Joe Bresee, deputy incident manager for the CDC’s pandemic response, said the agency hopes to flesh out the portion of cases that have evaded detection using three related studies.

The first, which has already begun, will be looking at blood samples from people never diagnosed as a case in some of the nation’s Covid-19 hot spots, to see how widely the virus circulated. Later, a national survey, using samples from different parts of the country, will be conducted. A third will look at special populations — health care workers are a top priority — to see how widely the virus has spread within them. Bresee said the CDC hopes to start the national survey in the summer; he gave no timeline for the health workers study.

“We’re just starting to do testing and we’ll report out on these very quickly,” Bresee said at a media briefing. “We think the serum studies will be very important to understand what the true amount of infection is out in the community.”