The disappearance of flu - because of COVID

I have mentioned this in some of the other COVID threads – the flu season of 2019-2020 was pretty quiet… but the 2020-2021 season was nonexistent.

Various people have theories – in this specific SciAm article, they think it was due to public health measures. Given the international spread of COVID, the international differences in public health measures, and the international universality of the disappearance of flu… I am skeptical about that explanation.

I think it’s viral interference.

This is from 2015:

A few graphs from the piece:

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This maybe presents a clearer argument that our NPIs did not eradicate flu:

That’s not an especially compelling argument. All you need to do is drive the reproduction rate of a virus far enough below 1 and it stops spreading. Maybe a little distancing, a little mask wearing, and a little staying home if you have symptoms or recently hung out with people who have symptoms is enough to drive the flu underground.

As for viral interference? The % of people who actively have covid at any time has been lower than the % of people wearing masks and “distancing” in pretty much every place the article talks about. What is the mechanism by which viral interference would stop the spread of flu but social-distancing wouldn’t?


If you are taking measures to prevent the spread of respiratory viruses, what you end up with is the most infectious virus being the one that slips through the most often. That started off as COVID in 2020, and in 2021 became the UK variant of it. That seems less due to “viral interference” and just a result that the reproductive rate of COVID being significantly higher than the flu.

Looking just at the US, I agree that not a lot of people were wearing masks in mid-March. It was very early in the pandemic, mostly a small but obviously growing out of control concern in nyc.

So I agree the US data doesn’t tell us anything about masks. That said, in mid-march also wasn’t much Covid in the US-- no matter how you count it was the very beginning of our outbreak and 99% did not have immunity.

What we did suddenly start doing in mid March is wfh more, travel less, wash hands more, and go out less, stay away from each other. So probably that did it.

To find out about masks you’d need other data. I don’t think country comparisons are all that good, especially random small countries.

Maybe the 2019 and prior flu morphed into Covid-19 and it is now our new flu and will keep morphing from there, eventually making it really deadly?

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I think viruses usually go the other way. Killing your host isn’t a very effective mechanism for spreading. The Spanish Flu is still around, for example.

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Hmm, didn’t know that. Thanks ND!

Well, specifically, viruses do poorly if they incapacitate their host before they can spread. One if the reasons covid is so successful is that it’s infectious for a few days before it produces any noticable symptoms. Sadly, that puts less evolutionary pressure on it to become milder.

I was reading over the weekend that the pandemic known as the “Russian flu” in 1889

is now thought to probably have been the first outbreak of one of the four coronaviruses that now causes “a cold”. It’s milder now mostly because everyone gets exposed to it as a child, and acquires some immunity.

This is not at all surprising.

The R0 for the flu, based on countless decades-long mutations is 1.38

For Covid-19, we are looking at 3.0 - 5.0 (depending on the variants) for the latest transmission numbers, so its not unusual that under a lockdown environment, or due to social distancing restrictions, the pathogen with the lower transmission factor essentially dies out.

My main argument is that flu has been squashed -everywhere-.

Lots of places did not have particularly strong lockdowns/ended them early/etc. They also have non-existent flu levels. Same as everywhere else.

If you want to show that it was lockdowns, you need to show that places that did -not- lockdown had detectable flu transmission. Or at least something worse than the places that did. But that’s not what we’re seeing.

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maybe covid destroyed regular flu. like sapiens did neanderthals.

Isn’t international / inter-regional propagation an element in the transmission of the flu?

Even in places that didn’t locally lock-down (or only saw limited lockdowns at particular times of the pandemic), long-distance travel was severely reduced starting early last year.

The fact that location X had a reduction in the flu but only limited COVID-related restrictions may not mean much if travel to location X had been constrained.


It’s not just a “reduction in the flu;” it basically disappeared -everywhere- in early March 2020 and has barely been heard from since, even in places without lockdowns and where travel has not been restricted. (I’ll grant that travel is likely down everywhere, even where not restricted.)

Who didn’t lock down? I mean, Sweden is often mentioned because they didn’t mandate a lot of stuff, but they still recommended that people travel less, distance more, etc., and the google location data shows that Swedes moved around a lot less, just like everyone else.

Understood. However, long-distance travel dropped to almost nil for a large part of last year. I’m offering that as a hypothesis for a contributing factor to the lack of flu during the COVID pandemic.

I suspect that the real answer is a combination of factors. The lack of travel might help explain why the incidence of flu dropped in places where lockdowns / pandemic protocols weren’t as robust.

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good thing I didn’t get the flu vaccine!

Hasn’t New Zealand been pretty much open after their initial lockdown…aside from their having closed their border?

That’s part of the reason for my long-distance travel hypothesis: to the extent that particular variants of the flu emerge someplace and then propagate…if there’s no travel, propagation is a lot more difficult.

Taiwan has been pretty much open too. It’s locked down as a country, but not within.

Taiwan has had mask mandates and has handed out masks to its residents.