had an average of three symptoms. The most common symptoms were headache and fatigue.

But a small subset of children had persistent symptoms such as fatigue, headaches, and loss of smell. Children between the ages of 12 and 17 were sicker longer than younger children and were more likely to have symptoms that lasted for at least four weeks.

“We hope that our results will be useful and up-to-date for doctors, parents and schools who are caring for these children – and of course the children themselves,” said Dr. Duncan.

The researchers also compared children who tested positive for the coronavirus with those who reported symptoms in the app but tested negative for the virus. Children who tested negative – and possibly had other illnesses like colds or flu – recovered faster and were less likely to have persistent symptoms than those with Covid. They were sick for an average of three days, and only 0.9 percent of the children had symptoms that lasted for at least four weeks.