The data from Johns Hopkins' Center for Systems Science and Engineering showed 51,400 new cases, a decline from just under 56,000 the day before. Prior to that, the national tally was under 50,000 for three days in a row.
The new data Friday showed there have been 5.254 million COVID-19 cases and 167,200 deaths in the United States to date.
COVID-19-related deaths totaled just under 1,100 Thursday, the data showed, a decline from 1,500 the day before -- the highest single-day toll since late May.
Robert Redfield, director of the Centers for Disease Control and Prevention, warned Thursday that the fall might be the "worst ever" for public health unless more Americans start to follow recommendations like face coverings, self-isolation and distancing.
Speaking to the health website WebMD in a video chat, Redfield said combining the pandemic with the flu season could create a substantial crisis.
"For your country right now and for the war that we're in against COVID, I'm asking you to do four simple things: wear a mask, social distance, wash your hands and be smart about crowds," he said.
Redfield also said he's "very cautiously optimistic" for a coronavirus vaccine before the end of the year.
"No one can predict scientific success, but we think from a scientific threshold, developing a COVID vaccine is not as complex as ... an HIV vaccine," he said. "I'm very optimistic that we're going to have one or more vaccines available."
In California, total cases surpassed 600,000 -- the first state to reach the mark. No state has seen more cases since the pandemic began early this year.
In the San Francisco Bay Area, Stanford University said Thursday it's canceled nearly all in-person instruction for the fall. President Marc Tessier-Lavigne said the school had suspended plans to provide on-campus housing to freshmen, sophomores and incoming transfer students.
"As eager as we have been to bring undergraduates back to campus and to pursue as normal a year as possible, we have concluded for reasons I will explain below that the public health situation due to COVID-19 simply does not make it feasible at this time," Tessier-Lavigne wrote in an update to students and parents.
The San Francisco Bay Area has been placed on a statewide "watch list" due to worsening public health indicators.
The National Park Service in San Francisco announced it will reopen outdoor areas of Alcatraz Island to visitors starting Monday. The island -- site of the infamous former federal prison -- will again be served by ferries, although with reduced capacity.
"The health and safety of our visitors and employees are our highest priority," said Laura Joss, general superintendent of Golden Gate National Recreation Area.
In Florida, deaths declined on Thursday after the state saw a record high the day before. More than 9,000 people have died in the state since the start of the pandemic.
The state, which saw dramatic spikes in cases in June and July, also recorded fewer than 10,000 new cases for the 19th consecutive day.