COMPLACENCY IS among the most potent forces that have aided the pandemic’s lethal advance — the pleasant and false idea that a modest infection rate or downward trend is here to stay.

That’s what prompted what turned out to be premature reopenings across the country, triggering spikes in the novel coronavirus’s prevalence from coast to coast. Now there’s cause to worry that the Washington area, specifically the Virginia suburbs, may be the latest region where complacency will afford the virus a new opening.

The District and its suburbs have been largely spared the explosion in new cases that has beset most states since Memorial Day. Having learned a lesson in the spring, when infection rates locally were among the highest in the nation, neither D.C. Mayor Muriel E. Bowser, a Democrat, nor Maryland Gov. Larry Hogan, a Republican, was eager to be at the vanguard of states rushing to reopen.

The odd man out was Virginia Gov. Ralph Northam, a Democrat who, heartened by the state’s relative success in driving down new cases in June, decreed that Phase 3 of its reopening would begin July 1. As it turns out, that was also the approximate date that much of the metropolitan area, of which Northern Virginia is a populous component, saw a leveling off in infections after weeks of steady decline. And now there are small but worrying signs of an uptick, including in Prince William and Loudoun counties, whose combined population is nearly 900,000.

Mr. Northam, a physician, was not heedless of the risk — he exempted bars and restaurant bar seating from the Phase 3 reopening, which continues to encourage mask-wearing, telework and limited work-related gatherings. However, shops and stores in Virginia are now fully open; indoor dining at restaurants is allowed with adequate separation between tables; and in-person gatherings of up to 250 people are permitted, up from the previous ceiling of 50.

It is folly to treat Northern Virginia as a separate epidemiological entity from the District and suburban Maryland, given that thousands of people cross between them daily. The virus does not stop at bridges or the Beltway.

D.C. Health Director LaQuandra Nesbitt told The Post that she asked Virginia officials to maintain the closure of big entertainment venues. But Mr. Northam’s Phase 3 rules allow concert halls and other places of entertainment to operate at half capacity or up to 1,000 people, whichever is lower. That seems like an unwarranted risk given the spikes elsewhere, including in Delaware, West Virginia and North Carolina.

The risk is compounded by the fact that thousands of federal workers are being ordered to return to work, including at the Departments of Energy, Interior and Defense. It’s unclear what makes the Trump administration believe that the national capital’s good luck will continue to hold even as the coronavirus gains momentum in nearly 40 states.

Fourteen states recorded single-day highs in cases of coronavirus in the first five days of this month. As of Monday, the rate of new infections was flat in other states and retreating in just one: New Hampshire. Those numbers are alarming and justify redoubled caution — including in states and regions that have so far been spared the worst.

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