As the United States continues to monitor the spread of the Omicron Covid-19 variant, scientists have been testing the nation’s wastewater to conduct early detection of the virus. In the past week, a team of researchers in California announced they had found traces of the variant in sewage treatment facilities across the state – suggesting that Omicron is already present in multiple cities.

The team from the Sewer Coronavirus Alert Network (SCAN), a collaboration between city officials and scientists at universities including Stanford, found that wastewater in Sacramento and Merced contained evidence of the Omicron variant.

“This is certainly suggestive – even though we haven’t had any clinical cases reported in those counties yet – that there is the virus circulating there,” Dr Erica Pan, the California state epidemiologist, said in a presentation earlier this week to the California Medical Association. “We definitely are seeing Omicron across the state,” she added.

The first confirmed case of Omicron in California was found last week and authorities across the country are concerned that the new variant could spread widely in the US as it has in countries such as South Africa and the United Kingdom. Multiple countries and US states have turned to wastewater testing as an early gauge of whether the variant is present in communities.

Colorado health officials announced on Tuesday that researchers had found traces of the Omicron variant in wastewater that likely indicated more than one case of the variant in the state. The Houston, Texas, health department said that same day that tests had detected the Omicron variant at eight wastewater treatment facilities throughout the city – about a fifth of its total facilities. Idaho’s Department of Health also announced on Thursday that it would be expanding its testing program amid concerns over Omicron, stating that it allowed the state to have earlier indications of new trends in coronavirus transmission. In California, officials said evidence of the Omicron variant was detected in wastewater samples taken as early as 25 November, a day before the World Health Organization named it and designated it a variant of concern.

Wastewater testing for Covid-19 emerged last year as a valuable warning system, giving officials a head start before hospitalizations rose, without relying on individuals to take Covid-19 tests. Sewage treatment facilities send vials of sample wastewater to scientists for testing, who can then quickly check for coronavirus in fecal matter. Although wastewater testing cannot confirm individual cases or provide more granular detail on how outbreaks have occurred, experts say the results are quick and there is always a constant source of samples available for testing.

In testing specifically for Omicron, Stanford professor Alexandra Boehm told NPR that scientists checked for the variant’s unique mutations that distinguish it from other types of Covid -19 such as the now-dominant Delta variant. After two rounds of tests over the course of the past two weeks, the Sewer Coronavirus Alert Network announced their findings.

Much is still unclear about the Omicron variant and how it may shift the course of the pandemic. Early indicators suggest that Omicron can rapidly spread, although researchers in South Africa have reported that it’s possible the variant is more transmissible but causes less severe illness than other forms of the virus. Lab studies on how existing vaccines hold up to Omicron are under way, with Pfizer and BioNTech reporting this week that tests of blood samples showed two doses of its vaccine were less effective against the variant, although receiving a third booster shot provided significant protection.

Scientists have said that travel bans from South Africa and southern African countries are ineffective at controlling the spread of disease, especially in light of evidence from wastewater that Omicron had reached the US before the government issued travel restrictions.

Health officials and the Biden administration have renewed a push for vaccinations as Omicron begins to spread throughout the country, while the US vaccination rate still lags behind other countries where the vaccine is readily available.