The number of new coronavirus cases is decreasing in Philadelphia, but officials are not sure why, Health Commissioner Thomas Farley said Tuesday.

After case averages slowly climbed over the course of July, the rate of people testing positive for the virus has dropped below 5% — the lowest the city has seen since the pandemic began.

But Farley cautioned that it was too early to tell whether the decline would continue and said it was “really unclear” why cases were decreasing. In the week that ended Saturday, the city saw an average of 123 new cases per day, a decrease from an average of 166 new daily cases for the previous week.

“They’re not falling by much, and this could be temporary,” Farley said. “I’m certainly pleased to see the numbers go down, but I can’t say whether they’re going to continue to go down.”

Reporting 106 new cases, he urged residents to continue social distancing and recommended vulnerable people stay home. Still, he said any decrease was “good news” and speculated that it could be a result of more people wearing masks.

Last week’s daily average came as a surprise after Farley had warned that cases were likely to continue rising. Test results have been delayed several days from some laboratories, making it difficult to determine trends in new cases and to trace and quarantine people exposed to the virus. But Farley said Tuesday that wait times from some large labs have started to improve.

Nationwide, the number of confirmed cases passed 4.7 million, encouraging results came out of preliminary studies on one coronavirus vaccine, and members of Congress continued negotiating a stalled coronavirus relief package.

The governors of six states, including Maryland, Virginia, and Ohio, announced a joint plan to acquire and deploy millions of coronavirus tests to their residents to circumvent the lack of a federal testing strategy. And Rhode Island was added to the travel advisory shared by New Jersey, New York, and Connecticut that asks people coming from certain states to quarantine.

Delaware announced its plan for the academic year on Tuesday: Schools will be permitted to reopen with a hybrid model of remote and in-person learning, with individual districts making their own plans, Gov. John Carney said.

Last week, Carney said he hoped the state’s coronavirus metrics might improve enough for him to approve fully in-person education. But on Tuesday, he said the state’s community spread and rate of positive tests remained in a moderate range, though hospitalizations are low.

“We don’t want anybody to come back if it’s not safe,” Carney said.

State Medical Director Rick Hong said that if schools feel they cannot provide a safe environment, they can opt to open fully virtually, but the state “would not recommend” providing fully in-person instruction.

Like in Pennsylvania and New Jersey, school districts and charter schools will be charged with deciding how to reopen safely. Carney noted every school system has different numbers of students, sizes of buildings, transportation requirements, and other variables.

The state will require face coverings for students in grades 4-12 and is “strongly encouraging” them for younger children. The state’s other health protocols for in-person instruction include increased ventilation and use of outdoor spaces.

Officials said they would focus on testing teachers, staff, and students for the virus. The state will supplement community-based testing with 20,000 saliva-based tests that can be mailed to homes, said Rick Pescatore, the state’s chief physician.

As August begins, numbers from July provided a snapshot of how, in many areas, the number of cases increased moderately after lows in June.

Pennsylvania reported 854 newly confirmed cases of the coronavirus on Tuesday, along with 23 deaths.

The state’s moving seven-day average for new daily cases has been decreasing since July 30 and was 804 on Tuesday, though the day’s number of new cases returned to the 800s after two weekend days of lower numbers.

Over the last seven days, results came back positive for 3.9% of people statewide who were tested for the virus, according to the Department of Health. And new cases in Allegheny County and Southwestern Pennsylvania have started to fall over the last two weeks after rising sharply in late June and mid-July.

But in Delaware County, the rate of new cases per capita over the last week was higher than that of Philadelphia or Allegheny Counties. It counted 738 new cases in June and 1,421 in July.

Chester and Bucks Counties also logged more new infections in July than June — Chester with 152 more new cases last month for a total of 1,134 in July, and Bucks with 662 cases in June and 1,158 in July. Burlington, Camden, and Gloucester Counties all had sharp drops in cases in June, with small increases in July.

New Jersey reported 416 newly confirmed cases and 11 deaths on Tuesday.

In Camden County, people in their 20s accounted for about one-third of recent cases, Freeholder Director Louis Cappelli Jr. said at a Tuesday news conference. The average age of coronavirus patients has dropped from the 50s to 31, he said.

Of the 103 cases confirmed in the first three days of August, 59 were in people age 20 to 49, and nine were in children and teens.

Younger people are catching the virus at the Shore, at parties, and at outdoor eateries where social distancing is ignored, Cappelli said. The county has also traced cases in young people involved in athletics, he said, advising parents and grandparents to keep their children out of sports if someone in their household is immunocompromised.

Cappelli urged those who are infected to be honest when speaking to contact tracers.

“Nobody is going to be punished for going to a house party or being too crowded at a bar,” he said. “What we really need is the information you have in order to make sure people who need to get tested, get tested and to make sure we can quarantine those who need to be quarantined, and stop the spread of the disease.”

Contributing to this article were staff writers Rob Tornoe and Ellie Silverman and staff graphic artists Dominique DeMoe and John Duchneskie.