Houston Public Health officials in collaboration with the Baylor College of Medicine and Rice University conducted voluntary antibody tests on 678 individuals in mid-September to better understand the prevalence of the virus in the city.
“The participation was really amazing,” said Dr. Loren Hopkins, chief environmental science officer with the Houston Health Department.
Given the results of the antibody survey, health officials estimate that up to 13.5% of the city’s population, or over 250,000 people, had contracted the coronavirus by September. At the time, about 57,000 cases had been reported, according to the health department.
To reach “herd immunity,” Houston Emergency Medical Director Dr. David Persse said the city will likely need to see 70% of residents immune to the virus at the same time.
The antibody test differs from diagnostic tests taken with mouth and nasal swabs because it detects a previous immune response to the virus rather than an active infection. This means it could find people who previously contracted the virus and were unaware or did not receive a diagnostic test. The study estimates as many as 40% of people infected with the virus show no symptoms but will develop antibodies.
Vaccine distribution will speed up the process of reaching herd immunity however if too many residents are hesitant to to receive the vaccine, it will impede the process, Persse said. Researchers do not yet know how long antibodies last in the immune system.
“If people become immune but only for a short period of time, the herd immunity is going to fluctuate. ... That is why it is so important to get this vaccine,” Persse said.
The study found that the prevalence of antibodies was higher in underserved communities, among women, residents under the age of 40 and Black and Hispanic residents.
The health department will conduct a second phase of the study in January to retest the first individuals who received the antibody tests along with testing another batch of 420 households.
By retesting the original residents, health officials will be able to measure the level of antibodies that remain present in those individuals and determine if the levels dropped at all over time.
The findings may help researchers far beyond Houston, Persse said.
“It is incredibly important,” Persse said. “This study is looking at the real world population in the 4th largest city in the US and is going to provide us with real world answers.”
The new tests will continue to help determine the prevalence of the virus’s spread in the city. Results are expected about three months after the study is conducted, officials said.
The city also reported 745 new cases and six newly reported deaths Dec. 14. The 14-day average positivity rate increased from 8.8% to 10.5%.