The Centers for Disease Control and Prevention has begun preliminary studies to try to determine how many Americans have already been infected with SARS-CoV-2, the virus that causes Covid-19, an agency official revealed Saturday. On Friday, the agency said nearly 240,000 people in the country have been infected with the virus and nearly 5,500 have died.

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.

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“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.”

These studies — called sero-surveys — involve drawing blood from people never diagnosed as a case to look for antibodies to the virus. They are conducted by taking a representative sample of people in a city, for instance, ensuring people from different age groups are included.

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It’s known that many people have mild infections when they contract Covid-19. Data from China and elsewhere suggests about 80% of people confirmed to have the infection have mild or moderate symptoms.

But it is also assumed that figure may in fact be low — that more people may have already encountered and fended off the virus than have been detected. There have been reports, for instance, from the Diamond Princess cruise ship involving people who tested positive who recounted having no symptoms at all. The ship was the first of several cruise ships on which the virus circulated widely; nearly 20% of passengers and crew on the ship eventually tested positive for the virus.

Getting a sense of how many mild and asymptomatic cases there are helps authorities plan for future responses to Covid-19 activity. If it’s known that a high percentage of people in a community were likely infected when the virus moved through during its first wave of infections, the response to a reappearance later might be tailored to protect only high-risk people, for instance.

This work is part of ramped-up coronavirus surveillance at the CDC. The agency has been adapting a number of surveillance systems used to record the toll of seasonal flu in the United States to get a near-real-time picture of SARS-CoV-2’s march across the country.

On Friday, the CDC published the first of what will be a weekly Covid-19 surveillance report, based on the model of its longtime influenza report, FluView. The report is based on data from the last week of March. It showed that in that week, nearly 76,000 Americans had been tested for the infection, with nearly 11,000, or 14.5%, testing positive.

It also showed that pneumonia and influenza deaths, which would normally be falling at this time of year as flu season starts to abate, are increasing. Pneumonia deaths have been rising sharply since the end of February — because of Covid-19.

The new surveillance systems will allow the CDC to add context to the daily reports of Covid-19 cases and deaths, said Lynnette Brammer, head of CDC’s domestic influenza surveillance system and COVIDView, the new weekly report. It will help the agency determine who is contracting the virus and being hospitalized because of Covid-19 infections, and who is dying from the disease.

“We’re starting to see different trends, but it will take us a while to get really comfortable interpreting this data,” Brammer said.

Correction: An earlier version of this story misstated that 80% of cases have mild or almost symptom-free disease.