Locator: 49965_FLU5.
Because of AI (chatbots), I have never felt so comfortable with my understanding of seasonal flu as I do now -- influenza A and B.
So, generally speaking there are two waves of influenza every season between October and May.
Today, a reply from Google Gemini said there were many reasons for those two waves, the lead reason being the "drift" of the antigenic strain over the season.
Google Gemini then said, as an example, the strain would drift from H3N1 to H1N2.
In fact, that is so wrong. First of all, H3N1 and H1N2 are not strains, but rather sub-types.
Influenza:
- A, B, C, D: the four types of influenza
- type D, only recently discovered, affects cattle, never affects humans (not yet, and probably never)
- type A: multiple sub-types, not strains; determined by surface antigens; HxNx; only two surface antigens: H and N;
- type B: has no subtypes, only two lineages, which further branch into specific strains; the two lineages: B/Yamagata and B/Victoria; B/Yamagata has not been seen since 2000 (interesting, the year of Covid-19)
- influenza B changes much more slowly and primarily affects humans
So, now, with that information we can follow "antigenic drift" for the rest of the 2025 - 2026 seasonal flu season.
First, week five (5) of 2026, ending February 7, 2026:
- where are we?
- end of the first wave
- beginning of the second wave, just beginning; off to a slow start
- overall, all influenza (A and B) has decreased significantly since the start of the season, last autumn, 2025, but is now picking up slowly;
- since last week:
- percent of influenza A has decreased very slightly week/week and influenza B has increased slightly, but at a rate higher than last week (again, as percentages but also as raw numbers);
- now this is the interesting part:
- earlier Google Gemini said the antigenic drift was due to subtype drift (H, N) so let's check that drift.
- subtyping (remember, the type is A or B) has remained relatively unchanged for Type A
- H1N1 (pdm09) [see below] -- absolutely unchanged from last week:
- September 28, 2025 (week 40): 11.5%
- Week 4, 2026: 14.6%
- H3N2 -- absolutely unchanged from last week:
- September 28, 2025 (week 40): 88.4%
- Week 4, 2026: 85.4%
- H3N2v: 0% at week 40 (2025) and week 5 (2026) [v = variant; usually swine]
- H5: 0% at week 40 (2025) and week 5 (2026) [A(H5) = avian]. See link.
So, minimal increase of H1N1 and slight decrease in H3N2. This is not antigenic drift. I had this slightly wrong last week.
This year's trivalent flu vaccine:
- H1N1, H3N2, and B/Victoria.
I finally get it although I continue to make content and typographical errors on the blog.
****************************
Now, The Graphs
So, far, the second wave is getting off to a very slow start. Is this due to warm winter? Folks spending less time indoors?
