Most children with Covid-19 recover within a week, but a small percentage have long-term symptoms, a study says.

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

The findings suggest that what has sometimes been called “long Covid” may be 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 at least four weeks and 4.5 percent had symptoms that lasted at least eight weeks.

“It is reassuring that the number of children experiencing long-lasting symptoms of Covid-19,” is low, Dr. Emma Duncan, an endocrinologist at King’s College London and lead author of the study, said in a statement. “Nevertheless, a small number of children do experience long illness with Covid-19, and our study validates the experiences of these children and their families.”

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

In most cases, the illness was mild and short. Children were sick for six days, on average, and experienced an average of three symptoms. The most common symptoms were headache and fatigue.

But a small subset of children experienced lingering symptoms, including fatigue, headache and a loss of smell. Children between 12 and 17 were sicker for longer than younger children and more likely to experience symptoms that lasted at least four weeks.

“We hope our results will be useful and timely for doctors, parents and schools caring for these children — and of course the affected children themselves,” Dr. Duncan said.

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 may have had other illnesses, such as colds or the flu — recovered more quickly and were less likely to have lingering symptoms than those with Covid. They were ill for three days, on average, and just 0.9 percent of children had symptoms that lasted at least four weeks.