We use U.S. county-level location data derived from smartphones to examine travel behavior and its relationship with COVID-19 cases in the early stages of the outbreak. People traveled less overall and notably avoided areas with relatively larger outbreaks. A doubling of new cases in a county led to a 3 to 4 percent decrease in trips to and from that county. Without this change in travel activity, exposure to out-of-county virus cases could have been twice as high at the end of April 2020.

Limiting travel-induced exposure was important because such exposure generated new cases locally. We find a one percent increase in case exposure from travel led to a 0.21 percent increase in new cases added within a county. This suggests the outbreak would have spread faster and to a greater degree had travel activity not dropped accordingly.

Our findings imply that the scale and geographic network of travel activity and the travel response of individuals are important for understanding the spread of COVID-19 and for policies that seek to control it.

我们使用从智能手机获取的美国县级位置数据来研究疫情爆发早期阶段的旅行行为及其与 COVID-19 病例的关系。人们总体上出行较少，尤其避开了疫情相对较大的地区。某个县的新病例翻倍会导致往返该县的出行量减少 3% 至 4%。如果没有旅行活动的这种变化，到 2020 年 4 月底，接触县外病毒病例的数量可能会增加一倍。

限制旅行引起的接触很重要，因为这种接触会在当地产生新病例。我们发现，旅行导致的病例增加 1%，就会导致县内新增病例增加 0.21%。这表明，如果旅行活动没有相应下降，疫情的传播速度可能会更快，程度也会更大。

我们的研究结果表明，旅行活动的规模和地理网络以及个人的旅行反应对于了解 COVID-19 的传播以及寻求控制它的政策非常重要。