The number of people infected with the coronavirus in different parts of the United States is two to 13 times greater than the rates reported for those regions, according to data released Tuesday by the Centers for Disease Control and Prevention.
The findings suggest that large numbers of people who had no symptoms or did not seek medical attention may have kept the virus circulating in their communities.
The study indicates that even the most affected area in the study, New York City, where nearly one in four people has been exposed to the virus, is not close to achieving collective immunity, the level of exposure the virus would leave to spread. a particular city or region Experts believe that 60 percent of people in an area would have had to have been exposed to the coronavirus to achieve collective immunity.
The assay, based on antibody testing, is the largest of its kind to date; A study of a subset of cities and states was launched last month.
“These data continue to show that the number of people who have been infected with the virus that causes Covid-19 far exceeds the number of reported cases,” said Dr. Fiona Havers, a CDC researcher who led the study. “It is likely that many of these people had no symptoms or mild illness and had no idea that they were infected.”
About 40 percent of infected people do not develop symptoms, but they can still pass the virus on to others. The United States now evaluates approximately 700,000 people per day. The new results highlight the need for many more tests to detect infection levels and contain viral spread in various parts of the country.
For example, in Missouri, the prevalence of infections is 13 times the reported rate, suggesting that the state lost most people to the virus that may have contributed to its outsized outbreak.
Dr. Havers emphasized that even those who do not know their infection status should wear cloth face covers, practice social distancing, and wash their hands frequently.
The researchers analyzed blood samples from people who underwent routine clinical tests or were hospitalized to determine if they had antibodies to the coronavirus, evidence of previous infection. They had released early data from six cities and states in June. The study published in the JAMA magazine on Tuesday expands that research by including four more regions. They also published data from later time periods for eight of those 10 sites on the CDC website on Tuesday.
The results indicate that in vast swaths of the country, the coronavirus has still touched only a small fraction of the population. In Utah, for example, just over 1 percent of people had been exposed to the virus in early June. The rate was 2.2 percent for Minneapolis-St. Paul as of the first week of June, 3.6 percent for the Philadelphia metropolitan region as of May 30 and 1 percent for the San Francisco Bay area as of April 30.
In some regions, the gap between estimated infections and reported cases decreased as testing and reporting capacity improved. New York City, for example, showed a 12-fold difference between actual infections and the rate reported in early April, and a 10-fold difference in early May.
“This is not a surprise or surprise to epidemiologists,” Carl Bergstrom, an infectious disease expert at the University of Washington in Seattle, said in an email. “All along, we have expected only 10 percent of cases to be reported.”
Tracking the numbers over time can provide useful information about the spread of the virus and about a region’s ability to cope with the epidemic, other experts said.
“The fact that they mark it over time and look at it longer will actually be very informative,” said Dr. Rochelle Walensky, a researcher at Harvard University who wrote an editorial accompanying the JAMA article.
For example, South Florida reached 2.9 percent as of April 24 from 1.9 percent just two weeks earlier. Missouri numbers barely moved from 2.7 percent as of April 26 to 2.8 percent as of May 30. The numbers for both regions are likely to be much higher in the next round of testing due to the increase in infections in those regions since then.
New York City showed the biggest jump in its rate, from 6.9 percent as of April 1 to 23.3 percent as of May 6, according to its outbreak.
The city’s estimate coincides with the prevalence of 22.7 percent found by a state survey, which evaluated customers in supermarkets from April 19 to 28.
Some experts criticized the state survey at the time because people who shopped during closing were more likely to be young, or might have recovered from illness and feel safe.
“These consistent results offer mutual support for two very different methods used,” Eli Rosenberg, an epidemiologist at the State University of New York at Albany and lead author of the state study.
The CDC study also has limitations, Dr. Walensky said, because many of the people who ventured out during test closings or were hospitalized would have been seriously ill and may not have been representative of the general population.
Each region also varied “in terms of where they were on their own epidemic curve and varied in terms of the number of tests they did,” he said.
The study also did not collect data on race, ethnicity, diagnosis and history of symptoms or preventive behaviors, Dr. Rosenberg said. “The approach used in the grocery store study enables these data collections by combining sample collection with a survey,” he said.
Still, experts said the findings were valuable, despite the limitations.
“This population may not be exactly representative of the population as a whole, but the hope is that it will be close enough to allow us to draw meaningful conclusions,” said Dr. Bergstrom.
Several recent studies have suggested that antibody levels, especially in people with mild or symptom-free symptoms, may decrease rapidly. If that’s true, surveys like those from the CDC could reflect only infected people in the past two to three months, Dr. Rosenberg said, “and complicate the interpretation of the results over time.”
