CDC begins studies for more accurate count of undetected Covid-19 cases

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

Joe Bresee, CDC’s deputy director of incidents for the pandemic response, said the agency hopes to flesh out some of the cases that have gone undetected using three related studies.

The first, which has already started, will be to examine blood samples from people who have never been diagnosed as cases in some of the country’s Covid-19 hotspots, to see how far the virus has circulated. Later, a national survey, using samples from different parts of the country, will be conducted. A third will look at specific populations – healthcare workers are a top priority – to see how far the virus has spread within them. Bresee said the CDC hopes to start the national investigation this summer; he gave no timetable for the study of health workers.

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“We are just starting to test and will find out very quickly,” Bresee said at a press conference. “We think serum studies will be very important in understanding the true amount of infection in the community.”

These studies – called serological surveys – involve drawing blood from people who have never been diagnosed as a case to look for antibodies to the virus. They are done by taking a representative sample of people in a city, for example, making sure that people of different age groups are included.

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

But it is also assumed that the figure may actually be low – that more people may have already encountered and repelled the virus than what was detected. There have been reports, for example, of the cruise ship Diamond Princess involving people who tested positive who reported having no symptoms. The ship was the first of several cruise ships on which the virus has circulated widely; almost 20% of the ship’s passengers and crew were eventually found to be positive for the virus.

Having an idea of ​​the number of mild and asymptomatic cases helps authorities plan for future responses to Covid-19 activity. If it is known that a high percentage of people in a community were likely infected when the virus crossed during its first wave of infections, the response to a later reappearance could be adapted to protect only those with high risk, for example.

This work is part of the increased coronavirus surveillance at CDC. The agency has adapted a number of surveillance systems used to record the seasonal flu toll in the United States to obtain a near real-time picture of the progress of SARS-CoV-2 across the country.



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