As economic activity grinds to a halt in the U.S. – with businesses and schools closing in response to the ongoing coronavirus pandemic – economists are widely predicting GDP growth will contract in the second quarter of the year, possibly dragging the U.S. into a recession.

But some analysts believe the recession may have already begun, effectively ending America's longest recovery on record.

Alan Blinder, a Princeton University professor and former Federal Reserve Board of Governors vice chairman, told CNBC last week that he "wouldn't be one bit surprised if, when we look back at the data," March is remembered as the month that the U.S. fell into recession.

Anirban Basu, chief economist at Associated Builders and Contractors, issued a statement on Tuesday speculating that "the broader economy is already in recession due to COVID-19."

And Gregory Daco, chief U.S. economist at Oxford Economics, wrote in a research note on Sunday that "the global coronavirus pandemic will lead to a profound, pervasive and persistent reduction in activity across the nation, with widespread reductions in travel and tourism activity, ongoing disruptions to supply chains and a pullback in discretionary outlays."

"The U.S. economy is in a recession," he said.

A recession is defined by the National Bureau of Economic Research as two consecutive quarters of GDP contraction. Officially, the NBER is responsible for designating when recessions begin and end, and it usually takes two quarters – or six months – of a slowdown before that designation can be reached.

Often, it takes longer than that. In late 2008, the NBER formally announced that the U.S. entered a recession in late 2007. In mid-2010, the bureau announced that the recession had ended a year earlier.

Delayed releases of economic data are also a factor. On Tuesday, a Bureau of Labor Statistics report showed the U.S. held 7 million job openings in January. When March's and April's openings are released in the summer, they will likely show a much different employment landscape.

It has thus far proven difficult to estimate the economic impacts of the coronavirus because it has hit the domestic economy fast and hard. Pennsylvania, New Jersey, New York, California and Connecticut are among a group of states that have introduced curfews, non-essential business shutdowns and even "shelter in place" declarations to help stem the spread of the virus.

Although these measures are believed to be effective at limiting person-to-person contact and thus slowing transmission of the disease, they are disastrous for local economies. Their effects, however, have yet to bleed meaningfully into many economic indicators, but they are being seen on a micro level.

"The self-quarantining and social distancing measures taken in response to COVID-19 have already led some businesses in affected industries to announce temporary layoffs and reduce working hours," a team of analysts at Oxford Economics wrote in a report published on Tuesday, noting that much of the domestic economy is "now facing a sudden stop in activity."

It is unlikely the U.S. will get a formal recession diagnosis for at least a few months – should that end up being where the economy falls. For now, analysts appear to be split between speculating that the economy has already entered a recession and saying that it will enter recessionary territory during the second and third quarters of the year – so, April through September.

David Berson, Nationwide's chief economist, predicted on Monday that GDP would shrink by as much as 5% in the second quarter and would "still probably be negative, but not significantly" in the third quarter, marking a sharp but short-lived recession.