A measure of Mountain View-area COVID-19 infections is showing that case numbers have dropped sizably from their peak last week.

The Santa Clara County Public Health Department has been testing wastewater from local treatment plants daily since October 2020, looking for two SARS-CoV-2 genes: the N gene and the S gene.

Testing since Jan. 1 shows that the amounts of the coronavirus genes found in human waste processed at the Palo Alto Regional Water Quality Control Plant peaked around Jan. 7 at more than twice the levels they had been at the start of the year.

However, by Jan. 11, levels had come back down to about 120% of the Jan. 1 levels. The quantity of the genes in wastewater as of Jan. 12, the last day for which data is available, showed a slight uptick from the previous day.

The Palo Alto Regional Water Quality Control Plant serves 213,968 people in Los Altos, Los Altos Hills, Mountain View, Palo Alto, Stanford University and the East Palo Alto Sanitary District.

Samples are collected seven days a week from four county wastewater treatment plants and results are typically ready within 24 hours.

Wastewater-based epidemiology has several potential advantages over test-based reporting because it includes asymptomatic individuals and people who are unable or unwilling to obtain clinical tests for a variety of reasons, according to the county. Increased cases of COVID-19 in the community have been associated with increased levels of SARS-CoV-2 in wastewater.

The county has been in a dramatic COVID-19 surge since late December, with infections at rates higher than any other time during the pandemic.

Read more local coronavirus news at Coronavirus central.