I'm confused how 'flu is at its highest level in 25 years' (which I saw some headlines about) but wastewater data seems to show lower levels compared to last winter peak. Seems like they're measuring "percentage of outpatient visits for influenza-like illness", but I'm not sure if that's a better measure of prevalence. The CDC also has lower positive tests of influenza than last year, maybe there's another flu-like disease spiking?
Thanks for your questions. I don't know precisely which metric(s) they're using for the claim that "flu-like activity in the US had reached the highest level on record since the agency started tracking about 30 years ago," but I would suspect it's based on some combination of numbers of influenza-positive tests reported and outpatient visits for ILI (https://www.cdc.gov/fluview/surveillance/2025-week-53.html#cdc_data_surveillance_section_4-outpatient-and-emergency-department-illness-surveillance ). Hospitalizations (at the same link) were also at their highest levels in week 52 at least since the 2018-2019 season, and perhaps in over 25 years as well.
You raise a great question: what's the better metric, wastewater, number of positive tests, % of outpatient visits that are for ILI, hospitalization rates per 100k population? It really depends on what you want to measure in the end.
So I look at it this way. We know this year's H3N2 strain is bad and that people have less immunity to it than they typically have for a circulating H3N2 strain. That means it's probably going to make more people sick. And, this particular strain, apparently, tends to make people sicker, on average. And the sicker people get, the more likely they are to seek healthcare, get tested and need hospitalization.
So it's possible that wastewater data could be decoupled from CDC surveillance data to some extent. It's possible that last winter, more people did get the flu than that have so far this season (which shows up in the wastewater data), but that people are just getting sicker when they get it this winter (which makes this winter look worse). That would be a little weird, and it would make me wonder about sampling or timing issues. But it's possible. As the season plays out, though, I will be very surprised if more people don't get sick overall this flu season than last flu season.
What wastewater data are you looking at specifically?
You write, "The CDC also has lower positive tests of influenza than last year, maybe there's another flu-like disease spiking?" I don't see that in the data. The number of positive tests reported by clinical labs for week 52 is higher this winter than last winter's week 52, and the reporting for public health labs for week 52 isn't complete yet. Maybe you were looking at last year's peak vs. this year's season to date? Flu's not done with us yet this year, sorry to say. And I also don't see any evidence that any other flu-like illness is spiking. Everything looks to be within typical bounds to me.
I'm confused how 'flu is at its highest level in 25 years' (which I saw some headlines about) but wastewater data seems to show lower levels compared to last winter peak. Seems like they're measuring "percentage of outpatient visits for influenza-like illness", but I'm not sure if that's a better measure of prevalence. The CDC also has lower positive tests of influenza than last year, maybe there's another flu-like disease spiking?
Thanks for your questions. I don't know precisely which metric(s) they're using for the claim that "flu-like activity in the US had reached the highest level on record since the agency started tracking about 30 years ago," but I would suspect it's based on some combination of numbers of influenza-positive tests reported and outpatient visits for ILI (https://www.cdc.gov/fluview/surveillance/2025-week-53.html#cdc_data_surveillance_section_4-outpatient-and-emergency-department-illness-surveillance ). Hospitalizations (at the same link) were also at their highest levels in week 52 at least since the 2018-2019 season, and perhaps in over 25 years as well.
You raise a great question: what's the better metric, wastewater, number of positive tests, % of outpatient visits that are for ILI, hospitalization rates per 100k population? It really depends on what you want to measure in the end.
So I look at it this way. We know this year's H3N2 strain is bad and that people have less immunity to it than they typically have for a circulating H3N2 strain. That means it's probably going to make more people sick. And, this particular strain, apparently, tends to make people sicker, on average. And the sicker people get, the more likely they are to seek healthcare, get tested and need hospitalization.
So it's possible that wastewater data could be decoupled from CDC surveillance data to some extent. It's possible that last winter, more people did get the flu than that have so far this season (which shows up in the wastewater data), but that people are just getting sicker when they get it this winter (which makes this winter look worse). That would be a little weird, and it would make me wonder about sampling or timing issues. But it's possible. As the season plays out, though, I will be very surprised if more people don't get sick overall this flu season than last flu season.
What wastewater data are you looking at specifically?
You write, "The CDC also has lower positive tests of influenza than last year, maybe there's another flu-like disease spiking?" I don't see that in the data. The number of positive tests reported by clinical labs for week 52 is higher this winter than last winter's week 52, and the reporting for public health labs for week 52 isn't complete yet. Maybe you were looking at last year's peak vs. this year's season to date? Flu's not done with us yet this year, sorry to say. And I also don't see any evidence that any other flu-like illness is spiking. Everything looks to be within typical bounds to me.
I hope this is helpful.