Just because this is the top comment right now, I'd like to point out that the conventional explanation works fine too. The interventions taken against covid worked even better against the flu and because diseases are exponential you only have to get the reproduction rate a bit below 1 to see huge impacts. Oh and also covid is currently a more dangerous disease so it would still be pretty bad if flu was replaced by it

Yeah, right, why do we still have a bunch of common cold viruses then? Any argument referring the interventions has to also explain why _only_ flu disappeared out of huge variety of respiratory viruses.

Some of the cold viruses are extremely infectious, certainly moreso than boring ol' Original Covid or Alpha, and some possibly more than Delta.

R number for influenza is normally reckoned at well under 1.5; it's really not that infectious.

> why _only_ flu disappeared out of huge variety of respiratory viruses.

Some of the viruses which cause the common cold may well have become rare or outright disappeared; we probably wouldn't notice.

But if you assume that measures are equally effective against flu, cold and covid, you'd probably expect flu to be suppressed first, then ancestral and alpha covid, then most colds, then delta.

Case in point: This summer my family all came down with Norovirus, aka the "winter" vomitting bug. This is much more infectious than COVID, each year infecting roughly 685 million people.

We're talking about a flu, not the flu. There are still plenty of influenza viruses out there, alive, well, and waiting. The difference with this particular lineage is that it seems to be human-only, with no known animal reservoir.

I've seen similar, and don't doubt it. I just keep wondering: how does that work, mechanistically? A coronavirus infection doesn't protect me from influenza: it should be perfectly possible to be infected by both. How do they compete?

Influenza and coronavirus 'compete' by forcing people to alter their behavior, thus removing potential carriers from the general population before the other virus can reach them. The more infectious virus wins this competition.

The flu isn't very infectious compared to even ancestral covid; its R number is normally reckoned at well below 1.5, so covid suppression techniques (which pushed covid's below 1 in many places last year) would have been pretty hard on it.