Detecting Mpox Cases Through Wastewater Surveillance - United States, August 2022-May 2023

Abstract

In October 2022, CDC's National Wastewater Surveillance System began routine testing of U.S. wastewater for Monkeypox virus. Wastewater surveillance sensitivity, positive predictive value (PPV), and negative predictive value (NPV) for Monkeypox virus were evaluated by comparing wastewater detections (Monkeypox virus detected versus not detected) to numbers of persons with mpox in a county who were shedding virus. Case ascertainment was assumed to be complete, and persons with mpox were assumed to shed virus for 25 days after symptom onset. A total of 281 cases and 3,492 wastewater samples from 89 sites in 26 counties were included in the analysis. Wastewater surveillance in a single week, from samples representing thousands to millions of persons, had a sensitivity of 32% for detecting one or more persons shedding Monkeypox virus, 49% for detecting five or more persons shedding virus, and 77% for detecting 15 or more persons shedding virus. Weekly PPV and NPV for detecting persons shedding Monkeypox virus in a county were 62% and 80%, respectively. An absence of detections in counties with wastewater surveillance signified a high probability that a large number of cases were not present. Results can help to guide the public health response to Monkeypox virus wastewater detections. A single, isolated detection likely warrants a limited public health response. An absence of detections, in combination with no reported cases, can give public health officials greater confidence that no cases are present. Wastewater surveillance can serve as a useful complement to case surveillance for guiding the public health response to an mpox outbreak.

FIGURE

Sensitivity, positive predictive value, and negative predictive value of wastewater surveillance for detecting persons shedding Monkeypox virus in a county in a week for different assumed shedding durations — United States, August 2022–May 2023 Abbreviations: MPXV = Monkeypox virus; NPV = negative predictive value; PPV = positive predictive value. * Sensitivity is the probability that MPXV was detected in wastewater when at least one person with mpox were shedding MPXV. PPV was defined as the probability that at least one person was shedding virus when a wastewater detection occurred. NPV was defined as the probability that no persons were shedding virus in the absence of wastewater detections. Probabilities for specific numbers of persons with mpox shedding MPXV when MPXV was and was not detected in wastewater are also shown. ^† Wastewater test results were combined for all sites serving a county: if at least one site serving a county detected MPXV in wastewater in a given sample collection week, that week was considered a detection for that county, and otherwise a nondetection. ^§ Persons with reported mpox were assumed to shed MPXV in their county of residence from the day of symptom onset until 25 days later. The number of persons with mpox shedding MPXV were summed to determine the number of persons with mpox shedding MPXV on each day in a given county. ^¶ Wastewater test results for a given sample collection week were compared to the average numbers of mpox cases shedding MPXV in that week in a given county. ** The assumed shedding duration was varied from 5 to 60 days in 5-day increments. Main results are shown with an assumed shedding duration of 25 days with 95% CIs. CIs were calculated using exact binomial tests.