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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.

  • It is truly terrifying that this didn’t start 3 weeks ago.

    What’s more terrifying is CNN referring to MD’s as “Real Doctors” over the past 48 hours.

    Allowing the difficult public policy decisions to be driven by clinicians and hyperbole in the media is not helpful.

    Instead of fear-mongering, these guys should have been advocating for the urgent need for epidemiological surveillance so that we we be able to have a better sense of the DENOMINATOR, and factors that impact the R0.

    MDs on front line have a very specific role and should not be relied upon to say anything useful about the broader population level implications.

    As evidenced by the IMHE model, the epidemiologists really aren’t very good at forecasting.

    We won’t know the counterfactual “do nothing” count of deaths, but I’d wager the Imperial College model was laughably wrong.

    UW, Imperial, Chris Murray and Neil Ferguson all got a TON OF PRESS.

    But their models were terrible,

  • The sooner those who had the virus (even asymptomatically) and have recovered (and are, therefore, now immune) can be identified and certified, the faster they can resume normal life, including rejoining the workforce. The sooner, too, there will be a serum with their antibodies that can help the rest of us: those who will test “positive” and need the serum to recover and those who will test “negative” and are, therefore, still vulnerable to the virus and will need the serum if they become ill.

    • A friend who I’ve known for 50+ years, a retired military helicopter pilot, has a friend who he’s known for 25+ years who went to a doctor in Chicago on March 17 with symptoms. The doctor told him it was the flu, and to rest at home and to call the rescue squad if it got worse.

      Two days later, on March 19, he did that and was taken to Northwest Memorial, a nationally-ranked hospital (and ranked #1 in Illinois) where he tested positive for COVID-19. They wanted to put him on a ventilator, and he refused. They gave him chloroquine in one arm and hydroxychloroquine in the other arm. A week later, on March 26, he walked out of the hospital, virus-free.

      He was Chicago’s #4 COVID-19 patient. Similar results are coming out of L.A. We are about to get a tidal wave of good news.

  • In fact, scientists in Taiwan have already studied the types of Covid-19 in the United States, the types of Covid-19 in Taiwan and the types of Covid-19 in China. The conclusion is that the United States has almost all types of Covid-19, while China has only one type Covid-19, Taiwan has two. And there are early Covid-19 among the types of viruses in the United States. The fact is that the virus is now outbreak in the United States and is considered to be flu. It was discovered after spreading to China. China made a huge sacrifice to control the virus, but it could not stop the rumours and unreasonable accusations of the Western media. After the outbreak of the United States, the United States The leaders’ inability to respond can only continue to discredit China to divert their attention. Shameless public opinion hegemony!

  • I hope they also let people know if they’re immune so they can get back to work (give them a certificate of some sort) and can donate convalescent serum. Also, like other comments here, a group of us in San Diego think we may have had COVID-19 in late January through February (negative for flu).

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