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Antibody tests show number of COVID-19 cases in Houston could be 4 times higher than reported

Based on the antibody survey, it's estimated about 250,000 residents had already been infected with the coronavirus by September.

HOUSTON — A survey by the Houston Health Department shows the number of Houstonians who've been infected with COVID-19 could be four times higher than their database shows. 

They did antibody tests in early September on nearly 700 volunteers from 420 homes chosen at random across the city. 

Based on the percentage of people who tested positive for the antibody, experts estimate approximately 250,000 residents -- or 13.5% -- had already been infected by September. At the time, only 57,000 COVID-19 cases had been confirmed through viral testing.

The purpose of the door-to-door survey was to figure out roughly how many people have been infected with COVID-19 without knowing it.

About 40% of people who get the virus have very mild symptoms or no symptoms at all, according to City of Houston Health Director Dr. David Persse. That means they're less likely to get tested so the database numbers are lower than the actual number of cases.

RELATED: Map: Keeping track of Houston-area coronavirus cases

The survey also turned up other interesting results:

  • More women than men had the antibody
  • More young people had the antibody
  • More Hispanics and Black people tested positive for the antibody than non-Hispanic White volunteers.
  •  A higher proportion -- or 18% -- of people with COVID-19 antibodies lived in high positivity rate areas compared to 10% in lower positivity areas.

The survey was conducted by the Houston Health Department, alongside the Centers for Disease Control and Prevention, Rice University, and Baylor College of Medicine.

"If anybody was positive, we will go back and test them in January if they agree to be tested,” Survey Co-Director Dr. Loren Hopkins said.

They’ll also do another random sample in January for phase two of the program.

“Along with tracking virus spread, the survey will provide insight on how much antibodies were made and how long they last over a 4-month interval (short-term duration) in blood circulation. In addition, the survey may help identify short-term immunity after infection,” said Dr. Pedro Piedra, professor of molecular virology and microbiology at Baylor College of Medicine

For more information on the City of Houston survey, click here.

Harris County began its own door-to-door COVID-19 antibody survey outside city limits in November.