When will we reopen? How Pennsylvania decides what’s in the red, yellow, and green phases.

It’s taken a lot of sacrifice. Businesses closed. Millions of people ordered to stay home. But the confirmed coronavirus case curve in the Philadelphia region is slowly flattening.

State and local officials are monitoring it closely. As the curve bends down, they are making plans to eventually lift the heavy restrictions that have all but shuttered the economy and brought normal life to a standstill since March.

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“I can’t put a date on when our activities can safely restart, but we can start to prepare,” Philadelphia Public Health Commissioner Thomas Farley said.

It will take time. And the decision to reopen counties depends on a number of factors. Many of those are data-based, such as the number of new cases. But others are qualitative, such as whether local industries are able to practice social distancing. In the end, it’s a judgment call.

Gov. Tom Wolf has laid out a system of three colored “phases,” with different orders in each.

Philadelphia and the surrounding region are still in the red phase: Only “life-sustaining” businesses are allowed to open, and residents are supposed to stay home except for essential trips.

But some counties are already moving into the yellow phase that allows most (but not all) businesses to reopen and puts limitations on social gatherings. No counties are yet in the unrestricted green phase.

How do they decide? Here’s what we know about how the state is making its reopening decisions.

Large-scale lockdowns are about what epidemiologists call “mitigation,” a strategy where a virus is spreading so widely and quickly that it’s impossible to contain. That’s what happened in March, when Pennsylvania’s outbreak quickly overtook our capacity to monitor and stop it.

How do you mitigate? By minimizing contact between people and stopping the spread of the coronavirus.

Under mitigation, we are told to stay home, skipping getting tested if we believe we are sick but with manageable symptoms.

Now, state and local governments are working to shift to a stage of “containment,” where they can catch and isolate individual cases of COVID-19.

Under containment, getting tested immediately is key to being able to do “contact tracing,” which means identifying any people who may have been exposed to the virus.

Containment means there needs to be:

That’s why state officials say reopening counties is about not only bringing down the number of cases; it means there need to be the resources to prevent and respond to outbreaks.

» READ MORE: Gov. Tom Wolf warns he will withhold funds from Pa. counties that violate coronavirus stay-at-home order

The geography of this can get tricky — viruses don’t respect political boundaries, but you also don’t want to put areas under restrictions that don’t make sense for them.

The Pennsylvania Department of Health divided the state into six regions and is looking at the whole region and each county when it considers reopening.

Deciding for the region is better because it allows for better coordination, said Lyndsay Kensinger, spokesperson for Wolf. “However, county data will also be reviewed as part of our analysis, affording us insight into whether a county should transition.”

» READ MORE: Can Gov. Wolf withhold coronavirus recovery money from counties that defy shutdown orders? Yes, but expect pushback, experts say.

How many new people are getting sick? Case counts are one of the most important things to look at because officials must be able to quickly respond to confirmed cases, and hospitals must have the capacity to treat them.

In order to reopen, the curve of new confirmed cases needs to fall low enough to what officials believe is a manageable number. Before a county can move to the yellow phase, the state looks at a two-week period and requires counties to have fewer than 50 new confirmed cases per 100,000 residents.

That means Philadelphia, with a population of 1.58 million people, would need to have fewer than about 800 new confirmed cases over two weeks. The city is nowhere near that right now.

The other counties in the Southeastern Pennsylvania region have lower new case rates but are still above the target.

Lack of testing has been a problem with the pandemic across the United States, and the state says that needs to change before counties can reopen.

For now, the focus is on diagnostic testing, the kind that identifies whether you have an active infection. Serology tests, which look for antibodies created by the immune system in fighting off an infection, will play a role later to help determine how many people have been infected and are potentially immune.

Widespread testing is key to protecting front-line workers such as health-care personnel and first responders, as well as for flagging cases as they pop up. With enough testing, counties can identify sick residents and determine whether they spread it to others, helping identify and stop the spread.

Officials and health-care workers have warned from the start of the outbreak about shortages of some personal protective equipment, including masks.

The state’s plan for reopening counties depends in part on the stockpile of protective equipment. Reopening counties means risking a spike in cases, and officials are wary of doing that if hospitals say they don’t have enough supplies to safely handle that.

The state and Carnegie Mellon University have built a dashboard that predicts what is likely to happen if restrictions are lifted, using data from a number of state agencies.

The dashboard, which is not available to the public, looks at the risk of exposure, the ability of hospitals to respond to cases, how the economy is affected, and supply chains, according to the state guidance.

But officials also look at population density, where state or local health departments are located, health-care capacity, and how close the area is to coronavirus hot spots in nearby counties.

In the end, there’s no one algorithm for deciding when to open back up. The dashboard and other data help guide the decision-making process, but there are qualitative factors that play a role, too.

For example, the state will look at what kinds of industries are operating in a county, and how they can reopen — some local economies are more damaged than others by shutdown orders, and different industries have different risks of virus exposure.

The state also wants there to be a plan for protecting high-risk places such as jails and prisons, nursing homes, and residential medical facilities. That needs to include a way to monitor employee health and screen visitors.

Many of these factors are subjective — do officials believe the right plans and procedures are in place? — and the decision to reopen is ultimately a judgment call.

If a county looks ready to reopen on paper, but the state worries about some other factor, it could choose to keep that county in the red phase. On the other hand, a county that seems borderline between red and yellow criteria could have other reasons for reopening.