Cells infected with covid-19 SARS virus particles (shown in yellow). An innovative test found around 30 infected people made use of the Long Beach sewer system over the Independence Day weekend.

LONG BEACH — An innovative sewage-water test suggests Fourth of July holiday visitors brought the covid-19 virus to the city of Long Beach, where two previous tests found no sign of the disease.

A wastewater sample taken from the Long Beach sewer system July 5 indicated about 30 people in Long Beach were shedding the virus in their stool, according to an analysis by Massachusetts-based company Biobot Analytics, Inc. This compares to 18 confirmed cases in all of Pacific County as of Tuesday, July 14.

In May, Long Beach began testing its sewer system for the presence of the SARS-CoV-2 virus, which causes covid-19. Long Beach City Administrator David Glasson said two previous wastewater samples showed no sign of the virus. With so many people in town for the three-day July 4 weekend, Glasson said he thinks it’s plausible there could have been 30 people in the city infected with the virus.

Long Beach is still waiting on results from a sample taken on June 22. The city wants to test the sewers again in two weeks to better understand what the July 5 result means.

“Did any of it get left here?” Glasson said.

Long Beach signed up to participate as part of Biobot Analytics’s discounted program for the first cities to sign up for the project. The company waived fees for three of the tests after the lab was delayed in processing the results. The first test cost the city $120 and was paid for with CARES Act funding.

Because Long Beach does not have anymore CARES Act funding to spend on testing its sewers, Pacific County will pay for the next two samples, said Kathy Spoor, Pacific County administrator.

Biobot Analytics case estimates may not be exact, Spoor said. But she said she was curious to see what the next sample will show.

“There was nothing detected, now there is something detected,” Spoor said. “In the next sample, will we go back to nothing now that the big crowd is no longer using the facilities?”

Biobot Analytics said it is a national leader when it comes to wastewater epidemiology — tracking diseases by analyzing sewer systems. It began its covid-19 mapping project after scientists detected SARS-CoV-2 in stool samples.

On April 7, the company published in medRxiv the findings of its first effort to estimate the number of people infected with covid-19 based on the levels of SARS-CoV-2 in sewage. The company collected samples from a “large metropolitan area” in Massachusetts, according to a study summary published on the company’s Medium page. On March 25, the sample area had about 446 confirmed cases of covid-19. Biobot’s sewage analysis showed an estimated 115,000 people infected in the same area.

The mapping program was created in collaboration with researchers at MIT, Harvard, and Brigham and Women’s Hospital.

The Biobot test samples only a fraction of local sewage. In south Pacific County, only Long Beach, Ilwaco and Seaview are served by wastewater treatment facilities. The majority of homes and businesses are on septic tanks.