White House Coronavirus Task Force Coordinator, Dr. Deborah Birx, answers a question during the daily briefing on the novel coronavirus, COVID-19, at the White House on March 18, 2020, in Washington, DC.

New York and New Jersey are seeing coronavirus attack rates at least five times higher than other parts of the country, a U.S. official in charge of the White House's pandemic response efforts said Monday.

"The New York metro area of New Jersey, New York City, and parts of Long Island have an attack rate close to one in 1,000," Dr. Deborah Birx, a physician and the White House coronavirus response coordinator, said at a press briefing Monday evening. The attack rate is the percentage of a population that gets the disease.

She said roughly 28% of the specimens submitted in that region have tested positive for COVID-19, while less than 8% have tested positive for the disease in the rest of the country.

"To all of my friends and colleagues in New York, this is the group that actually needs to social distance and self-isolate," she said. "Clearly, the virus had been there for a number of weeks."

New York is currently the hardest-hit state in the country, ahead of New Jersey, California and Washington state. New York City, alone, accounts for 12,305 of the 20,875 confirmed infections in the state as of Monday morning, New York Gov. Andrew Cuomo said earlier in the day.

On Friday, Cuomo ordered nonessential businesses to keep 100% of their workforce at home and put in place stringent new restrictions on New Yorkers.

Cuomo estimated up to 80% of the state's more than 19.4 million residents will get the coronavirus. Last week, Cuomo estimated there are likely "tens of thousands" of COVID-19 cases in the state among residents who didn't know they had it.

"Many people will get the virus but few will truly be in danger," he said Monday.

Birx also said Monday the disease appears to be less severe in children, adding that no child under the age of 15 "has succumbed" to the virus in Europe. About 99% of deaths in Europe are from people over the age of 50 or with underlying health conditions, she said, citing data that's in line with recent studies.

Last week a study looking at cases in children in China found more than 90% of them were asymptomatic, mild or moderate cases. However, nearly 6% of the children's cases were severe or critical, compared with 18.5% for adults.