Why is my influent testing negative when there are reported cases in my area? Why is my report showing a “not-detected” result when the virus has already been detected before?
A result of “non-detect” means the levels of SARS-CoV-2 virus genetic material in the influent sample was below our limit of detection.
There can be a number of reasons for this. For example, if the number of new infections in your community is small, the virus concentration in your wastewater may hover around our limit of detection.
The location of reported cases can also be a factor. If individuals who have been infected with Covid-19 do not reside in an area connected to your catchment area, then they also will not be represented in the wastewater sample.
Lastly, If the composite sampler is collecting sewage once per hour (vs. every few minutes) we may miss the virus.
What is your current limit of detection (LOD)?
Our current protocol has a LOD of 3,600 SARS-CoV-2 virus copies / Liter of sewage (3.6 copies/mL). We reliably (>99%) detect the virus when there is at least 1 infected person in a population of 6,500 people. In other words, our methods are sensitive enough to detect even 1 infected person in a community of 6,500 people or smaller.
How should I calculate the change since my last sample or plot my own data?
If you are interested in plotting the data yourself or doing additional analyses on these results, we highly recommend using the normalized concentration. We’ve found that normalizing the data to a virus universally found in stool is the best way to control for dilution and variability in sampling. To interpret trends in your community, we recommend looking at consistent trends across multiple samples, rather than sample-to-sample changes.
Why is my normalized concentration so different from the raw concentration? Should I be concerned?
There is no need to worry if your normalized concentration is different from the raw concentration. In fact, that’s to be expected since normalization corrects for any sample dilution.
For example, if your samples tend to be more dilute than the average sample in our dataset, you’ll see consistently higher normalized values than your raw concentration. That’s because the dilution in your samples will lead to lower PMMV than average, and so your sample concentrations will be corrected upwards in the normalization process.
How well does your data reflect Covid-19 cases?
Our dataset shows a consistent agreement between wastewater concentrations and confirmed new Covid-19 cases reported in each county. For example, we’ve seen locations with peaks in cases over the summer which were also reflected in the wastewater measurements. The nationwide increase in Covid-19 in the late fall of 2020 has been reflected across the board in our data. We have also seen a good correlation between geographic patterns of state-level Covid-19 cases and respective average wastewater concentrations.
How do you estimate Covid-19 cases?
Our latest Covid-19 incidence estimation model is built from Biobot’s dataset, the largest Covid-19 wastewater dataset in the world to date. We mined this dataset to derive an empirical relationship between the amount of virus in sewage samples and the number of cases reported in the associated communities over the next 7 days. This means that our model provides an estimate of the number of cases that will be reported in your community in the next week.
How do you derive the shedding parameter?
We directly model the relationship between viral loads in sewage and reported cases using our dataset on SARS-CoV-2 in wastewater rather than relying on published clinical studies reporting clinical viral shedding in stool. More specifically, we correlate the total virus load in each wastewater sample with the number of reported COVID-19 cases in the respective communities for 7 days following the date of sample collection. This empirical relationship allows us to determine the parameter linking a given amount of SARS-CoV-2 in wastewater with associated reported cases in the community.
What factors affect your calculation for Covid-19 cases? What information do you need from customers to improve your case estimates?
The factors affecting our estimate of Covid-19 cases are the virus concentration, the flow rate on your sampling date, and viral shedding per reported case. The most important information you can provide is the daily flow on the day of sampling. The more accurate this number is, the better our estimates will be.
What contributes to the variability in the data? / Why is my data so variable?
Our analyses indicate that variability in the data is primarily due to lab processing and sampling. We continuously review and improve our lab processes to ensure high-quality data. Because 24-hour composite samples are not 100% continuous, we expect some additional variability contributed by the non-continuous sampling. Our team has built hydraulic simulations to model these effects, which indicates that the most important factor to improve data quality is the pumping frequency (i.e. pumping interval during 24-hour composite sampling). Therefore, if your data and case estimates are highly variable, we recommend increasing your pumping frequency to the maximum setting (e.g. pumping every 5 minutes.
Do the mRNA vaccines affect our measurement of the virus in wastewater?
No. At this time, there is no risk of mRNA vaccines affecting the amount of virus detected in wastewater samples. mRNA vaccines, such as the Pfizer/BioNTech and the Moderna ones used to combat Covid-19, are a new type of vaccine that can protect populations against infectious diseases. To trigger an immune response, many vaccines put a weakened or inactivated germ into our bodies. Not mRNA vaccines. Instead, they teach our cells how to make a protein—or even just a piece of a protein—that triggers an immune response inside our bodies. That immune response produces antibodies, which protect us from getting infected if the real virus enters our bodies.
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Does your testing detect the new Covid-19 strains?
Biobot testing will indiscriminately detect known SARS-CoV-2 variants. A variant is characterized by one or more signature changes (mutations) to the virus's genetic code (for example, B.1.1.7 has 17 mutations). Similar to standard clinical Covid-19 testing, our testing detects the SARS-CoV-2 virus by targeting the N1 & N2 regions of the genetic code (as designed by the CDC). These two regions are unchanged in the known SARS-CoV-2 variants. Therefore our testing will detect the virus variants (and original variant) all the same.