When 5,000 students moved to the University of Arizona campus, there were no signs of an outbreak. Everyone felt fine and no symptoms were reported. But unbeknownst to the students, a couple of them were already infected — and asymptomatic.

University officials warned students that there would be regular coronavirus tests — but it wasn’t only nose swabs that they were referring to. Each dorm has its own sewage, and officials regularly screen sewage water for traces of coronavirus; on Thursday, university officials announced that the technique worked. They picked up traces of the virus coming from one dorm and when they tested the students, they found two asymptomatic students.

It worked like this: first, they detected traces of coronavirus in the sewage water in one dorm. Then, they tested all the 311 individuals in that dorm, and found two that tested positive. The two are now in quarantine, and the potential outbreak was stopped before it even started.

“With this early detection, we jumped on it right away, tested those youngsters, and got them the appropriate isolation where they needed to be,” said Richard Carmona, a former U.S. surgeon general who is directing the school’s reentry task force, in a news conference.

Wastewater treatment has been previously discussed as a method to trace outbreaks in their early stages. The advantage is that you can track the virus before any symptoms show up, even in asymptomatic people. The major downside is that you don’t know who and how many people have the virus. The levels of the virus can offer some clues about the extent of the outbreak, but you’ll never really be sure how many people have the virus.

However, if you can follow it up with localized testing, it’s an excellent tool The Director of the University of Arizona West Center, Dr. Ian Pepper, says the method can pick up one single case in sewage from 10,000 people, two to three weeks before the patients would be diagnosed otherwise — and two to three weeks in which the virus could circulate freely within the community.

“Sewage surveillance is a leading indicator as opposed to deaths, that’s a lagging indicator. That’s the last thing you see,” said Dr. Pepper.

“You think about if we had missed it, if we had waited until they became symptomatic and they stayed in that dorm for days, or a week, or the whole incubation period, how many other people would have been infected?” Carmona also said.

The wastewater monitoring approach is extremely useful in small communities like on campus, but there are already wider-scale programs in several countries, including Spain, China, Canada, Singapore, New Zealand, Netherlands, and the US.

As our battle with the coronavirus pandemic is set to enter a long winter stage, detecting and stopping outbreaks as early as possible is key — and it’s tools like this might help us do that.