Estimating the effective reproduction number from wastewater (R_t): A methods comparison

The effective reproduction number (R_t) is a dynamic indicator of current disease spread risk. Wastewater measurements of viral concentrations are known to correlate with clinical measures of diseases and have been incorporated into methods for estimating the R_t.

We review wastewater-based methods to estimate the R_t for SARS-CoV-2 based on similarity to the reference case-based R_t, ease of use, and computational requirements. Using wastewater data collected between August 1, 2022, and February 20, 2024, from 205 wastewater treatment plants across New York State, we fit eight wastewater R_t models identified from the literature. Each model is compared to the R_t estimated from case data for New York at the sewershed (wastewater treatment plant catchment area), county, and state levels.

We find a high degree of similarity across all eight methods despite differences in model parameters and approach. Further, two methods based on the common measures of percent change and linear fit reproduced the R_t from case data very well and a GLM accurately predicted case data. Model output varied between spatial scales with some models more closely estimating sewershed R_t values than county R_t values. Similarity to clinical models was also highly correlated with the proportion of the population served by sewer in the surveilled communities (r = 0.77).

While not all methods that estimate R_t from wastewater produce the same results, they all provide a way to incorporate wastewater concentration data into epidemic modeling. Our results show that straightforward measures like the percent change can produce similar results of more complex models. Based on the results, researchers and public health officials can select the method that is best for their situation.