TABLE. Factors contributing to COVID-19 acceleration and corresponding public health actions — United States, January–April 2020.

Factor contributing to acceleration	Examples	Public health actions
Continued travel-associated importations of the virus	Travelers arriving from countries or cruise ships with ongoing transmission	Travel health notices, traveler screening (including risk assessment, public health management and monitoring), travel restrictions, federal isolation and quarantine orders, educating travelers and clinicians regarding symptoms and evaluation
Large gatherings	Social, cultural, and professional gatherings where persons convene and then disperse over broad areas	Restricting mass gatherings; global travel restrictions and domestic travel recommendations, recommending transition to virtual events
Introductions into high-risk workplaces/settings	Long-term care facilities, hospitals, correctional facilities, and homeless shelters	Restricting visitor access, establishing cohort units or facilities for residential settings, vigorous contact tracing around persons with confirmed cases, increased infection control, environmental surface cleaning, use of recommended personal protective equipment
Crowding and high population density	Densely populated areas, crowded workplaces, schools, and public spaces	Stay-at-home orders, recommendations for hand washing and social distancing, cloth face covering guidance, school dismissals, extended telework, environmental surface cleaning
Cryptic transmission	Presymptomatic or asymptomatic spread, limited testing, co-occurrence with circulation of other respiratory viruses	Increased testing, COVID-19–specific surveillance, cloth face covering guidance, aggressive contact tracing accompanied by quarantine and/or testing of asymptomatic contacts, stay-at-home orders

Abbreviation: COVID-19 = coronavirus disease 2019.