Almost 280 000 school-aged children were infected with the novel coronavirus between 1 March and 19 September, according to detailed data released on Monday in a report by the US Centers for Disease Control and Prevention (CDC).
The figure accounted for roughly four percent of the total US caseload over this period, with children aged 12-17 approximately twice as likely to be infected as those aged 5-11.
The rate of new cases rose steadily during the spring and then shot up over the summer, peaking on 19 July with an average weekly incidence of 37.9 per 100 000.
The new cases then plateaued for several weeks before declining in late August – though it appears they are now rising again towards summer levels.
The authors wrote that the data helped establish a baseline for monitoring trends of Covid-19 infection as some schools return to in-person learning now and in coming months.
“School studies suggest that in-person learning can be safe in communities with low SARS-CoV-2 transmission rates, but might increase transmission risk in communities where transmission is already high,” they said.
The report provides more data for trends that were already known, such as that Hispanic and black minority groups are at heightened risk of developing severe Covid-19, as are children with underlying conditions.
Overall, 277 285 children were infected, 3 240 hospitalised (or 1.2 percent); 404 admitted to intensive care (0.1 percent); and 51 died (0.01 percent).
The real rates of hospitalisation, intensive care admission and death are likely to be even lower in reality because the actual number of children infected was probably far higher and not recorded, given the number of asymptomatic cases.
Though the risk to children is relatively low, health experts are more worried about the onward transmission risk children could present to their parents or grandparents.
Children in the 5-11 group accounted for 101 503 cases and 20 deaths while those in the 12-17 group accounted for 175 782 cases and 31 deaths.
Chronic lung disease – which includes asthma and chronic obstructive pulmonary disease (COPD), for example – was the most prevalent underlying condition.
The rates of infection varied greatly from region to region and the authors emphasised that local authorities should closely monitor the situation to arrive at the best decision about in-person learning.