Limiting indoor capacity to 20% can greatly reduce COVID-19 infections, study says
Limiting the number of people who are close together indoors for an extended period of time — such as in gyms, hotels, cafes, religious centers and restaurants — can reduce new coronavirus infections by about 80%, according to a new study from Stanford University.
Researchers tracked the movements of 98 million Americans with anonymous cell phone location data in 10 major cities from March to May through half a million indoor establishments. They then developed a computer model that accurately predicted the spread of COVID-19 by analyzing where people went, how long they stayed there and how many people shared the same space at the same time.
The model’s predictions “tracked closely with actual reports from health officials,” according to a university news release, suggesting it can serve as a tool for policymakers to determine the trade-offs between new infections and lost sales if businesses were to close again or reduce capacity limits due to spikes in COVID-19 cases across the country.
The study not only “offers the strongest evidence yet” that stay-at-home mandates from earlier in the pandemic prevented new infections, the researchers said, but it also revealed that people of color have about double the risk of contracting COVID-19 than white people when visiting indoor spaces, such as grocery stores.
A paper was published Tuesday in the journal Nature.
“In the past, these disparities have been assumed to be driven by preexisting conditions and unequal access to health care, whereas our model suggests that mobility patterns also help drive these disproportionate risks,” study co-author David Grusky, a professor of sociology at Stanford’s School of Humanities and Science, said in the news release.
“Because the places that employ minority and low-income people are often smaller and more crowded, occupancy caps on reopened stores can lower the risks they face,” Grusky added.
Demographic information from Census data combined with cell phone tracking information showed that grocery stores visited by predominantly lower-income people of color had 59% more hourly visitors per square foot than wealthier locations. Marginalized communities also spent 17% more time shopping in these stores.
The model combined data collected from March 8 to May 9 from 10 of the nation’s largest metropolitan areas: New York, Los Angeles, Chicago, Dallas, Washington, D.C., Houston, Atlanta, Miami, Philadelphia and San Francisco.
The researchers developed a series of equations to calculate the probability of coronavirus spread, or infection rates, at different times, places and scenarios. When the team fed the computer the number of COVID-19 infections actually reported in each city each day, the model’s predictions nearly matched.
For example, it found that capping indoor maximum capacity to 20% in the Chicago metro area could cut down new infections by 80% while losing 42% of overall visits — “and we observed similar trends across other metro areas,” the researchers said in their study.
Also in Chicago, just 10% of the places visited accounted for 85% of predicted infections, with certain locations contributing more to viral spread than others. The study points to full-service restaurants and hotels as some of the more risky locations.
“It corresponds to what we thought from the beginning, that there are certain activities that lead to spread more than other activities,” Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security who was not involved in the study, told STAT. “When you are thinking of targeted public health interventions, it’s important to focus on those activities where that’s occurring and not being overly blunt and blocking and stopping all activities that may not necessarily have been a major contributor to spread.”
But experts say that circumstances have changed since May when data collection for the study came to an end.
Now, there are more mitigation measures in public, indoor places. New research is also pointing to smaller gatherings in homes as a major driver in new infections, especially as winter nears.
This story was originally published November 12, 2020 at 12:26 PM with the headline "Limiting indoor capacity to 20% can greatly reduce COVID-19 infections, study says."
