"So we went in, we tested everyone, we found out the virus had come back into the shelter and we were able to stop it a second time," said Dr. David Persse of the Houston Health Department.
On Thursday, Persse and Houston Mayor Sylvester Turner shared more details about the innovative partnership between the city, Rice University and Baylor College of Medicine to track the spread of the coronavirus through testing Houston’s wastewater.
Back in May, Rice University and city health officials said they were undertaking a project to determine if by tracking No. 2 residue they might be able to stop the coronavirus in its tracks, as even the feces of asymptomatic carriers of COVID-19 carries detectable amounts of the disease in sewage.
This week, Turner said that the goal of the city's wastewater testing program is “to help develop an early warning system, allowing the health department to identify the city’s COVID-19 hotspots sooner.”
Both Persse and Turner said that wastewater data shows that COVID-19's spread has decreased across Houston over the past several months. That backs up the downward trend shown by the city's falling testing positivity rate, a metric that some have called into question due to week-to-week fluctuations based on data reporting delays, and because of the decreasing number of Houstonians getting tested since the summer.
Persse explained that the wastewater results provide a more real-time picture of the prevalence of the coronavirus in a given area than data from traditional nasal swab COVID-19 tests of individuals.
Wastewater sampling “occurs weekly, and we get the results within that same week,” Persse said, in contrast to swab test results where in a given week “as much as 40 percent or more” can be over two weeks old. “So right there, you can see that we’re getting much more rapid answers,” Persse said.
Since May, the Houston Public Works Department has taken weekly samples from the 39 wastewater treatment facilities spread across the Houston area. That water is then quickly tested by labs at Rice and BCM, which provides a snapshot of how prevalent COVID-19 is in different parts of the city.
By comparing data from those samples week over week, the Houston Health Department can determine which zip codes are seeing an increase or decrease in the spread of COVID-19. Persse said that allows the health department to focus its resources on parts of Houston where the virus is spreading more quickly.
Dr. Lauren Stadler, a Rice professor of civil and environmental engineering and the project’s academic lead, said that wastewater testing is a faster and cheaper way to monitor the overall level of COVID-19 in the community than traditional testing, and that the viral load in wastewater is “an early indicator of one to two weeks” compared to a given area’s positivity rate due to test positivity data reporting lags.
She gave an example of a similar wastewater testing program at the University of Arizona that earlier this summer was able to detect the presence of COVID-19 in a student dorm, which led the school to make sure everyone in that building got tested. In that case, university officials “were able to identify two asymptomatic students before an outbreak occurred,” Stadler said.
“We could have talked about this a lot sooner,” Persse said, but the reason the city waited was to make sure the data was accurate.
After weeks of crunching the numbers, the city and its research partners found a 97 percent correlation between the up-to-date, corrected COVID-19 testing positivity rate for different parts of Houston with the rate of COVID-19 in those same areas found from wastewater testing. That’s why the health department is now so confident that the wastewater data can be trusted, said Dr. Loren Hopkins, the Houston Health Department’s chief environmental science officer and a statistics professor at Rice.
Wastewater testing for diseases isn’t a new phenomenon in Houston. Dr. Anthony Maresso from Baylor College of Medicine said that in 1962, a BCM scientist used wastewater to detect the spread of polio in Houston, which that year led public health officials to bump up the distribution of the polio vaccine from the fall to the summer which saved countless lives. “That effectively prevented the polio outbreaks, and we haven’t really had them since,” Maresso said.
Persse was careful to caution that traditional individual COVID-19 testing is still extremely important. He said one of the main benefits of the new program is that the health department will know which communities they need to visit to encourage residents to get tested, because people have to know they are sick in the first place to be able to isolate to prevent spreading COVID-19 any further.
“This will give us that early warning that may have otherwise missed, so we can empower people to take care of themselves,” Persse said.