The graph lines are climbing again.

Cases of the novel coronavirus have surged in Maryland, the District and Virginia after more than a month of declines and plateaus, prompting fears of a surge similar to what is happening elsewhere in the country.

Washington-area officials are concerned and taking action: The District expanded its mask mandate and ordered people coming from hot spots to quarantine themselves for 14 days, Virginia stepped up enforcement, Maryland’s largest city, Baltimore, suspended indoor dining, and Anne Arundel County, also in Maryland, reimposed size caps for social gatherings.

Experts agree that the numbers are alarming, and they say governments would be wise to shut down bars, stop indoor dining and require face coverings in most parts of the region. But they say it is too early to tell whether the three jurisdictions are heading toward a deadly resurgence of the virus — driven mostly by localities outside the immediate Washington area — that would warrant more-drastic measures.

The uptick is partly the result of more testing, which is why the real metric to watch is the rate of positive tests, said Tom Inglesby, the director of the Johns Hopkins Center for Health Security and an adviser on the pandemic to Maryland Gov. Larry Hogan (R).

So far, Maryland’s rate has stayed below the 5 percent mark that the World Health Organization considers safe for resuming some normal activities. But it is higher in certain counties and for specific demographics: Test positivity for people younger than 35 was above 6.5 percent in Maryland last week, officials say. Virginia’s rate crept from a low of 5.8 percent in late June to 7.7 percent last week.

If rates continue to accelerate, officials should act swiftly to pull back on the reopening, Inglesby said, noting that governments in the South were slow to do so.

Virginia, Maryland and the District reported 2,601 new coronavirus cases Saturday, the highest daily increase in infections reported since mid-May. The bulk of recent infections have been observed in specific places — the Baltimore metropolitan area and along the coasts of Maryland and Virginia — and among younger adults, who are less likely to experience severe symptoms of covid-19, the disease caused by the novel coronavirus.

Hospitalization and mortality rates so far have not increased, which could in part be because it usually takes weeks for the virus to seriously sicken and kill people. In addition, young people generally tolerate the virus better than older people. But if those young people infect their parents, grandparents and neighbors, hospitalization rates also could jump.

A rise in case numbers was not unexpected, experts say, given the loosening of restrictions after a prolonged economic shutdown. But how the region will react — and be affected — hangs in the balance.

“What we’re trying to avoid is the situation where cases shoot up uncontrollably,” said Anne Arundel Health Officer Nilesh Kalyanaraman. “The challenge is we can only really tell in retrospect where that tipping point is.”

Crowded boardwalks

Ocean City, in Maryland’s Worcester County, was among the first places in the region to reopen, in mid-May, attracting droves of visitors. Daily case numbers plateaued for six weeks, then surged in July: Nearly 150 county residents tested positive for the coronavirus in the first three weeks of the month compared with 56 in the first three weeks of June.

Business owners say the infections are driven by out-of-state vacationers and young adults drawn to the beach by warm weather and social activity.

When Delaware ordered beachside bars to close ahead of the Fourth of July weekend, thousands of visitors flooded Ocean City’s boardwalk. Soon, dozens of restaurants — about 8 percent, said the local chamber of commerce — closed because employees had contracted the virus.

“A lot of people are coming to Ocean City because it’s more open than other resort areas,” said David Schroder, a manager at Ocean Gallery, a framing business on the boardwalk. “To me, it’s more people than should safely be here.”

On busy days, social distancing “does not exist,” on the boardwalk, said Timur Unal, manager of the pizza restaurant Piezano’s, which chose not to allow dine-in services even when the restriction was lifted in June.

As the weather warms, families are joining younger revelers on the beach, Unal said. Many are in “vacation mode,” he added, reluctant to wear masks or keep their distance from others.

According to research from the Maryland Transportation Institute (MTI), which uses cellphone data to track human traffic, out-of-state travel in and out of Worcester County was about 18 percent of the total trips logged in April, during the shutdown, but 41 percent of total trips in July.

As states have reopened, researchers have seen the number of trips, frequency of out-of-state travel and miles traveled per person trend upward, sometimes exceeding what they were in the weeks preceding the pandemic, said Aref Darzi, a project researcher at the MTI.

In Virginia and Maryland, the percentages of out-of-state travel in July have surpassed what they were in the first two weeks of March, before coronavirus restrictions were put in place.

'Being 20-something'

But it is not just outside visitors who are driving new cases, experts say. Struck by quarantine fatigue, local residents are traveling more and spending more time at bars, restaurants and parties.

In Carroll County, the site of a major nursing home outbreak, the caseload had consisted primarily of infections in congregate settings. That changed during the week of July 13, when all 37 new cases were reported within the community, said Health Officer Ed Singer. The median age of infected individuals this month has fallen from 63 to 57.

“People are going out of state and bringing it back here,” Singer said, adding that those testing positive often report having recently attended weddings, parties or other gatherings outside Carroll County.

