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Estimated 35 Million US COVID-19 Cases Without Social Distancing
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Mark Mascolini
Without four common social-distancing measures applied in March and April, 2020, researchers estimate the United States would have had 35 million COVID-19 cases on April 27 rather than the nearly 1 million it actually had [1]. The investigators believe their findings illustrate "the potential danger of exponential spread in the absence of interventions, providing relevant information to strategies for restarting economic activity."
Researchers at the University of Kentucky and other centers recorded the daily growth rate in COVID-19 cases in all counties across the United States for 58 days from March 1 to April 27, 2020. The later date marks the first removal of one of the four types of social distancing analyzed: (1) large event bans, (2) school closures, (3) closures of entertainment-related businesses (including restaurant dining rooms, bars, and gyms), and (4) shelter-in-place orders. Case numbers came from the 2019 Novel Coronavirus COVID-19 Data Repository at the Johns Hopkins Center for Systems Science and Engineering.
The investigators calculated daily exponential growth rate in cases as the natural log of cumulative daily COVID-19 cases minus the log of cumulative daily cases on the previous day. Because they multiplied the case growth rate by 100, it can be read as percentage point changes. The researchers used an event-study design to assess the impact of the four distancing measures on case growth rate. Analysis of each of the four distancing strategies included 7 variables: whether it was implemented 1-5, 6-10, 11-15, 16-20, or more than 20 days ago; and whether it will be implemented 5-9 or 10 or more days later.
Across the United States, confirmed COVID-19 cases grew from only 30 on March 1, 2020 to 978,047 on April 27. On March 1 no US county had implemented any of the 4 distancing policies studied. By March 29 all 4 policies were in place for about 65% of the US population, and by April 7 about 95% of the population had all 4 policies.
Compared with a reference category of 0-4 days before implementation, shelter-in-place orders alone led to statistically significant (P < 0.01) drops in the COVID-19 case growth rate of 3.0 percentage points after 6-10 days, 4.5 percentage points after 11-15 days, 5.9 percentage points after 16-20 days, and 8.6 percentage points from day 21 on.
Compared with 0-4 days before implementation, combining all 4 distancing measures cut the COVID-19 growth rate by 5.4 percentage points in days 1-5 after implementation, by 6.8 percentage points 6-10 days after implementation, by 8.2 percentage points 11-15 days after implementation, by 9.1 percentage points 16-20 days after implementation, and by 12.0 percentage points 21 or more days after implementation.
If the researchers held the amount of voluntary social distancing constant, the results imply 10 times greater spread of COVID-19 by April 27 (10 million cases) without shelter-in-place orders, and more than 35 times greater COVID-19 spread by April 27 (35 million cases) with none of the 4 distancing measures.
The research team urges caution in taking the predicted number of COVID-19 cases avoided too literally because "simulations that use estimated parameters to predict outside the range of observed policy variation are inherently subject to a high level of uncertainty that is difficult to quantify." The investigators believe their findings "provide credible empirical evidence" that social distancing had its intended effect of slowing the epidemic in the United States.
An unanswered question, the authors add, is whether "cases averted" will turn into "cases delayed." They believe "a premature return to light measures would make this more likely."
Reference
1. Courtemanche C, Garuccio J, Le A, Pinkston J, Yelowitz A. Strong social distancing measures in the United States reduced the COVID-19 growth rate. Health Aff (Millwood). 2020 May 14:101377hlthaff202000608. doi: 10.1377/hlthaff.2020.00608. https://www.healthaffairs.org/doi/10.1377/hlthaff.2020.00608
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