Northern Virginia’s jump to Phase 3 in relaxing covid-19 restrictions, earlier than its neighbors, risks stalling or reversing the region’s progress in containing the disease, according to Maryland and District officials and public health experts.

The region has had noteworthy success so far in reducing infection rates, in sharp contrast with many parts of the country in the south and west.

That’s partly because jurisdictions here have moved mostly together as they gradually emerge from the strict shutdowns imposed in March. Both Northern Virginia and the Maryland suburbs of Montgomery and Prince George’s counties waited longer than other parts of their states to reopen because of high infection rates at the time in the Washington area.

But that regional unity appears to have come to an end, as Northern Virginia agreed to go along with the rest of Virginia in moving to Phase 3 on Wednesday. The District and Maryland suburbs show no signs of matching that in the immediate future. Montgomery officials say they will wait until at least a week after July 4 to see if holiday festivities lead to a spike in cases.

The divergence is significant. Among other changes, under Phase 3, Northern Virginia restaurants can now be fully occupied indoors, providing people distance physically. Gatherings of up to 250 people are permitted. The rest of the region, under Phase 2, allows restaurants only 50 percent occupancy indoors and limits gatherings to 50 or 100 people.

Given that many residents cross the Potomac regularly for work or entertainment, the relaxed rules on one side of the river are worrisome, the officials and experts said.

“It is concerning because we try to make sure that we all move at the same speed and at the same level that things are open,” Travis A. Gayles, Montgomery’s public health chief, said. “The counties in Northern Virginia have more in common with D.C. and the counties in Maryland [than with the rest of Virginia], because our populations move back and forth.”

Said a senior District official, who spoke on the condition of anonymity because the matter was politically sensitive: “With Virginia moving to Phase 3, any loosening of restrictions increases the risk of spread. . . . We operate as a region, so that’s why we’re concerned.”

The official said the District is “okay being cautious,” and added “We’re not rushing to get to the next phase.”

Top experts at two local universities also were uneasy about Northern Virginia’s move.

“Most of those counties — Fairfax, Arlington, Loudoun — have had pretty low rates over a period of several days, but they are part of a larger metro area,” said Lynn R. Goldman, dean of the George Washington University School of Public Health. “There is a lot of potential for transmission to happen if they’re not careful, as we’ve seen happen in Texas and Florida when they’ve opened up too quickly.”

Said Boris Lushniak, a former acting U.S. surgeon general who is dean of the University of Maryland School of Public Health: “It’s not the best public health measure to break the region apart in this manner. . . . I would have preferred for this to be done in a more coordinated fashion.”

Alena Yarmosky, spokesperson for Virginia Gov. Ralph Northam (D), said the Phase 3 reopening was justified by numerous factors, including falling numbers of covid cases and hospitalizations, increased supplies of masks and other protective equipment, and expanded capacity of testing and contact tracing.

“Finally, I’d note that the governor has made it clear he is closely monitoring what is happening in other states and is prepared to tighten restrictions should that be necessary,” Yarmosky said.

Northern Virginia officials said they did not ask Northam to let their jurisdictions wait to enter Phase 3 — as they did with the earlier moves to Phases 1 and 2 — because the data showed that their jurisdictions had met criteria for advancing.

“If you look at the data, we’re in pretty good shape,” Arlington County Board Chair Libby Garvey (D) said Friday. “Nobody’s enthusiastic [about the situation], but we need to figure out how to live with and be with this virus, and using the numbers seems to be the best way to do it.”

Fairfax Board of Supervisors Chair Jeff C. McKay (D-At Large) said: “It would have been pretty difficult for us, based on the data, to suggest that we shouldn’t reopen.” Still, McKay said he regretted that the region’s jurisdictions were not at the same place.

“Of course I’m concerned that when we do something that D.C. and Maryland are not doing, there’s always a potential risk of spread [and] of confusion to our residents and business owners,” McKay said.

Northam made one last-minute concession when he agreed to keep bars closed indoors, after hearing appeals from area officials including McKay and D.C. Health Director LaQuandra Nesbitt.

In general, the Washington region has performed well in responding to the novel coronavirus, health experts said. Using strict shutdowns and a comparatively slow reopening, it appears to be following the path of some western European nations that have driven down infection rates.

So far, it has avoided the resurgence that is gripping states that adopted partial shutdowns and reopened quickly — although numbers have ticked up slightly in Virginia over the last two weeks.

“Our region is looking like western Europe, while, honestly, Arizona, Texas and Florida are showing some of the patterns that we were seeing back in March, with exponential growth of the epidemic,” GWU’s Goldman said.

Said U-Md.’s Lushniak: “What I’m really proud of in the capital region in general is that there’s been wise people making relatively good decisions for the good of the population’s health.”

He continued: “Northern Virginia pushing ahead into Phase 3 is of potential concern. . . . You’re going to have to have the strength and wisdom to pull back if the metrics start changing.”

Some local officials and experts said they would support following the example of New York and some other states in urging visitors from heavily affected states to undergo 14-day quarantines.

“I would fully support that,” McKay said. “The problem, obviously, is how do you enforce it?”

Gayles, of Montgomery, said that county and some other local jurisdictions were considering such a measure.

“Those discussions are happening here,” Gayles said. “It’s definitely on the radar.”