A look back at the main drivers behind early COVID-19 outbreaks in more than 200 US counties attests that social distancing was, as it is now, among the most effective interventions to reduce the spread of the disease. Research by DBEI and PolicyLab at Children’s Hospital of Philadelphia (CHOP), published in JAMA Network Open, portrays multiple local stories of the pandemic in the US dating back to February of this year. Confirming the interim findings of their innovative model, the team found that the benefits of spring-like temperatures faded when local communities—particularly those in densely populated areas—gradually reopened.
The results show in retrospect that more cautious re-openings might have reduced case counts, and they show why local communities’ efforts to limit gathering sizes, reduce occupancy in places of business, and require universal masking have been critical to containing viral transmission. Crucial to the findings, the study uses actual local case data to project the epidemic. “Our model is unique in that it considers the impact of local-area effects—including temperature, humidity, social distancing, population density and population characteristics—on COVID-19 transmission and risk of resurgence,” says Jing Huang, PhD, who leads statistical development. The key factor, social distancing, is measured as the percent change in visits to non-essential businesses (e.g., restaurants, hair salons) within each county.
The dynamic model allows the team to observe trends within counties and regions in real time, as they evolve, and has revealed some disturbing new facts. “We could see the earliest signs of recent upticks in various regions, as the disease spread along travel corridors in California, Texas, and Florida,” Dr. Huang says. “We have also seen resurgence is beginning in the Upper Midwest and Northeast and more serious commitment on social distancing may be needed to reverse the tide,” she adds. “As we’ve seen across the country in recent weeks, higher temperatures alone were never going to be enough to stop the spread of this dangerous virus,” comments Gregory Tasian, MD, MSc, MSCE, a study co-leader. A recent blog post explores the newest findings.
The team continually adjusts to ensure they are generating an accurate picture. One important addition, says Dr. Huang, was to incorporate daily testing positivity rates, or proportions of daily tests that are positive in an area. “Growing rates are often related to infectious surge in an area, but when we see increasing cases with flat or declining positivity rates, it may actually result from increased testing capacity or contact tracing.”
Challenges remain—and chief among them is to capture the effects of mask wearing, Dr. Huang says. The team has manually collected data on masking policies for all counties, including state, county and city level mandates. “It’s hard to isolate the true effect without data on compliance,” she comments, “however we think mask use is also key to mitigating transmission, as it is closely related to distancing.”
David Rubin, MD, MSCE, PolicyLab’s director and lead author on the study, agrees: “As the pandemic resurgence continues, we must commit to social distancing and to universal masking nationwide in order to gain control of this epidemic and avoid a potentially catastrophic fall and winter season.” In another recent blog post, the team considers the merits of a national strategy.
Each week, users can find new four-week case projections—now for 520 counties, representing 71% of the U.S. population and 88% of all identified coronavirus cases—by the COVID-Lab: Mapping COVID-19 in Your Community project.