Wastewater surveillance may be able to predict flu outbreaks and distinguish which strain is predominant roughly a week before official surveillance data are published, according to a new study led by researchers from the University of Osaka in Japan.
The findings, published in the Water and Environment Journal, suggest that wastewater-based epidemiology could help hospitals and public health officials prepare for surges in flu activity, including anticipating demand for hospital beds and other healthcare resources.
A, B strains detectable in wastewater
The researchers measured influenza A virus (IAV) and influenza B virus (IBV) RNA concentrations in weekly wastewater samples collected from three treatment plants in Osaka Prefecture from April 2023 through April 2025. The treatment plants served about 884,000 residents, or roughly 10% of the prefecture’s population. Overall, 303 samples were collected—101 samples from each treatment plant.
Then the team measured concentrations of IAV and IBV RNA in the untreated wastewater samples and compared them with surveillance data collected through Japan’s sentinel healthcare system.
The findings showed that flu activity rose in several waves during the study period, with the biggest surge occurring at the end of 2024. Reported influenza cases peaked at about 22 cases per monitoring site per week in late 2023, climbed again to 30 cases per site in early 2024, and then reached 66 cases per site in late 2024.
The wastewater data followed a similar pattern. Levels of IAV RNA in wastewater peaked during the late-2023 surge, while influenza B virus levels peaked earlier, in the first part of 2024, reaching a maximum in early 2025. The type of flu circulating also shifted over time. Influenza A was the dominant strain in late 2023 and again in late 2024, while influenza B accounted for more infections in early 2024, before falling to low levels after early 2025.
Viral traces surfaced before patients
The researchers concluded that the wastewater samples could predict type-specific flu trends with high accuracy about one week before patient data became publicly available, potentially helping hospitals and public health officials make proactive decisions about how to allocate healthcare resources such as clinical and hospital staff and hospital beds.
Flu viruses were also detected in wastewater during non-outbreak periods, suggesting that wastewater surveillance may capture community transmission that is missed outside peak flu season.
The researchers cautioned that the study was conducted in a single region, so it may not be possible to generalize to other regions or when flu seasons are dominated by different strains.
Still, lead author Michio Murakami, PhD, said in a news release, “By measuring influenza virus in wastewater, we found that community influenza outbreaks can be estimated by type, separating influenza A and B, about one week earlier than publicly available patient report data. While this paper reports results through April 2025, we have continued monitoring since then and have confirmed that our model continues to estimate outbreak trends with high accuracy.”
“We expect these findings will support earlier preparedness of healthcare systems, such as securing hospital beds in anticipation of increased admissions as influenza cases rise,” he added.
