According to a Centers for Disease Control (CDC) and Prevention antibody survey, infection rates for the coronavirus in the Philadelphia region are estimated to be between four and seven times higher than reported.
A Patch report indicates that he latest data from the second round of the survey, released this week, shows that as much as 5 percent of the Philadelphia region’s population may have contracted the coronavirus as of late May. Based on the data, the CDC estimates between 2.4 and 5 percent of the Philadelphia region’s population has been infected with the virus at some point.
The CDC joined commercial laboratories for a “large-scale geographic seroprevalence survey,” which assesses blood specimens from 10 states or metro areas in order to determine the prevalence of the virus in those regions.
The survey was conducted in two rounds, with the goal of determining what percentage of people already have antibodies against SARS-CoV-2.
Confirmed COVID-19 cases likely represent only a fraction of all infections, the CDC has said. This is due to the fact that many people have mild symptoms, or do not seek medical care.
The study used blood specimens from people tested for reasons unrelated to COVID-19, such as for a routine or sick visit. It was conducted in Connecticut, Louisiana, Minnesota, Missouri, New York City, Philadelphia, San Francisco, South Florida, Utah and Western Washington State.
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The first round, which found a 3.2 percent seroprevalence in Philadelphia, assessed 824 samples between April 13 and 25. For the first round, samples were taken from Bucks, Chester, Delaware, Lancaster, Montgomery, and Philadelphia counties in Pennsylvania, as well as Cumberland County in New Jersey.
At that point in time, there were 22,987 confirmed cases in the survey territory. But based on the test results, the CDC estimates more than 157,000 people had SARS-CoV-2 antibodies. Based on the data, the CDC believes at that time, between 1.7 and 5.2 percent of the population had antibodies.
That means the virus was likely seven times more prevalent than the data at the time showed.
The second round, which found a 3.6 percent seroprevalence, assessed 1,743 samples between May 25 and 30. In the second round, the sample group was expanded to include Berks, Dauphin, Lehigh, Northampton, and York counties.
At that time, there were 56,318 confirmed cases reported in the survey region. But, according to the CDC survey, it’s estimated there were 245,000 infections. Between 2.4 and 5 percent of the population had antibodies at that time, the CDC estimates.
The CDC notes that results from seroprevalence surveys should not be interpreted to mean that people who have tested positive for having SARS-CoV-2 antibodies are immune to future bouts with the virus.
“We do not know whether having SARS-CoV-2 antibodies provides protection against getting infected again. Other studies are planned to learn more about SARS-COV-2 antibodies, including how long they last, whether or not they provide protection against getting infected again, and if you get infected again, whether or not they can make that illness milder,” the CDC said.
Additionally, it’s possible that antibody levels may decrease over time. Some people who previously had detectable SARS-CoV-2 antibodies may no longer have antibody evidence of past infection, the CDC said.