With its entire membership of 51,000 professional actors and stage managers out of work, Actors’ Equity Association on Tuesday announced the health and safety guidelines it expects theaters to follow before it will authorize its members to return to the stage.

The list of “core principles” devised by David Michaels, a former federal public health official retained by Actors’ Equity, is a statement of four broad concerns the union wants theater producers and companies to address. It covers such issues as widespread testing and contact tracing; the quick isolating of individuals symptomatic or ill with the coronavirus; modifying theaters and productions to minimize the spread of the pathogen; and a process in which the union is included in any decision-making about how and when to reopen.

In a Zoom news conference, union president Kate Shindle and executive director Mary McColl made it clear that conditions for productions to resume have not been met anywhere in the country. They confirmed they were advising their members at this point not to go back to work at any theater that has announced plans to reopen. The schedules for the vast majority of theaters remain sketchy, although a few companies with summer programs, including the Muny in St. Louis and Barrington Stage in western Massachusetts, have disclosed tentative plans to reopen in July and August, respectively.

“We believe when we reopen we have one opportunity to get it right,” McColl said.

Michaels — an epidemiologist who was an assistant secretary of labor in the Obama administration and is now a professor at George Washington University’s Milken Institute of Public Health — noted the singularity of health and safety obstacles faced by live venues.

“Our understanding of our primary tools to reduce exposure — they’re challenging in some situations and impossible in others,” he said.

Because of the many challenges that close physical contact presents to actors and other stage personnel, Michaels stressed that no single approach to ameliorating the threat would guarantee safe conditions. He likened the proposed remedies to slices of Swiss cheese placed one on top of another, with each layer covering up some of the holes in the slices below.

The release of the core principles — which the union said it will pass on to theaters and producers, as well as to its members — underscored some of the unique and urgent issues affecting workers in the arts and entertainment industry. The public unveiling grew partly out of a concern that much of the attention about guaranteeing safety in theaters was being directed at audience members.

As McColl pointed out, performers “have unique problems from other workplaces” that exacerbate airborne transmission of the coronavirus, believed to be the main method of its spread. They take jobs, for instance, that require them to sing, to “talk loudly in enclosed spaces,” and to be in close contact onstage and backstage.

Still, the guidelines are fairly general, and a mechanism for how they might be applied remains unclear.

Equity’s requirement that the “epidemic must be under control” before a theater reopens, for example, stipulates there must be “few if any new cases in the area.” In a city as complex as New York or Chicago, that standard could be unmet for many months, if not years — or until a vaccine is widely available.

Even theaters that are testing the waters for a quick reopening are hedging their bets. The more than century-old Muny, an 11,000-seat outdoor amphitheater, has plans to begin performances of “Chicago” on July 20. On its website, though, it cautions it will push the summer season back to 2021 “if by June 8 conditions for a July 20 start have not been deemed safe and positive.”