Mass General study shows social distancing slowed growth of COVID-19

Government-issued physical distancing orders in the United States significantly slowed the novel coronavirus (COVID-19), leading to an estimated reduction of more than 600,000 cases within three weeks of implementation, according to a new study by clinicians at Massachusetts General Hospital in Boston and published in the journal PLOS Medicine.

The MGH researchers, in collaboration with colleagues at University College London, the Harvard T.H. Chan School of Public Health, and the Perelman School of Medicine at the University of Pennsylvania,  analyzed data from the first five months of the COVID-19 epidemic in the U.S. They collected data on government-issued orders on statewide physical distancing measures and compared changes in COVID-19 cases and COVID-19-attributed deaths in states that implemented physical distancing measures before and after implementation.

The results show the average daily COVID-19 case growth rate began declining approximately one incubation period after implementation of the first statewide physical distancing measures. The period required for the number of cases to double increased from approximately four days to eight days within three weeks of implementation. These findings are consistent with other recently published work, the researchers said.

According to the researchers, what is unique to this new study is they found that the average daily COVID-19-attributed death rate also began declining after the implementation of physical distancing measures, which prior to this study had not been analyzed.

The study looked at a wide array of measures, including school and business closures, restrictions on public gathering, and shelter-in-place orders. Most combinations of these orders appeared to have similarly beneficial effects. Because the different types of physical distancing measures were generally implemented in close temporal proximity to each other, the research team said it was unable to determine specifically which types of physical distancing measures were most effective.

The findings of the model suggest that statewide physical distancing measures reduced the total number of reported COVID-19 cases by approximately 1,600 cases by one week after implementation and, due to the exponential growth of the spread, by approximately 621,000 cases by three weeks after implementation, the researchers said.

One of the most significant limitations of the study is that the implementation of statewide physical distancing measures was not a controlled experiment. If state governments intensified physical distancing measures in response to worsening local epidemics, the analysis would likely have shown that the policies were less effective. The authors said that, in many states, people may have begun spontaneously changing their behavior in response to a worsening local epidemic even prior to any statewide measures.

"Many have strongly suspected that physical distancing policies helped interrupt COVID-19 transmission during the early days of the U.S. epidemic," said Mark Siedner, MD, MPH, an infectious diseases physician at MGH and Associate Professor of Medicine at Harvard Medical School, who co-authored the research, in a statement. "This study adds clear evidence to support those suspicions. The results show the timing of government-issued orders correlated strongly with reductions in both cases and deaths. In short, these measures work, and policy makers should use them as an arrow in their quivers to get on top of local epidemics where they are not responding to containment measures."

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