Locator: 49879INFLUENZA.
In a long note like this, there will by typographical and content errors. Not proofread prior to posting.
There are two main types of human seasonal flu:
- influenza A
- influenza B
Both circulate annually
- A: often leading to more severe illness and pandemics
- B: generally milder, later-season outbreaks
Timing:
- A: generally appears first, earlier in the season; think -- before the New Year
- B: generally appears second, later in the season; think -- after the New Year
- explains why the double spike in flu cases annually; there may be other reasons, also
Host:
- A: animal hosts, particularly birds and pigs
- B: only human; no other animal host
Main types
- A: subtyped (like H1N1, H3N2) because it rapidly mutates
- subtypes are further typed by clades
- because this year's K clade is particularly virulent, we'll probably be hearing more about "clades" in the future
- B: strains, like Yamagata (not seen since 2000, link here; first seen in 1988) and Victoria (currently circulating);
- is Covid somehow related to demise of Yamagata?
- Yamagata, Japan: far north of Tokyo on main island; west of Sendai
"Strains" / main types:
- A: subtypes, divided by surface proteins (H & N); e.g.:
- A(H1N1 and A(H3N2)
- mutates rapidly and often; leads to new strains and pandemics
- subtypes are further typed by clades
- because this year's K clade is particularly virulent, we'll probably be hearing more about "clades" in the future
- B: lineages, not subtypes
- long history of lineages; seldom mutates; no need to subtype
- seldom mutates
- only two lineages in recent history: Yamagata, Victoria
- Yamagata: believed extinct; hasn't been seen since 2000; some disagree;
- Victoria circulating now
- evolved separately, primarily in their HA (or H) protein
- thus vaccinating for one won't protect against both
Current season, 2025 - 2026
- influenza A: A(H3N2)
- subclade K; new, n ot seen before; seems to be particularly virulent
- influenza B: Victoria strain
Current vaccine, 2025 - 2026
- all flu vaccines are now trivalent
- exception: nasal FluMist quadrivalent
- target three different types of influenza:
- two types of influenza A
- A(H3N2), J.2 subclade
- subclade K: J.2.4.1 -- but subclade "migrated" antigenically from vaccine
- A(H1N1)
- one type of influenza B (since Yamagata strain has not been seen since 2000)
- up until recently, the vaccines had been quadrivalent (to also cover Yamagata)
Fluzone and Fluzone High-Dose
- most common vaccine; probably the one you were given; link here;
- Fluzone High-Dose: specifically designed for folks >65 years of age
- preservative-free, single-dose
- contains 4x the amount of antigen compared to standard-dose flu vaccines
- A/Victoria/4897/2022 (H1N1)pdm09-like virus
- A/Croatia/10136RV/2023 (H3N2)-like strain
- B/Austria/1359417/2021-like (B/Michigan/01/2021-like, B Victoria lineage) virus
Effectiveness:
- Subclade K "drifted" from vaccine shortly after vaccine developed
- Recent 2025 data published in The Lancet showed Sanofi's high-dose vaccine significantly reduced the risk of hospitalizations for pneumonia/influenza and cardio-respiratory events in older adults.
Spanish Flu:
- exceptionally deadly
- H1N1, not further subtyped?
- first seen:
- Camp Funston, Kansas (USA), March, 1918, possibly earlier in Haskell County, Kansas
- Etaples, France, or Aldershot, England in late 1916 / early 1917
- northern China: late 1917
- how it could have spread: Chinese laborers were then mobilized and transported across North America to the Western Front, potentially spreading the virus along the way.
- why "Spanish Flu"?
- this flu was incredibly deadly (particularly in the West) at the same time major fighting (WWI)
- due to deaths and WWI, military authorities censored release of flu data; did not want to give enemies intel regarding effect of flu on fighting forces
- Spain was a neutral country; thus no censorship; and reported cases and high number of cases reported relative to other countries, led it to be called "Spanish Flu"; in fact, it appears there was nothing "Spanish" about it.
- based on all we know about seasonal flu now, the Chinese source seems most likely
Which was worse? Covid-19 or Spanish flu
- this is interesting (note: most of the information below was as of 2021 -- when the Covid 19 outbreak was still significant):
- globally: Spanish flu -- 50 to 100 million deaths
- America: it took awhile but apparently Covid-19 is now considered the deadliest disease even in America, surpassing that of the 1918 Spanish flu; link here.
- numbers:
- Spanish flu, 1918: 675,000 US deaths stretched over two years; no vaccine; burned itself out on its own
- Covid-19: as of September 20, 2021, 675,400 US deaths
- of great concern considering how aggressive the US was managing Covid:
- U.S. deaths make up roughly 14% of the nearly 4.7 million fatalities that have been reported worldwide in this pandemic to date, even though the country’s population comprises only about 4.2% of the global population. Could be simply a reporting phenomenon, but still 4% vs 14% is a huge anomaly.
- US Civil War: generally accepted, 620,000 deaths, but new data suggests upwards of 750,000 deaths in the US Civil War
- Population growth in the US: Comparing events that happened more than a century apart has its perils. For instance, the population of the United States in 1918 was a third of what it is now. So as a percentage of the national population, the Spanish flu deaths still has the lead on Covid-19.
- Mean age of those dying:
- 1918, Spanish flu: age 28
- Covid-19: mostly the elderly
- Bottom line: thus, in terms of cumulative years of life lost, the Spanish flu’s impact thus remains greater.
Swine flu, pandemic, 2009
- "swine flu" is an influenza A(H1N1) virus -- H1N1 was originally from pigs, but now a common seasonal flu
- at the time it was a novel influenza A(H1N1) -- caused the first flu pandemic in over 40 years
- "the A(H1N1)pdm09 strain became seasonal H1N1, and a specific swine H3N2 variant later incorporated a gene from that H1N1 strain"
Confusing the issue:
A(H3N2) variant viruses emerged in pigs with H1N1 genes: A separate swine-origin H3N2 variant virus, known as A(H3N2)v, emerged in humans around 2011.
This specific H3N2v virus was concerning because it had acquired the matrix (M) gene from the 2009 pandemic H1N1 virus, which was thought to contribute to its transmissibility to humans. This variant virus is different from the standard seasonal human H3N2 virus.
