This CDC report discusses various factors that contributed to the accelerated spread of COVID-19 in the US during February–March 2020.
These factors include continued travel-associated importations, large gatherings, introductions into high-risk workplaces and densely populated areas, and cryptic transmission resulting from limited testing and asymptomatic and presymptomatic spread.
The authors suggest that targeted and community-wide mitigation efforts, like those imposed in March, were needed to slow transmission. They also caution that interventions that were critical in the early stages, such as quarantine and airport screening, may have less impact now when transmission is widespread in the community. Nevertheless, they argue that many elements of the mitigation strategies used during the acceleration phase will still be needed in later stages of the outbreak, which are forthcoming.