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Spring stay-at-home orders linked to a significant slowing down of COVID-19’s spread

Researchers calculated the pandemic’s doubling time — how long it took for confirmed COVID-19 cases to double — before and during the stay-at-home periods.

stay-at-home
The 45 states that implemented stay-at-home orders added an average of twelve days to their doubling time, suggesting a significant slowing of the virus’ spread.
Photo by Annie Spratt on Unsplash

The stay-at-home orders that most states put into place last spring appear to have had a significant impact on slowing down the spread of the coronavirus, both nationally and in individual states, according to a study published in the Journal of Infectious Diseases.

A team of researchers, led by Mark Lurie, a professor of epidemiology at Brown University, looked state by state and nationally at how quickly COVID-19 spread before and after stay-at-home orders were implemented. They did this by calculating the pandemic’s doubling time — how long it took for confirmed COVID-19 cases to double.

“Increased doubling time indicates a slowing of the epidemic — more days are required for the cumulative number of cases to double,” they explain in their paper.

At the state level, the researchers began by calculating the pandemic doubling time during the period prior to each state’s stay-at-home orders, starting on March 4 (the day when 100 cumulative cases were reported nationwide). They then compared it to the doubling time in a period that began 14 days after each state’s orders were implemented — through April 30.  (The 14 days permitted a “buffer” to allow for the minimum period of time for policy changes to impact the number of new cases.) For the five states that didn’t enact stay-at-home orders (Arkansas, Iowa, Nebraska, North Dakota and South Dakota), the researchers calculated the doubling time over the entire period.

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To determine the national doubling time, they compared the doubling time of March 4-April 4 with that of April 5-April 30.

Key findings

The data revealed that before the lockdowns became widespread (March 4-April 4), the number of COVID-19 cases in the U.S. doubled in less than three days (2.68 days), indicating a rapid spread of the virus. After the lockdowns, the doubling time expanded by 460 percent— to an average of 15 days.

This map highlights the percentage increase in epidemic doubling time between pre-stay-at-home order and during stay-at-home order time periods, by state.
Brown University
This map highlights the percentage increase in epidemic doubling time between pre-stay-at-home order and during stay-at-home order time periods, by state.
All states saw their doubling time increase, but the rate of the increase varied. The 45 states that implemented stay-at-home orders added an average of twelve days to their doubling time, suggesting a significant slowing of the virus’ spread. The states that didn’t have some form of lockdown, however, added only about six days to their doubling time.

Minnesota had the smallest increase in its doubling rate among the states that issued stay-at-home orders. Its pandemic doubling time went from every 5.89 days before March 27, when Gov. Tim Walz issued a stay-at-home order for the state, to every 9.20 days afterward — a 56 percent increase.

Louisiana had the biggest increase in its doubling rate, from every 2.99 days before its lockdown to every 29.41 days afterward — an 884 percent increase.

“These findings suggest that stay-at-home orders combined with varied levels of implementing CDC-recommended practices of testing, tracing and isolation, as well as travel restrictions, likely played a key role in significantly reducing the epidemic growth rate,” the study’s authors conclude.

Limitations and implications

The study relied on available COVID-19 data, which, as the researchers point out, likely under-reports the true number of cases in the U.S. In addition, the study is observational, which means it can’t prove a cause-and-effect relationship between the lockdowns and the increase in the COVID-19 doubling times.

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Still, these findings are supported by previous studies that have made similar observations. Research published in May in the American Journal of Infection Control reported that the COVID-19 doubling rate increased from every 5 or 6 days to every 14 days after states locked down last March and April.

Also in May, University of Minnesota researchers reported in the Journal of the American Medical Association (JAMA) that stay-at-home orders were associated with a significant reduction in the number of Americans hospitalized for COVID-19

“We hope that these findings contribute to a growing body of evidence aimed at studying the full course of COVID-19 in America,” says Joe Silva, a co-author of the current study and a Ph.D. student at Brown’s School of Public Health, in a released statement.

“This study does not imply stay-at-home orders were the sole factor that drove the observed increase in epidemic doubling time, but the data may be representative of the impact of multiple public health measures,” he added.

FMI: You can read the full study on the website for the Journal of Infectious Diseases.