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.
“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.
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.
On Friday, the CDC released the first of what will be a Covid-19 weekly surveillance report, based on the model of its long-standing flu report, FluView. The report is based on data from the last week of March. It showed that during that week, nearly 76,000 Americans had been tested for the infection, of which nearly 11,000, or 14.5%, were positive.
It also showed that deaths from pneumonia and the flu, which are expected to decrease at this time of year as the flu season begins to decrease, are increasing. Deaths from pneumonia have increased sharply since late February – because of Covid-19.
New surveillance systems will allow CDC to add context to daily reports of Covid-19 cases and deaths, said Lynnette Brammer, head of CDC’s national influenza surveillance system and COVIDView, the new weekly report . This will help the agency determine who gets the virus and who is hospitalized with Covid-19 infections, and who dies from the disease.
“We are starting to see different trends, but it will take some time for us to be really comfortable interpreting this data,” said Brammer.
Correction: An earlier version of this story incorrectly stated that 80% of cases had a mild or almost symptomatic illness.