https://www.cdc.gov/coronavirus/2019-ncov/php/public-health-recommendations.html

Public Health Guidance for Community-Related Exposure | CDC

*For the purpose of this guidance, fever is defined as subjective fever (feeling feverish) or a measured temperature of 100.4°F (38°C) or higher. Note that fever may be intermittent or may not be present in some people, such as those who are elderly, immunocompromised, or taking certain fever-reducing medications (e.g., nonsteroidal anti-inflammatory drugs [NSAIDS]).

** Data to inform the definition of close contact are limited. Factors to consider when defining close contact include proximity, the duration of exposure (e.g., longer exposure time likely increases exposure risk), and whether the exposure was to a person with symptoms (e.g., coughing likely increases exposure risk). While research indicates masks may help those who are infected from spreading the infection, there is less information regarding whether masks offer any protection for a contact exposed to a symptomatic or asymptomatic patient. Therefore, the determination of close contact should be made irrespective of whether the person with COVID-19 or the contact was wearing a mask. Because the general public has not received training on proper selection and use of respiratory PPE, it cannot be certain whether respiratory PPE worn during contact with an individual with COVID-19 infection protected them from exposure. Therefore, as a conservative approach, the determination of close contact should generally be made irrespective of whether the contact was wearing respiratory PPE, which is recommended for health care personnel and other trained users, or a mask recommended for the general public.

***Data are insufficient to precisely define the duration of time that constitutes a prolonged exposure. Recommendations vary on the length of time of exposure, but 15 minutes of close exposure can be used as an operational definition. Brief interactions are less likely to result in transmission; however, symptoms and the type of interaction (e.g., did the infected person cough directly into the face of the exposed individual) remain important.

Integration of these definitions and actions into communications and actions of public health authorities can be guided by CDC's "Implementation of Mitigation Strategies for Communities with Local COVID-19 Transmission"