Thread by @SGriffin_Lab

Waves of infection are caused by exposure of a susceptible population to an infectious agent.
For endemic/seasonal infections, we see one or two waves per year as this susceptible pool is replenished by young (unvaccinated) kids, and older/other folks losing their immunity...

In addition, for viruses that change relatively rapidly to avoid our adaptive immune response, e.g. influenza A virus, this susceptible pool is larger because a proportion of otherwise "immune" people have responses that are unable to stop the virus. We are "used to this"...🙄

Seasonality is also influenced by other factors like behaviour, weather etc., but it's not as straightforward as some may think. AGAIN, see this excellent review by @VirusesImmunity to know more, incl. why tropical countries don't have seasonality!

annualreviews.org/doi/10.1146/an…

First and foremost, do NOT underestimate seasonal/endemic infections, they cause profound harm annually, and flu is a major issue that we cope with badly...BUT, we are "used to this", and as long as excess mortality doesn't change, we seemingly just tolerate it...🤦‍♂️

Madness.

Seasonal flu is a BIT like a pandemic in a microcosm in that it mutates and presents a new target to our immunity, although NOT the same as being completely immune naive, otherwise we'd have annual pandemics!
We counter this with our vaccines, tried and tested, pretty good...

BUT, if the vaccine match is poor (despite massive worldwide surveillance and time to prepare for a single annual wave), or uptake low, we have a "bad" flu season, sometimes >20K deaths, albeit mainly due to 2ndry bacterial pneumonia...again we are "used to this"🙄

Makes you wonder why we don't give out more vaccines, really...
Which reminds me...so, what happened during the time when we actually paid attention to mitigating infectious diseases for once on account of an ACTUAL pandemic?!
We are "NOT used to this!"
Well, flu disappeared 👀

Seasonal flu isn't as infectious as SARS-CoV2, so the measures taken in terms of staying at home, hygiene, masks etc really worked - indeed, one of the flu B lineages almost went extinct! Almost...

So, what does this mean? Well, the number of folks catching flu dropped suddenly.

Obvious, eh?
As a consequence the number of people that haven't seen flu, survived (CRITICAL POINT!!!), and then became relatively "immune" plummeted from 2021 to 2022...
But, I hear you say, don't worry, we know 'flu will return, so we'll just vaccinate everyone in preparation!

Alas not...
The flu peak in 2022/23 was BAD...it even caused more hospitalisations than SARS-CoV2 for about a week...not playing virus top trumps, just setting context...
Flu coincided with SARS2, and RSV. Then came Strep A outbreaks...but, why?! "We aren't used to this!!!"

Not to worry, the immunology departments from certain tabloids...and some that should really know better...had the answer...

IMMUNITY DEBT!!!!!!😬😬😬😜

What does this mean?
Well, those of you paying attention to my drivel will clearly presume this refers to an increased susceptible pool resulting from failing to supplement population immunity by vaccines...we of course know flu can promote GAS and iGAS by dysbiosis...simples😜

This makes sense, more infections = more severe disease, just timed extremely badly, no effort to increase vax, then of course nothing preventative was done to stop scarlet fever or other GAS nasties spreading like wildfire amongst kids, and, tragically, causing some deaths...💔

But, no...apparently, it's a twisted version of the debunked hygiene hypothesis, where every time we get sick is actually a "good thing" because we're sending lymph nodes on some f*****g Krypton Factor assault course, flexing those TCR muscles, and "having healthy immune systems"

People genuinely thought that the lives saved by NOT having an influenza wave but then making up for it by not vaccinating more was explained by "living too cleanly".
We MUST immediately all resort to licking toilet seats in public conveniences and eating toenail scrapings...🙄

What really takes the smegging biscuit though, is that now, in 2024, two years since the last set of COVID restrictions, this is apparently happening again, despite flu in 22, RSV in 21/22, and everything else...
It even applies to countries that didn't have restrictions, again!

Let's be clear, disease is a function of immunity, virulence and environment.
Your immunity is preprogrammed genetically.
Fighting TB does NOT make you better at fighting whooping cough.
A first encounter with a pathogen is usually the same, regardless if it's delayed by a year.

The trick is to ensure the first encounter incurs the least possible risk...hence why that Jenner chap became so popular and people started avoiding scraping that puss into their skin...
Your 2nd response is stronger and quicker, and for some diseases can = sterilising immunity!

Yet, we still see references to this ridiculous term, which according to learned colleagues, was first referenced at the start of the pandemic...
Eejits railing for freedom to become infected actually consider this a reason to, well, get infected with stuff.
Best of luck, folks.

I see countless stories and quotes...from folks that should know better...that disease is inherently more severe because of the "immunity debt", this is, of course 💩.
Some may then rethink and say they meant the susceptible pool thing, but they REALLY need to mind what they say.

The reason I detest this crap SO much is that it all feeds the notion that infection is a better way of instilling immunity, rather than vaccines.
This underpins current policy, it's complete crap. There's ALWAYS survivorship bias, and SARS2 evolves too quickly for you-know-what.

Lastly, the simple binary that you get infected, then immune, ignores the plight of the vulnerable, as well as both long COVID and other sequelae that cost people their quality of life, or their lives. It's harmful, callous, and amoral.

Tragically tho, we ARE "used to this" 💔