We test whether earlier social distancing affects the progression of a local COVID-19 outbreak. We exploit county-level rainfall on the last weekend before statewide lockdown. After controlling for state fixed-effects, temperature, and historical rainfall, current rainfall is a plausibly exogenous instrument for social distancing. Early distancing causes a reduction in cases and deaths that persists for weeks. The effect is driven by a reduction in the chance of a very large outbreak. The result suggests early distancing may have sizable returns, and that random events early in an outbreak can have persistent effects on its course.
From the paper:
Counties where more people left home on the pre-shutdown weekend are no more likely to have a moderate case count, but are slightly more likely to have a big outbreak. This result is what might be expected given that differences in the number of infections on the eve of a statewide lockdown will either vanish or be drastically amplified depending on how well the county succeeds in lowering the viral reproduction rate below 1.
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Subject to caveats discussed in the final section, our results suggest moving even a few days more quickly could make a measurable difference. Our results also suggest that completely random events early in the course of a local outbreak can have surprisingly persistent effects on its size.
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