ESCANABA — The amount of detectable DNA from the virus responsible for COVID-19 in Escanaba’s sewer water has continued to rise, following a brief dip in detection of the virus before Christmas.

According to the Sentinel Wastewater Epidemiology Evaluation Project, commonly known a “SWEEP,” the city’s two most recent samples contained more viral DNA than 93% and 90% of samples collected at the wastewater treatment plant since the city began participating in the project in August of 2021. The two samples represent the amount of viral DNA found on Dec. 28, 2023 and Jan. 3, 2024, respectively.

SWEEP samples are collected twice a week to minimize inconsistencies caused by sample or reporting errors.

Shortly before Christmas, it appeared the city would be able to welcome the new year with an average, or even below-average, amount of the virus detectable. The sample collected on Dec. 13, 2023 was exactly average for the city, containing more viral DNA than only 50% of all samples collected. At the time, the city’s 15-day trend showed a decreasing amount of viral activity.

According to SWEEP, the city’s 15-day trend switched from a meager drop in viral activity on Dec. 30 to a 1,000% or more increase in detectable viral DNA for the 15-day period ending Jan. 6.

The upward trend following Christmas may have been the result of more residents traveling or gathering together indoors to celebrate the holidays.

It is difficult to know how many people are sick in the city and surrounding areas based just on Escanaba’s wastewater for a number of reasons. Some people who are actively infected with the virus show no symptoms, and the viral load can vary greatly even among individuals. In addition, the ten-day incubation period of the COVID-19 virus means some people who are carrying the virus may not have developed the disease even if they will in the future.

It is also difficult to gauge how many people are infected with the disease using other metrics. At the beginning of the pandemic, all testing was done by healthcare professionals and sent to laboratories, where epidemiologists could track and report infection trends for the general public. Nearly all of the reporting systems put in place at the beginning of the pandemic have been phased out, and those that remain are largely inaccurate because home tests are not included in the data.

Still, the sewer data does coincide with historical infections trends. According to SWEEP, a spike in viral detection in January 2022 — which was one of the highest periods recorded for the city — coincided with the highest number of cases reported for Escanaba to the Michigan Department of Health and Human Services, with more than 350 cases per 100,000 people. Despite a nearly-identical viral load in November of 2023, shortly before December’s dip, only about 60 cases per 100,000 people were reported to MDHHS.