A boy plays on a swing at a playground in New York, the United States, on Aug. 9, 2020. (Xinhua/Wang Ying)

The sharp increase in the number of cases among children is mainly concentrated in COVID-19 "hotspots" such as California, Florida and Arizona.

WASHINGTON, Aug. 12  -- The number of COVID-19 cases among children in the United States has increased sharply recently as a new report by the American Academy of Pediatrics (AAP) and the Children's Hospital Association (CHA) showed a 90-percent surge in child cases over four weeks.

Leading experts told Xinhua on Wednesday that factors including high COVID-19 infections among adults, increasing gatherings among teenagers and returning to school during the pandemic may contribute to the high infections among children.

"The sharp increase in the number of cases among children is mainly concentrated in COVID-19 'hotspots' such as California, Florida and Arizona. The surge in adult cases in these states also led to an increase in children infections," Zhang Zuofeng, professor of epidemiology and associate dean for research with the school of public health at University of California, Los Angeles, told Xinhua.