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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.

“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.”

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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.


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.

  • To all of you eager to be tested with hopes that you already had the virus and, therefore, are immune, I hope your interest is not “merely” to get back to work and resume your life, but also to be able to donate blood so that your antibodies can help create a serum to help those who become sick with the disease. One person’s antibodies can create enough serum to help three sick people. It’s the best we can do (in addition to other therapies) until there is a vaccine, probably in early 2021. If you’re interested in being tested to see if you can donate blood, I know they are doing this at Mount Sinai Medical Center in New York City. You can give them a call and follow their instructions. Good luck–

  • I live in Missouri and I was very sick for two weeks at the end of March and had every symptom but was able to recover at home. They weren’t doing much testing here at that time so I never even considered being tested but I would be interested in helping with this study as well.

  • Detecting infected persons not discovered by testing should be possible by taking reported additional (new) deaths on a given day, and an assumed mortality of x % (e.g., 1 % if China is an example), and then take the average length from infection to death, say 21 days, that should give you a figure of newly infected on that day x-21. It there have been new deaths reported in the US of 3.500 on day x, the number of newly infected on x-21 would 350.00, or 175.000 if the death rate should be 2 %, or – more likely – if the rate is below 1 %, an even higher number. – Greetings from Germany – Alex

  • I’m interested in the antibody testing I would like to partake multiple family members suffered the symptoms last fall and the doctors told us no worry just a flu but now we’re wondering did we really have a flu or did we have covid-19 please get back to me because we all would like to help out in this study thank you

  • Yes, I also live in Ohio and would like to know if testing is available in our area. Not knowing leaves us sitting on the sideline. If we know we have the antigens then we can return to work and contribute to helping others.

  • I have never had the flu of any type I’m now 65) but early November 2019 I got sick. Looking back I had every symptom of COVID-19 with the exception of loss of smell. I was sick for a couple of weeks followed by fatigue and almost seemingly a relapse but the fact is it was a slow recovery. I did fully recover but I can say that was the sickest I have ever been.

  • I was very sick the end of October, thru November and first week of December. It went into my chest and lungs right away, which usually doesnt happen when I get sick. I had problems breathing and it actually hurt to breath for a couple of weeks. Had a nasty cough thought it might be bronchitis then I actually started thinking I had walking pnuemonia which Ive never had. Never went to the doctor because I was scheduled for eye surgery 12/23 and wanted to get the surgery over with and was afraid it would be canceled. Could this have been an early Covid-19 case? Is there a test that would show what I had. I live outside Sacramento CA.

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