With so many people using home tests (which mostly go unreported), how can we rely on COVID-19 data to understand community levels of transmission?

Data and Metrics Testing and Contact Tracing

A: With more people using home rapid tests interpreting local COVID-19 data does become murkier, but a useful addition is wastewater surveillance!

TL; DR Keeping an eye on local wastewater surveillance can help round out understanding of community transmission as it doesn’t rely on testing or reporting of test results.

✳️Home testing is great because it’s another tool for finding out whether you have COVID-19 and lets people know they need to take action to protect others around them, so what’s the problem?

Well, how many of you have taken a rapid test in the past month? When you got your results (whether negative or positive), did you report them to anyone? If your test was negative, you most likely did not report your result anywhere (though some health departments do have systems in place for doing so, so worth checking on!). If your test was positive, you might have confirmed with a PCR test, the results from which do get reported. Or, you might have just started isolating, we get it.

✳️What does this mean for tracking new COVID-19 cases?

Well, it is unlikely that all new cases are being captured or that all the tests being taken getting accounted for. As a result, total new cases and metrics such as percent positivity (positive tests/total tests) have become a bit harder to interpret, though they should still be tracked.

✳️What else can we do to monitor transmission of COVID-19 in our community, then?

Another handy tool that has become more widely used over the course of the pandemic is wastewater surveillance! What is wastewater surveillance, you ask? It is a technique in which samples of wastewater (i.e. the water that gets flushed from our toilets 🚽 or enters the sewage system from other sources) are collected from a sewer shed before it enters the treatment plant served by a given community. Those samples are then sent off to a lab to be tested for levels of SARS-CoV-2 virus. Higher levels of SARS-CoV-2 in a sample is an indicator of more shedding of the virus into the water system and thus more people infected in a community.

✳️But how does the virus get into wastewater, you ask?

SARS-CoV-2 replicates in the digestive system and therefore exits our bodies via our 💩, often before we even develop symptoms or know we even have COVID-19. We know, kinda gross 🤢. But(t), this means that increasing levels of SARS-CoV-2 in wastewater can serve as an early warning sign that more people are becoming infected in a community. Importantly, this form of surveillance doesn’t rely on people getting tested or results being reported, just on…well…💩…which we all do!

This is not to say there is no longer any use in counting new cases or keeping an eye on percent positivity. We also shouldn’t get too cheeky 🍑, and rely solely on wastewater surveillance (for example, a big storm can flood the sewer system and cause a sudden drop in virus levels). The best option is to keep our eyes 👀 on multiple sources of data with local wastewater surveillance sites being a useful addition to the list!

For more on CDC wastewater surveillance efforts, see here.

For more on how wastewater surveillance has been used over the course of the pandemic as well as potential limitations, see here.

For a couple of our past posts on wastewater surveillance, see here OR see here.

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