People under 40 make up 66 percent of new D.C. cases since July 1, up from 41 percent in the months prior. D.C. Health Department Director LaQuandra Nesbitt said a growing share of infections seems to be linked to young residents engaging in nonessential travel.

“They are going to places like Florida, Texas, North Carolina . . . and then participating in some of those things that are part of our Phase 2 reopening, such as activities in restaurants, and more of the family gatherings,” Nesbitt said.

“It’s the nature of being 20-something,” said Maura J. Rossman, the health officer for Howard County, where cases are rising sharply among those under 35. “They socialize differently. They enjoy going outside, going to restaurants and bars, hanging with their friends.”

At the same time, younger adults are more likely to work in jobs considered essential, which makes it harder for them to avoid community spread of the virus, Rossman said.

At Magruder’s grocery and wine store in Northwest Washington on Thursday, a steady stream of customers walked out with shopping carts filled with beer and other beverages.

Daniel Crystal, 22, carried a case of Bud Light and a few other beverages — enough for the gathering he would attend in a friend’s backyard, where a giant screen projector would be set up for the Washington Nationals’ home opener.

Crystal — living with his parents in Maryland after graduating from the University of Virginia last month — said he expected that all attendees would keep their distance from one another. But he admitted that, until recently, he had not been vigilant about wearing a mask or limiting his exposure.

“I think D.C. is still doing pretty well,” he said about the recent uptick in infections. “. . . But I’m also not being super reckless.”

Dupont Circle resident Dani Hupper, 25, said she has not been to bars or parties since the pandemic began but went camping with her boyfriend on the Fourth of July weekend at Pohick Bay on the Potomac River in Virginia, which is where she suspects she contracted the virus.

The campsite was “packed,” she said. Even though she did not speak to other people, she used a public restroom where other campers were not wearing masks.

After she tested positive, two of her D.C. roommates left to quarantine themselves elsewhere. Hupper said she informed everyone with whom she had had contact that she had the virus, then put their test dates into her calendar so she could track their results. Some of her friends canceled travel plans; others argued with partners over the possible exposure.

It has been a period of high anxiety, she said, even though her symptoms have been minor and none of her contacts have yet tested positive for the virus.

“I truly lived in fear these past couple weeks that I was going to get a text message that my friends or their parents were feeling symptomatic,” Hupper said. “There’s a social toll.”

Changing behavior

The spikes have triggered calls for more-sweeping action.

Virginia Gov. Ralph Northam (D) has restricted the use of bar areas inside restaurants and pubs to small groups that are eating and plans to make the cutoff time for alcohol sales earlier than midnight, while D.C. Mayor Muriel E. Bowser (D) has imposed one of the strictest mask mandates in the country and, on Friday, issued a quarantine order for those coming to the city for nonessential travel from places where the virus is spiking.

Hogan so far has left it up to individual counties and cities to decide on tightening restrictions. But officials in Howard, Anne Arundel and Baltimore counties and Baltimore City questioned that approach, saying a state-led strategy was necessary, given the degree of travel within Maryland.

“The reality is we’re not an island,” said Rossman, the Howard County health officer. “It’s not going to be effective if we all take different actions.”

Michael Ennis, assistant general manager at Kelsey’s Restaurant, Irish Pub & Banquet Room in Howard County, said that most restaurants in the county close by 10 p.m. but that younger residents often travel to nearby Baltimore City for bars that stay open longer.

The Baltimore metropolitan area recently overtook Maryland’s most populous counties — Montgomery and Prince George’s — in the number of new daily infections. The city’s positivity rate has jumped to 6.8 percent, from 5.2 percent earlier this month. Some neighborhoods, including the trendy Canton and Patterson Park, have positivity rates as high as 22 percent.

Baltimore Health Commissioner Letitia Dzirasa said the city has seen a “dramatic rise” in cases among those under age 40, a “disproportionately high” positivity rate in the southeast section of the city and a doubling of the city’s caseload in the past month.

The city suspended indoor dining as of Friday evening, adding to the hardship facing the restaurant industry.

“We had just started getting the momentum going,” said Charlie Gjerde, whose five restaurants in the city include Huck’s American Craft in the hard-hit Brewer’s Hill area. “Now, all that goes away.”

If infections continue to trend upward, more restrictions could be on the horizon — a fate that public health experts hope to avoid by sounding alarm bells early.

Dennis Kistner, who co-owns Mahaffey’s Pub in Canton, said that when restrictions were lifted, locals were bar hopping, and people held barbecues and went to house parties. Some of his pub’s patrons were “standoffish” about wearing masks or keeping distance from others, he said.

In recent days, however, as city officials reported surging cases in Canton, people have started behaving differently, Kistner said. His pub is quieter, and masks are more prevalent.

“Epidemiologically, we knew we were going to see another peak,” said Singer, the Carroll County health officer. “Right now, we don’t know where that peak is going to end. A lot of that depends on what people do next.”

Ovetta Wiggins, Fenit Nirappil, Emily Davies and Teddy Amenabar contributed to this report.

A previous version of the chart in this story miscounted the average number of new Baltimore-area cases. It has been corrected.