National case surveillance data are constantly changing. As new information is gathered about previously reported cases, health departments provide updated data to CDC. As a result, surveillance data and trends from a previously reported time window might keep changing. Another key challenge is that some people infected with the virus that causes COVID-19 have mild or no symptoms. If testing and health care services are not needed, those people are less likely to be reported as cases. Similarly, people who have had severe outcomes – such as hospitalization, intensive care unit (ICU) admission, and death – are more likely to be reported as cases. These challenges can limit analysis and interpretation of the data.