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Pima County to use wastewater data as COVID-19 ‘early warning sentinel’ | Local news

COVID-19 cases are climbing again in Pima County, just as local scientists are figuring out how to gauge infection levels faster and warn the community sooner.

The solution flows in our sewers, where wastewater researchers are focused on establishing baseline SARS-CoV-2 levels.

Here’s how it works: Infected people shed the virus in feces, even if they don’t have symptoms, and that can then be detected in wastewater.

“If we know the community shedding rate of a (sewage) utility service area, we can use that to predict the real number of infections in the community,” said Ian Pepper, an environmental microbiologist and director of the University of Arizona research lab that runs wastewater testing. Pepper and his team will have a paper published on this topic in the next few weeks in “Science of The Total Environment.”

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As early as July, the county will be launching a new data dashboard on these findings. It will be similar to one created in Tempe earlier in the pandemic. With shedding rates established, Pepper said, it will be possible to give a same-day estimate of the total number of infections.

And when COVID-19 concentration levels increase in sewage, the health department will alert the community so those at high-risk are aware, said Mary Derby, chief epidemiologist with the county’s health department.

“We will provide those communities at the highest risk of bad health outcomes with education, testing, vaccination and treatment options,” she said. “Wastewater surveillance provides additional information that the public needs, especially during this time of ‘COVID-19 fatigue.’”

County’s rates rising

That pandemic fatigue has contributed to the county’s case rate rising, up from 74 per 100,000 the week of May 11 to 104 per 100,000 for the week of May 18. This week, it’s risen to 162 cases per 100,000. The tipping point for alarm: 200 cases per 100,000.

The county is already reaching a negative marker with its positivity rate, which ideally should be below 10% but this week was as high as 16%.

Hospitalizations, the final bellwether, are also increasing, but slowly: 27 COVID-19 admissions for the week ending May 21.

While the current subvariant of omicron — called BA.2.12.1 — is not leading to as many hospitalizations, an increasing number of people are getting sick. That’s partly because people have become complacent about things like masking in high-risk environments, said Dr. Theresa Cullen, the county’s director of public health.

“If they are in a social situation inside, they should be wearing a mask,” Cullen said of the county’s current risk levels and protecting public health for everyone. “None of us can really predict what we’re going to see going forward. It’s worrisome that more people are getting sick.”

People might also be somewhat confused by the county’s website, which was changed to minimize confusion and now matches the one used by the Centers for Disease Control and Prevention. The new CDC site measures COVID-19 illness in health-care systems to ensure space for those still most at risk: a community’s older citizens, immunocompromised people and those living with disabilities.

“What they want to do is ensure that the hospital system is able to respond,” Cullen said of the CDC site’s focus. “From a public-health perspective, yes, we can respond, but from an individual perspective, people are at risk. Your risk is high right now.”

Understand potential risk

Hospitalization rates during the pandemic are considered a “lagging indicator” of what’s happening, while wastewater monitoring is a predictor of not only hospitalization but now — with SARS-CoV-2 wastewater baselines being established — also of infection.

This is vital now that fewer people are reporting test results.

“As an epidemiologist, the advantages of wastewater surveillance has been impressive. We are getting data on both symptomatic and nonsymptomatic individuals,” said Derby, the county’s lead epidemiologist. “Now with more people using home tests, which may not be reported, we can understand the potential risk to the community as changes in community-level infection occur.”

Going forward, the county will be able to monitor other health issues through wastewater, she said.

“We look forward to expanding wastewater surveillance to monitor for possible other community health threats, such as estimating opioid use, measuring antibiotic-resistant bacteria or detecting a novel strain of influenza that could led to another respiratory pandemic,” she said.

Wastewater surveillance has already been used extensively in developing countries as a way to track polio rates. Once detected, she said, vaccine campaigns are started to stop its spread.






Research technicians Nick Betts-Childress and Erika Stark analyze samples at the WEST Center laboratory in Tucson.




There are seven wastewater treatment plants in Pima County and draws are currently happening at four, an effort that’s being coordinated with help from the CDC. The CDC in September 2020 developed a National Wastewater Surveillance System to track SARS-CoV-2.

Early warning sentinel

Pepper, with the UA, said while monitoring wastewater when the pandemic is raging is critical, it’s even more important when cases are low “in case you can see the virus coming back.”

Researchers working with Pepper through the Water & Energy Sustainable Technology, or WEST Center, are also looking at shedding rates in Yuma as well as some communities in Florida, coordinated with help from one of Pepper’s former students.

Viruses are usually seen in wastewater samples a week or more before people start showing symptoms.

“It is an early warning sentinel for awareness that a virus is present, increasing, or waning in a community,” said Jeff Prevatt, an analytical chemist and deputy director of the county’s Regional Wastewater Reclamation Department.

The benefits extend beyond public health, he said, and will potentially help prevent the kind of economic shutdowns that happened at the beginning of this pandemic.

“It was a disaster what we did to our community, the economic impacts were astounding,” he said of the COVID-19 pandemic. “With wastewater data, that could be better managed and better monitored.”

When data is concerning, quickly notify the vulnerable areas, he said, and increase testing and prevention measures.

That’s what Pepper and others carried out at the UA to reopen during the pandemic. Next, they hope to make it work around the county.

Contact reporter Patty Machelor at 806-7754 or

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