WASHINGTON, March 2 (Xinhua) -- The U.S. National Institutes of Health (NIH) has launched a new research effort to understand how COVID-19 affects children, who have accounted for roughly 13 percent of total confirmed cases in the country.

The research will investigate why some children are at greater risk for SARS-CoV-2 infection than others, why symptoms vary among children who are infected, and how to identify children at risk for severe illness from SARS-CoV-2 infection, according to a release of the NIH on Tuesday.

Research will also focus on multi-system inflammatory syndrome in children (MIS-C), a life-threatening condition marked by severe inflammation of one or more parts of the body, including the heart, lungs, kidneys, brain, skin, eyes and gastrointestinal organs.

Based on current data, most children with SARS-CoV-2 infection do not develop serious illness. However, those who do go on to develop MIS-C can experience prolonged fever and severe abdominal pain and may progress to shock, according to the NIH.

About 3.17 million children in the United States have tested positive for COVID-19 since the onset of the pandemic, according to a latest report of the American Academy of Pediatrics (AAP) and the Children's Hospital Association.

A total of 3,168,274 child COVID-19 cases had been reported across the country as of Feb. 25, and children represented 13.1 percent of all confirmed cases, according to the report.

The overall rate was 4,209 cases per 100,000 children in the population.

Over 64,000 new child COVID-19 cases were reported last week. This marked the sixth consecutive week with a decline in new cases, according to the AAP.

Over the two weeks from Feb. 11 to Feb. 25, there were 134,904 new child COVID-19 cases reported across the country, an 4 percent increase.

Children accounted for 1.3 percent to 3 percent of total reported hospitalizations, and 0 to 0.19 percent of all COVID-19 deaths, said the report.

"At this time, it appears that severe illness due to COVID-19 is rare among children. However, there is an urgent need to collect more data on longer-term impacts of the pandemic on children, including ways the virus may harm the long-term physical health of infected children, as well as its emotional and mental health effects," the AAP said in the report.

The United States has recorded over 28.67 million cases with more than 515,300 related deaths as of Tuesday noon, according to the real-time count kept by Johns Hopkins University.