The sewer knew about the flu before the hospital did

Wastewater surveillance proved itself on SARS-CoV-2. Whether it works for the
other winter respiratory viruses was the open question, and influenza and RSV
were the obvious candidates: **both are quantifiable in wastewater, both cause
substantial illness and death every year, and both arrive in annual outbreaks
that vary in timing and duration.**

During the 2022–23 season, Wisconsin measured influenza A and RSV in the
wastewater of its three largest cities and compared it against
pathogen-specific emergency department visits.

They agree

Correlation between wastewater concentration and emergency department visits
(Kendall's tau, across the three cities):

Higher concentrations, more visits — in all three cities, for both viruses.

And the wastewater is earlier, and later

**Detections of both influenza A virus and RSV in wastewater often preceded a
rise in associated ED visits**, and **virus material remained detectable in
wastewater for up to 3 months after pathogen-specific ED visits declined.**

**Weeks of warning before the wave, and up to three months of signal after it
looks finished.**

Both ends are useful in different ways. The lead time is what a hospital wants
in order to staff for what is coming. The tail is the more interesting one: a
season that appears over by the clinical data is still present in the
community, in people who are infected and not going to hospital.

Why it sees something different

The reason is in one clause:

**Wastewater surveillance is independent of health care access or testing
biases.**

Emergency department data counts people who were ill enough to go, able to go,
and then tested. Wastewater counts everyone connected to the sewer. The two
numbers disagree not because one is wrong but because they are measuring
different populations — and the gap between them is exactly the group
conventional surveillance never sees.

What it is for

The recommendation is careful to say complement, not replace:
**incorporating wastewater surveillance into established surveillance systems
might improve local preparedness and response to seasonal respiratory virus
outbreaks.**

Wastewater says how much virus is in a city. It cannot say who is ill, how
severe it is, or who is in hospital. The clinical data answers those and
arrives late; the sewage answers the first and arrives early.

Source: Centers for Disease Control and Prevention, MMWR.