Experts warn many schools should delay reopening, as data indicates virus resurgence across much of country

As communities across the country grapple with whether to bring students back into the classroom, a new COVID-19 data projection dashboard and school reopening guidance released Thursday by the PolicyLab at Children's Hospital of Philadelphia seek to help school districts and parents make informed decisions and lead to sustained school reopenings.

The Children's Hospital of Philadelphia was granted access to Health and Human Services and Centers for Disease Control and Prevention data pertaining to county positivity rates for the project. The hospital uses its projections to provide ongoing information to the federal coronavirus task force, and its data is used to update states with information about the spread of the virus within their various communities.

Many areas are struggling to contain the spread of the infection, as PolicyLab projections continue to show concerns for virus resurgence across much of the country. In an interview on ABC's "Good Morning America," Dr. David Rubin, director of PolicyLab at CHOP and professor of pediatrics at the University of Pennsylvania's Perelman School of Medicine, said that he is particularly concerned about the impact of Labor Day travel, with Americans potentially bringing infections back into metropolitan areas, and then schools.

PolicyLab's data modeling project tracks and forecasts COVID-19 transmission for 750 of the larger counties across the United States, using concrete metrics such as temperature, humidity, population density and social distancing, while also accounting for population characteristics, such as age, insurance status, crowding within homes and others. These counties represent 80% of the U.S. population.

It is sensitive to local community contexts, predicting micro-level trends of cases and positivity rates that each county could experience over the next four weeks, thus allowing public officials to effectively use the latest data when determining whether opening schools is safe or if a delay is necessary.

Although early evidence suggests children are at lower risk for severe complications from COVID-19, the risk is not zero. Some children who do become sick require hospital-level care, including a very small subset who develop an inflammatory syndrome following SARS-CoV-2 infection.

Evidence also suggests that symptomatic children of all ages can spread COVID-19, and older children have similar transmission risks as adults.

The most important inputs to PolicyLab's models are the rate of growth in the number of cases, as well as the test positivity rates, and when these are "north of 8% to 10%, you know you're in trouble," Rubin said.

However, each state has to be measured against its own bar. In areas like New York, for example, with its tremendous testing capacity, rates of positivity will likely not be higher than 3% to 4%, given the high number of people getting tested. Hence the importance of looking at county-specific historical trends of case incidence and testing positivity.

"Schools have to be very careful," said Rubin. "It's a national tragedy that we have not been able to get our schools open in time after Labor Day. But we are where we are right now."

For a district to have full or hybrid reopening, the threshold set by PolicyLab is to have a stable or declining number of 10 cases per 100,000 per week, and less than 5% positivity.

If a district has 10 to 35 weekly cases per 100,000, and less than 5% positivity, the recommendation is an incremental reopening strategy, with special needs and/or elementary-age children returning first to the classroom.

A district with a stable number of cases, and 5% to 9% positivity can "cautiously" continue its in-class instruction, if started, but only if there is no evidence of virus transmission.

Lastly, districts with a 9% positivity rate or above should revert to remote learning.

For high-contact sports to take place, the weekly case incidence must be less than 10 per 100,000, with test positivity under 1%. Competitions should only take place with teams from places of similar positivity rates and case incidences.

If the test positivity rate is between 1% and 5%, and there is a stable or declining rate of weekly case incidence, only moderate-contact sports, such as field hockey, soccer and lacrosse, will be allowed, with competition limited to a few teams from areas with similar positivity rates and case incidences.

When test positivity is between 5% and 9%, and there is a stable weekly case incidence, all sports may do individual drills and distanced or masked group training. Lower-contact sports, such as swimming, track, golf, baseball and softball, can compete, but only with a few partner schools experiencing similar numbers.

If test positivity is above 9%, all team competition and practices should revert to individual or online training.

The threshold for full or hybrid reopening is strict, according to Rubin, and one that very few communities are actually able to meet today. Of the 750 counties, "only a few dozen of those counties would be prepared to open without exception."

Further, he said, "Most of the school districts in the larger cities in this country are not prepared to reopen based on our thresholds."

"This conservative threshold represents the incidence at which much of Europe and Southeast Asia reopened their schools -- the only lasting successes we've seen to date," Rubin said. "The exceptions were countries like Denmark and Belgium, which opened within our intermediate threshold of up to 35 weekly cases per 100,000."

"Less than 20% of the counties in our study would meet the intermediate thresholds as of today," said Rubin, while the other 2,000-plus counties in more sparsely populated areas are likely in a better position to open after Labor Day.

"Some people would argue that the threshold is too low," acknowledged Rubin. "But our job is to identify the thresholds that would be safe, and this also has sustained opening. If you open your classroom, and you are closing in two weeks, how are you going to reopen them? What will that next reopening look like? You're going to lose the confidence of your teachers, your parents. And so what we're trying to do is to provide a sustained pathway to safety and success."

There is much concern about the virus continuing to spread throughout the country, particularly with the reopening of many colleges and universities. Georgia, Indiana and North Carolina are among the states seeing an increasing number of clusters.

According to Rubin, this week's projections have worsened significantly in California, particularly throughout the Bay Area, the Central Valley and the Sacramento region.

"California is raging right now, raging even worse than it was a couple weeks ago. In many ways, California looks like Arizona did a few weeks ago; the Central Valley continues to astound us with even worse rates," Rubin said.

In the Midwest, from Minnesota to Kansas to Ohio, forecasts show limited progress and, in some cases, troubling upward trends.

However, because of mitigation strategies, such as mask mandates implemented in certain states and counties, transmission rates have dropped and even stayed down. Louisiana, Alabama and Mississippi and Orange County in Florida have all shown improvement.

Ultimately, Rubin concluded, "The goal has always been to get our case incidents down within our communities, because the safest way to protect our teachers, our families and our children is to ensure that very little virus gets into the building."

ABC News' Nicholas Coulson contributed to this report.