Thread by @TRyanGregory

I noted recently that claims about exceptionally high rates of RSV this year, caused by "immunity debt" and explaining the high number of children in hospital, is an empirical claim that requires data. Well, we now have this for Canada thanks to @Wikisteff (follow him btw!). 🧵

Here is a plot of RSV rates since winter 2013/2014. As you can see, it's a very regular seasonal pattern for all the years up to the start of the COVID pandemic. There was essentially no RSV two years ago when schools were closed (winter 2020/2021).

And it's not just RSV. There was almost no flu or colds caused by other coronaviruses (not SARS2) two years ago either.

So, we can conclude two things right away: 1) mitigations worked extremely well and absolutely crushed respiratory virus transmission, and) the first assumption of "immunity debt" holds, namely that there was a season without kids being infected with RSV or flu.

BUT, look at last year. Winter 2021/2022 was a record season for RSV. It is simply not accurate that we are seeing three years' worth of RSV cases this year. RSV was back last year after many mitigations were lifted.

Last year, there were lots of cases of RSV but hospitals were not overwhelmed like they are right now. This year, RSV season has started early but the case numbers are not exceptionally high yet. Still, many kids are in the hospital already.

So, this doesn't fit with the simple "immunity debt" hypothesis. Something else seems to be going on. Of course, high rates of severe infection without more cases is consistent with the "immunity theft" hypothesis in which past COVID infection impairs immune function.

But there is another interesting observation pointed out by @Prassas, namely that flu was NOT back last year the way RSV was. Is there something about RSV and flu co-infections that matters? Immunity debt to flu but not RSV? Is flu more severe this year?

We still don't know what exactly is happening. What is pretty clear, though, is that assuming that it's simply "immunity debt" after two years of no RSV infections is false in Canada. This is why we need to test multiple hypotheses and why we need data, not just-so stories.

In the meantime, we *know* that mitigations work against the viruses that are leading to kids becoming very ill and pediatric hospitals being overwhelmed. Whatever the explanation, the right thing to do *right now* is to reduce transmission of these viruses in children. #MaskUp

Again, huge thank you to @Wikisteff for the data sleuthing -- this is exactly what we need. Also, as he noted:

For a bit more on "immunity debt" vs. "immunity theft" (the latter being only a slightly modified-for-catchiness version of @FurnessColin's "immunity robbery").

Don't forget to check out this fantastic discussion on @TheAgenda. Note that a couple of the claims made by Dr. McGeer (three years of RSV all at once after two years without, this only being a problem with pediatric hospitals in Canada) are not accurate.

It bears noting that there *is* a mechanism by which COVID infection can impair defence against RSV.

journals.plos.org/plospathogens/…

frontiersin.org/articles/10.33…

mdpi.com/1999-4915/12/1…

nature.com/articles/s4142…

There was also plenty of RSV in the US last year.